I was shocked when I read this (from the OpenGL wiki):
glTranslate, glRotate, glScale
Are these hardware accelerated?
No, there are no known GPUs that
execute this. The driver computes the
matrix on the CPU and uploads it to
the GPU.
All the other matrix operations are
done on the CPU as well :
glPushMatrix, glPopMatrix,
glLoadIdentity, glFrustum, glOrtho.
This is the reason why these functions
are considered deprecated in GL 3.0.
You should have your own math library,
build your own matrix, upload your
matrix to the shader.
For a very, very long time I thought most of the OpenGL functions use the GPU to do computation. I'm not sure if this is a common misconception, but after a while of thinking, this makes sense. Old OpenGL functions (2.x and older) are really not suitable for real-world applications, due to too many state switches.
This makes me realise that, possibly, many OpenGL functions do not use the GPU at all.
So, the question is:
Which OpenGL function calls don't use the GPU?
I believe knowing the answer to the above question would help me become a better programmer with OpenGL. Please do share some of your insights.
Edit:
I know this question easily leads to optimisation level. It's good, but it's not the intention of this question.
If anyone knows a set of GL functions on a certain popular implementation (as AshleysBrain suggested, nVidia/ATI, and possibly OS-dependent) that don't use the GPU, that's what I'm after!
Plausible optimisation guides come later. Let's focus on the functions, for this topic.
Edit2:
This topic isn't about how matrix transformations work. There are other topics for that.
Boy, is this a big subject.
First, I'll start with the obvious: Since you're calling the function (any function) from the CPU, it has to run at least partly on the CPU. So the question really is, how much of the work is done on the CPU and how much on the GPU.
Second, in order for the GPU to get to execute some command, the CPU has to prepare a command description to pass down. The minimal set here is a command token describing what to do, as well as the data for the operation to be executed. How the CPU triggers the GPU to do the command is also somewhat important. Since most of the time, this is expensive, the CPU does not do it often, but rather batches commands in command buffers, and simply sends a whole buffer for the GPU to handle.
All this to say that passing work down to the GPU is not a free exercise. That cost has to be pitted against just running the function on the CPU (no matter what we're talking about).
Taking a step back, you have to ask yourself why you need a GPU at all. The fact is, a pure CPU implementation does the job (as AshleysBrain mentions). The power of the GPU comes from its design to handle:
specialized tasks (rasterization, blending, texture filtering, blitting, ...)
heavily parallel workloads (DeadMG is pointing to that in his answer), when a CPU is more designed to handle single-threaded work.
And those are the guiding principles to follow in order to decide what goes in the chip. Anything that can benefit from those ought to run on the GPU. Anything else ought to be on the CPU.
It's interesting, by the way. Some functionality of the GL (prior to deprecation, mostly) are really not clearly delineated. Display lists are probably the best example of such a feature. Each driver is free to push as much as it wants from the display list stream to the GPU (typically in some command buffer form) for later execution, as long as the semantics of the GL display lists are kept (and that is somewhat hard in general). So some implementations only choose to push a limited subset of the calls in a display list to a computed format, and choose to simply replay the rest of the command stream on the CPU.
Selection is another one where it's unclear whether there is value to executing on the GPU.
Lastly, I have to say that in general, there is little correlation between the API calls and the amount of work on either the CPU or the GPU. A state setting API tends to only modify a structure somewhere in the driver data. It's effect is only visible when a Draw, or some such, is called.
A lot of the GL API works like that. At that point, asking whether glEnable(GL_BLEND) is executed on the CPU or GPU is rather meaningless. What matters is whether the blending will happen on the GPU when Draw is called. So, in that sense, Most GL entry points are not accelerated at all.
I could also expand a bit on data transfer but Danvil touched on it.
I'll finish with the little "s/w path". Historically, GL had to work to spec no matter what the hardware special cases were. Which meant that if the h/w was not handling a specific GL feature, then it had to emulate it, or implement it fully in software. There are numerous cases of this, but one that struck a lot of people is when GLSL started to show up.
Since there was no practical way to estimate the code size of a GLSL shader, it was decided that the GL was supposed to take any shader length as valid. The implication was fairly clear: either implement h/w that could take arbitrary length shaders -not realistic at the time-, or implement a s/w shader emulation (or, as some vendors chose to, simply fail to be compliant). So, if you triggered this condition on a fragment shader, chances were the whole of your GL ended up being executed on the CPU, even when you had a GPU siting idle, at least for that draw.
The question should perhaps be "What functions eat an unexpectedly high amount of CPU time?"
Keeping a matrix stack for projection and view is not a thing the GPU can handle better than a CPU would (on the contrary ...). Another example would be shader compilation. Why should this run on the GPU? There is a parser, a compiler, ..., which are just normal CPU programs like the C++ compiler.
Potentially "dangerous" function calls are for example glReadPixels, because data can be copied from host (=CPU) memory to device (=GPU) memory over the limited bus. In this category are also functions like glTexImage_D or glBufferData.
So generally speaking, if you want to know how much CPU time an OpenGL call eats, try to understand its functionality. And beware of all functions, which copy data from host to device and back!
Typically, if an operation is per-something, it will occur on the GPU. An example is the actual transformation - this is done once per vertex. On the other hand, if it occurs only once per large operation, it'll be on the CPU - such as creating the transformation matrix, which is only done once for each time the object's state changes, or once per frame.
That's just a general answer and some functionality will occur the other way around - as well as being implementation dependent. However, typically, it shouldn't matter to you, the programmer. As long as you allow the GPU plenty of time to do it's work while you're off doing the game sim or whatever, or have a solid threading model, you shouldn't need to worry about it that much.
#sending data to GPU: As far as I know (only used Direct3D) it's all done in-shader, that's what shaders are for.
glTranslate, glRotate and glScale change the current active transformation matrix. This is of course a CPU operation. The model view and projection matrices just describes how the GPU should transforms vertices when issue a rendering command.
So e.g. by calling glTranslate nothing is translated at all yet. Before rendering the current projection and model view matrices are multiplied (MVP = projection * modelview) then this single matrix is copied to the GPU and then the GPU does the matrix * vertex multiplications ("T&L") for each vertex. So the translation/scaling/projection of the vertices is done by the GPU.
Also you really should not be worried about the performance if you don't use these functions in an inner loop somewhere. glTranslate results in three additions. glScale and glRotate are a bit more complex.
My advice is that you should learn a bit more about linear algebra. This is essential for working with 3D APIs.
There are software rendered implementations of OpenGL, so it's possible that no OpenGL functions run on the GPU. There's also hardware that doesn't support certain render states in hardware, so if you set a certain state, switch to software rendering, and again, nothing will run on the GPU (even though there's one there). So I don't think there's any clear distinction between 'GPU-accelerated functions' and 'non-GPU accelerated functions'.
To be on the safe side, keep things as simple as possible. The straightforward rendering-with-vertices and basic features like Z buffering are most likely to be hardware accelerated, so if you can stick to that with the minimum state changing, you'll be most likely to keep things hardware accelerated. This is also the way to maximize performance of hardware-accelerated rendering - graphics cards like to stay in one state and just crunch a bunch of vertices.
Related
I'm looking into whether it's better for me to stay with OpenGL or consider a Vulkan migration for intensive bottlenecked rendering.
However I don't want to make the jump without being informed about it. I was looking up what benefits Vulkan offers me, but with a lot of googling I wasn't able to come across exactly what gives performance boosts. People will throw around terms like "OpenGL is slow, Vulkan is way faster!" or "Low power consumption!" and say nothing more on the subject.
Because of this, it makes it difficult for me to evaluate whether or not the problems I face are something Vulkan can help me with, or if my problems are due to volume and computation (and Vulkan would in such a case not help me much).
I'm assuming Vulkan does not magically make things in the pipeline faster (as in shading in triangles is going to be approximately the same between OpenGL and Vulkan for the same buffers and uniforms and shader). I'm assuming all the things with OpenGL that cause grief (ex: framebuffer and shader program changes) are going to be equally as painful in either API.
There are a few things off the top of my head that I think Vulkan offers based on reading through countless things online (and I'm guessing this certainly is not all the advantages, or whether these are even true):
Texture rendering without [much? any?] binding (or rather a better version of 'bindless textures'), which I've noticed when I switched to bindless textures I gained a significant performance boost, but this might not even be worth mentioning as a point if bindless textures effectively does this and therefore am not sure if Vulkan adds anything here
Reduced CPU/GPU communication by composing some kind of command list that you can execute on the GPU without needing to send much data
Being able to interface in a multithreaded way that OpenGL can't somehow
However I don't know exactly what cases people run into in the real world that demand these, and how OpenGL limits these. All the examples so far online say "you can run faster!" but I haven't seen how people have been using it to run faster.
Where can I find information that answers this question? Or do you know some tangible examples that would answer this for me? Maybe a better question would be where are the typical pain points that people have with OpenGL (or D3D) that caused Vulkan to become a thing in the first place?
An example of answer that would not be satisfying would be a response like
You can multithread and submit things to Vulkan quicker.
but a response that would be more satisfying would be something like
In Vulkan you can multithread your submissions to the GPU. In OpenGL you can't do this because you rely on the implementation to do the appropriate locking and placing fences on your behalf which may end up creating a bottleneck. A quick example of this would be [short example here of a case where OpenGL doesn't cut it for situation X] and in Vulkan it is solved by [action Y].
The last paragraph above may not be accurate whatsoever, but I was trying to give an example of what I'd be looking for without trying to write something egregiously wrong.
Vulkan really has four main advantages in terms of run-time behavior:
Lower CPU load
Predictable CPU load
Better memory interfaces
Predictable memory load
Specifically lower GPU load isn't one of the advantages; the same content using the same GPU features will have very similar GPU performance with both of the APIs.
In my opinion it also has many advantages in terms of developer usability - the programmer's model is a lot cleaner than OpenGL, but there is a steeper learning curve to get to the "something working correctly" stage.
Let's look at each of the advantages in more detail:
Lower CPU load
The lower CPU load in Vulkan comes from multiple areas, but the main ones are:
The API encourages up-front construction of descriptors, so you're not rebuilding state on a draw-by-draw basis.
The API is asynchronous and can therefore move some responsibilities, such as tracking resource dependencies, to the application. A naive application implementation here will be just as slow as OpenGL, but the application has more scope to apply high level algorithmic optimizations because it can know how resources are used and how they relate to the scene structure.
The API moves error checking out to layer drivers, so the release drivers are as lean as possible.
The API encourages multithreading, which is always a great win (especially on mobile where e.g. four threads running slowly will consume a lot less energy than one thread running fast).
Predictable CPU load
OpenGL drivers do various kinds of "magic", either for performance (specializing shaders based on state only known late at draw time), or to maintain the synchronous rendering illusion (creating resource ghosts on the fly to avoid stalling the pipeline when the application modifies a resource which is still referenced by a pending command).
The Vulkan design philosophy is "no magic". You get what you ask for, when you ask for it. Hopefully this means no random slowdowns because the driver is doing something you didn't expect in the background. The downside is that the application takes on the responsibility for doing the right thing ;)
Better memory interfaces
Many parts of the OpenGL design are based on distinct CPU and GPU memory pools which require a programming model which gives the driver enough information to keep them in sync. Most modern hardware can do better with hardware-backed coherency protocols, so Vulkan enables a model where you can just map a buffer once, and then modify it adhoc and guarantee that the "other process" will see the changes. No more "map" / "unmap" / "invalidate" overhead (provided the platform supports coherent buffers, of course, it's still not universal).
Secondly Vulkan separates the concept of the memory allocation and how that memory is used (the memory view). This allows the same memory to be recycled for different things in the frame pipeline, reducing the amount of intermediate storage you need allocated.
Predictable memory load
Related to the "no magic" comment for CPU performance, Vulkan won't generate random resources (e.g. ghosted textures) on the fly to hide application problems. No more random fluctuations in resource memory footprint, but again the application has to take on the responsibility to do the right thing.
This is at risk of being opinion based. I suppose I will just reiterate the Vulkan advantages that are written on the box, and hopefully uncontested.
You can disable validation in Vulkan. It obviously uses less CPU (or battery\power\noise) that way. In some cases this can be significant.
OpenGL does have poorly defined multi-threading. Vulkan has well defined multi-threading in the specification. Meaning you do not immediately lose your mind trying to code with multiple threads, as well as better performance if otherwise the single thread would be a bottleneck on CPU.
Vulkan is more explicit; it does not (or tries to not) expose big magic black boxes. That means e.g. you can do something about micro-stutter and hitching, and other micro-optimizations.
Vulkan has cleaner interface to windowing systems. No more odd contexts and default framebuffers. Vulkan does not even require window to draw (or it can achieve it without weird hacks).
Vulkan is cleaner and more conventional API. For me that means it is easier to learn (despite the other things) and more satisfying to use.
Vulkan takes binary intermediate code shaders. While OpenGL used not to. That should mean faster compilation of such code.
Vulkan has mobile GPUs as first class citizen. No more ES.
Vulkan have open source, and conventional (GitHub) public tracker(s). Meaning you can improve the ecosystem without going through hoops. E.g. you can improve\implement a validation check for error that often trips you. Or you can improve the specification so it does make sense for people that are not insiders.
I'm planning to do some optimization on my OpenGL program (it doesn't need optimizing, but I'm doing it for the sake of it). Out of curiosity, how expensive are OpenGL drawing functions compared to basic logic code? At the moment, I'm making the start of a game where the screen is filled with squares, to represent a 2D blocky landscape. This means that the draw call for a square(two triangles) is called many times. At the moment, I'm planning to add in some code that looks at the positioning of blocks in the current frame, and groups them together. For example, if there is a column that is 7 blocks high, instead of doing 7 separate drawBlock() functions (which contain the glDrawElements() calls) I could call one function, that draws a rectangle that is 1 x 7, and so on, throughout the screen.
I won't bother doing this if the code that calculates what to draw, actually uses up more of the CPU than just drawing the blocks individually would.
The cost of glDrawElements (or any other OpenGL rendering command) cannot really be estimated. This is because its cost depends a great deal on what OpenGL state you changed between draw calls. The cost of calling an OpenGL state changing function (basically, any OpenGL function that isn't a glGet of some form or a glDraw of some form) will be relatively quick. But it will make the next draw call slower.
This video on OpenGL performance shows which state changes are more costly at draw time than others. The really good part starts around 31 minutes in.
Draw calls are relatively fast if you haven't changed any OpenGL state between draw calls. Different pieces of state have different effects on draw calls. From fastest to slowest (according to NVIDIA's presentation above, so take it with a grain of salt):
Non-UBO uniform updates
Vertex buffer bindings (without changing formats)
UBO binding
Vertex format changes
Texture bindings
Fragment post-processing state changes
Shader program changes
Render target switches
Now, a draw call will be more expensive than "basic logic". They're not cheap, even without state changes between them. If efficiency is important to your code, then grouping your squares is a good idea.
The actual numbers are highly platform and vendor dependent. Driver architectures on different operating systems vary substantially, and some of them are more efficient than others. On top of that, driver implementations and hardware can cause large performance differences. For example, I've seen 10-20 times higher draw call throughput for one vendor compared to another vendor, on the same platform and with comparable hardware.
Based on this, any numbers below are just a very rough order of magnitude. You really need to measure this yourself on the configurations you care about.
With all these disclaimers, I would expect that a draw call could be processed in the range of 100 instructions (CPU cycles). This is for the case where you just make back to back draw calls, and there are no other bottlenecks in the pipeline.
As #NicolBolas already pointed out, the most expensive part of handling draw calls is normally processing deferred state changes. And most of the time, you will have state changes between draw calls. In this case, for relatively cheap state changes (like binding a texture or buffer, or changing some attributes), a few 100 instructions are typical.
Switching frame buffers is generally quite expensive, and very expensive on some platforms. Other than that, the numbers I measured in the past while optimizing and benchmarking state changes showed an order that is quite different from the list in #NicolBolas' answer. But again, this is highly platform and vendor/hardware dependent.
There are a couple more aspects that makes this somewhat tricky to measure:
Most of the CPU time might not be consumed in your thread. Many drivers are multi-threaded, meaning that most of the work needed to process OpenGL calls is offloaded to a secondary thread. If your application does not use all CPU cores, and you're not throttled by power/thermal limits, this means that a lot of the driver work can happen in parallel, without slowing down your application much. But particularly on mobile devices and laptops, performance is often limited by power consumption, so the driver overhead will still slow you down.
CPU time consumed by the driver is only part of what can slow your application code down. Another consideration is cache pollution. If cache content used by your application is evicted while the OpenGL implementation processes your draw calls, your own code will get more cache misses, and will run slower. So measuring the time spent inside the OpenGL calls only shows part of the picture.
For the past month I've been messing with WebGL, and found that if I create and draw a large vertex buffer it causes low FPS. Does anyone know if it be the same if I used OpenGL with C++?
Is that a bottleneck with the language used (JavaScript in the case of WebGL) or the GPU?
WebGL examples like this show that you can draw 150,000 cubes using one buffer with good performance but anything more than this, I get FPS drops. Would that be the same with OpenGL, or would it be able to handle a larger buffer?
Basically, I've got to make a decision to continue using WebGL and try to optimise by code or - if you tell me OpenGL would perform better and it's a language speed bottleneck, switch to C++ and use OpenGL.
If you only have a single drawArrays call, there should not be much of a difference between OpenGL and WebGL for the call itself. However, setting up the data in Javascript might be a lot slower, so it really depends on your problem. If the bulk of your data is static (landscape, rooms), WebGL might work well for you. Otherwise, setting up the data in JS might be too slow for your purpose. It really depends on your problem.
p.s. If you include more details of what you are trying to do, you'll probably get more detailed / specific answers.
Anecdotally, I wrote a tile-based game in the early 2000's using the old glVertex() style API that ran perfectly smoothly. I recently started port it to WebGL and glDrawArrays() and now on my modern PC that is at least 10 times faster it gets terrible performance.
The reason seems to be that I was faking a call go glBegin(GL_QUADS); glVertex()*4; glEnd(); by using glDrawArrays(). Using glDrawArrays() to draw one polygon is much much slower in WebGL than doing the same with glVertex() was in C++.
I don't know why this is. Maybe it is because javascript is dog slow. Maybe it is because of some context switching issues in javascript. Anyway I can only do around 500 one-polygon glDrawArray() calls while still getting 60 FPS.
Everybody seems to work around this by doing as much on the GPU as possible, and doing as few glDrawArray() calls per frame as possible. Whether you can do this depends on what you are trying to draw. In the example of cubes you linked they can do everything on the GPU, including moving the cubes, which is why it is fast. Essentially they cheated - typically WebGL apps won't be like that.
Google had a talk where they explained this technique (they also unrealistically calculate the object motion on the GPU): https://www.youtube.com/watch?v=rfQ8rKGTVlg
OpenGL is more flexible and more optimized because of the newer versions of the api used.
It is true if you say that OpenGL is faster and more capable, but it also depends on your needs.
If you need one cube mesh with texture, webGL would be sufficient. However, if you intend building large-scale projects with lots of vertices, post-processing effects and different rendering techniques (and kind of displacement, parallax mapping, per-vertex or maybe tessellation) then OpenGL might be a better and wiser choice actually.
Optimizing buffers to a single call, optimizing update of those can be done, but it has its limits, of course, and yes, OpenGL would most likely perform better anyway.
To answer, it is not a language bottleneck, but an api-version-used one.
WebGL is based upon OpenGL ES, which has some pros but also runs a bit slower and it has more abstraction levels for code handling than pure OpenGL has, and that is reason for lowering performance - more code needs to be evaluated.
If your project doesn't require web-based solution, and doesn't care which devices are supported, then OpenGL would be a better and smarter choice.
Hope this helps.
WebGL is much slower on the same hardware compared to equivalent OpenGL, because of the high overheard for each WebGL call.
On desktop OpenGL, this overhead is at least limited, if still relatively expensive.
But in browsers like Chrome, WebGL requires that not only do we cross the FFI barrier to access those native OpenGL API calls (which still incur the same overhead), but we also have the cost of security checks to prevent the GPU being hijacked for computation.
If you are looking at something like glDraw* calls, which are called per frame, this means we are talking about perhaps (an) order(s) of magnitude fewer calls. All the more reason to opt for something like instancing, where the number of calls is drastically reduced.
What features make OpenCL unique to choose over OpenGL with GLSL for calculations? Despite the graphic related terminology and inpractical datatypes, is there any real caveat to OpenGL?
For example, parallel function evaluation can be done by rendering a to a texture using other textures. Reducing operations can be done by iteratively render to smaller and smaller textures. On the other hand, random write access is not possible in any efficient manner (the only way to do is rendering triangles by texture driven vertex data). Is this possible with OpenCL? What else is possible not possible with OpenGL?
OpenCL is created specifically for computing. When you do scientific computing using OpenGL you always have to think about how to map your computing problem to the graphics context (i.e. talk in terms of textures and geometric primitives like triangles etc.) in order to get your computation going.
In OpenCL you just formulate you computation with a calculation kernel on a memory buffer and you are good to go. This is actually a BIG win (saying that from a perspective of having thought through and implemented both variants).
The memory access patterns are though the same (your calculation still is happening on a GPU - but GPUs are getting more and more flexible these days).
But what else would you expect than using more than a dozen parallel "CPUs" without breaking your head about how to translate - e.g. (silly example) Fourier to Triangles and Quads...?
Something that hasn't been mentioned in any answers so far has been speed of execution. If your algorithm can be expressed in OpenGL graphics (e.g. no scattered writes, no local memory, no workgroups, etc.) it will very often run faster than an OpenCL counterpart. My specific experience of this has been doing image filter (gather) kernels across AMD, nVidia, IMG and Qualcomm GPUs. The OpenGL implementations invariably run faster even after hardcore OpenCL kernel optimization. (aside: I suspect this is due to years of hardware and drivers being specifically tuned to graphics orientated workloads.)
My advice would be that if your compute program feels like it maps nicely to the graphics domain then use OpenGL. If not, OpenCL is more general and simpler to express compute problems.
Another point to mention (or to ask) is whether you are writing as a hobbyist (i.e. for yourself) or commercially (i.e. for distribution to others). While OpenGL is supported pretty much everywhere, OpenCL is totally lacking support on mobile devices and, imho, is highly unlikely to appear on Android or iOS in the next few years. If wide cross platform compatibility from a single code base is a goal then OpenGL may be forced upon you.
What features make OpenCL unique to choose over OpenGL with GLSL for calculations? Despite the graphic related terminology and inpractical datatypes, is there any real caveat to OpenGL?
Yes: it's a graphics API. Therefore, everything you do in it has to be formulated along those terms. You have to package your data as some form of "rendering". You have to figure out how to deal with your data in terms of attributes, uniform buffers, and textures.
With OpenGL 4.3 and OpenGL ES 3.1 compute shaders, things become a bit more muddled. A compute shader is able to access memory via SSBOs/Image Load/Store in similar ways to OpenCL compute operations (though OpenCL offers actual pointers, while GLSL does not). Their interop with OpenGL is also much faster than OpenCL/GL interop.
Even so, compute shaders do not change one fact: OpenCL compute operations operate at a very different precision than OpenGL's compute shaders. GLSL's floating-point precision requirements are not very strict, and OpenGL ES's are even less strict. So if floating-point accuracy is important to your calculations, OpenGL will not be the most effective way of computing what you need to compute.
Also, OpenGL compute shaders require 4.x-capable hardware, while OpenCL can run on much more inferior hardware.
Furthermore, if you're doing compute by co-opting the rendering pipeline, OpenGL drivers will still assume that you're doing rendering. So it's going to make optimization decisions based on that assumption. It will optimize the assignment of shader resources assuming you're drawing a picture.
For example, if you're rendering to a floating-point framebuffer, the driver might just decide to give you an R11_G11_B10 framebuffer, because it detects that you aren't doing anything with the alpha and your algorithm could tolerate the lower precision. If you use image load/store instead of a framebuffer however, you're much less likely to get this effect.
OpenCL is not a graphics API; it's a computation API.
Also, OpenCL just gives you access to more stuff. It gives you access to memory levels that are implicit with regard to GL. Certain memory can be shared between threads, but separate shader instances in GL are unable to directly affect one-another (outside of Image Load/Store, but OpenCL runs on hardware that doesn't have access to that).
OpenGL hides what the hardware is doing behind an abstraction. OpenCL exposes you to almost exactly what's going on.
You can use OpenGL to do arbitrary computations. But you don't want to; not while there's a perfectly viable alternative. Compute in OpenGL lives to service the graphics pipeline.
The only reason to pick OpenGL for any kind of non-rendering compute operation is to support hardware that can't run OpenCL. At the present time, this includes a lot of mobile hardware.
One notable feature would be scattered writes, another would be the absence of "Windows 7 smartness". Windows 7 will, as you probably know, kill the display driver if OpenGL does not flush for 2 seconds or so (don't nail me down on the exact time, but I think it's 2 secs). This may be annoying if you have a lengthy operation.
Also, OpenCL obviously works with a much greater variety of hardware than just the graphics card, and it does not have a rigid graphics-oriented pipeline with "artificial constraints". It is easier (trivial) to run several concurrent command streams too.
Although currently OpenGL would be the better choice for graphics, this is not permanent.
It could be practical for OpenGL to eventually merge as an extension of OpenCL. The two platforms are about 80% the same, but have different syntax quirks, different nomenclature for roughly the same components of the hardware. That means two languages to learn, two APIs to figure out. Graphics driver developers would prefer a merge because they no longer would have to develop for two separate platforms. That leaves more time and resources for driver debugging. ;)
Another thing to consider is that the origins of OpenGL and OpenCL are different: OpenGL began and gained momentum during the early fixed-pipeline-over-a-network days and was slowly appended and deprecated as the technology evolved. OpenCL, in some ways, is an evolution of OpenGL in the sense that OpenGL started being used for numerical processing as the (unplanned) flexibility of GPUs allowed so. "Graphics vs. Computing" is really more of a semantic argument. In both cases you're always trying to map your math operations to hardware with the highest performance possible. There are parts of GPU hardware which vanilla CL won't use but that won't keep a separate extension from doing so.
So how could OpenGL work under CL? Speculatively, triangle rasterizers could be enqueued as a special CL task. Special GLSL functions could be implemented in vanilla OpenCL, then overridden to hardware accelerated instructions by the driver during kernel compilation. Writing a shader in OpenCL, pending the library extensions were supplied, doesn't sound like a painful experience at all.
To call one to have more features than the other doesn't make much sense as they're both gaining 80% the same features, just under different nomenclature. To claim that OpenCL is not good for graphics because it is designed for computing doesn't make sense because graphics processing is computing.
Another major reason is that OpenGL\GLSL are supported only on graphics cards. Although multi-core usage started with using graphics hardware there are many hardware vendors working on multi-core hardware platform targeted for computation. For example see Intels Knights Corner.
Developing code for computation using OpenGL\GLSL will prevent you from using any hardware that is not a graphics card.
Well as of OpenGL 4.5 these are the features OpenCL 2.0 has that OpenGL 4.5 Doesn't (as far as I could tell) (this does not cover the features that OpenGL has that OpenCL doesn't):
Events
Better Atomics
Blocks
Workgroup Functions:
work_group_all and work_group_any
work_group_broadcast:
work_group_reduce
work_group_inclusive/exclusive_scan
Enqueue Kernel from Kernel
Pointers (though if you are executing on the GPU this probably doesn't matter)
A few math functions that OpenGL doesn't have (though you could construct them yourself in OpenGL)
Shared Virtual Memory
(More) Compiler Options for Kernels
Easy to select a particular GPU (or otherwise)
Can run on the CPU when no GPU
More support for those niche hardware platforms (e.g. FGPAs)
On some (all?) platforms you do not need a window (and its context binding) to do calculations.
OpenCL allows just a bit more control over precision of calculations (including some through those compiler options).
A lot of the above are mostly for better CPU - GPU interaction: Events, Shared Virtual Memory, Pointers (although these could potentially benefit other stuff too).
OpenGL has gained the ability to sort things into different areas of Client and Server memory since a lot of the other posts here have been made.
OpenGL has better memory barrier and atomics support now and allows you to allocate things to different registers within the GPU (to about the same degree OpenCL can). For example you can share registers in the local compute group now in OpenGL (using something like the AMD GPUs LDS (local data share) (though this particular feature only works with OpenGL compute shaders at this time).
OpenGL has stronger more performing implementations on some platforms (such as Open Source Linux drivers).
OpenGL has access to more fixed function hardware (like other answers have said). While it is true that sometimes fixed function hardware can be avoided (e.g. Crytek uses a "software" implementation of a depth buffer) fixed function hardware can manage memory just fine (and usually a lot better than someone who isn't working for a GPU hardware company could) and is just vastly superior in most cases. I must admit OpenCL has pretty good fixed function texture support which is one of the major OpenGL fixed function areas.
I would argue that Intels Knights Corner is a x86 GPU that controls itself.
I would also argue that OpenCL 2.0 with its texture functions (which are actually in lesser versions of OpenCL) can be used to much the same performance degree user2746401 suggested.
In addition to the already existing answers, OpenCL/CUDA not only fits more to the computational domain, but also doesn't abstract away the underlying hardware too much. This way you can profit from things like shared memory or coalesced memory access more directly, which would otherwise be burried in the actual implementation of the shader (which itself is nothing more than a special OpenCL/CUDA kernel, if you want).
Though to profit from such things you also need to be a bit more aware of the specific hardware your kernel will run on, but don't try to explicitly take those things into account using a shader (if even completely possible).
Once you do something more complex than simple level 1 BLAS routines, you will surely appreciate the flexibility and genericity of OpenCL/CUDA.
The "feature" that OpenCL is designed for general-purpose computation, while OpenGL is for graphics. You can do anything in GL (it is Turing-complete) but then you are driving in a nail using the handle of the screwdriver as a hammer.
Also, OpenCL can run not just on GPUs, but also on CPUs and various dedicated accelerators.
OpenCL (in 2.0 version) describes heterogeneous computational environment, where every component of system can both produce & consume tasks, generated by other system components. No more CPU, GPU (etc) notions are longer needed - you have just Host & Device(s).
OpenGL, in opposite, has strict division to CPU, which is task producer & GPU, which is task consumer. That's not bad, as less flexibility ensures greater performance. OpenGL is just more narrow-scope instrument.
One thought is to write your program in both and test them with respect to your priorities.
For example: If you're processing a pipeline of images, maybe your implementation in openGL or openCL is faster than the other.
Good luck.
I'm curious how expensive functions like:
glViewPort
glLoadIdentity
glOrtho
are in terms of both the work done on the CPU and the work done on the GPU.
Where is this documented?
This kind of thing is probably pretty dependent on your platform. Your best bet is probably to use a profiler yourself if you're worried about it.
As Alex O'Konski mentions, this is highly dependent on the platform.
That said, if you're interested in recent graphics cards of the PCs, You should know that most of them don't "do work" on the GPU. they set up state for future draw calls.
This is important because their cost is more related with how well the GPU can pipeline them between various draw calls that flow through the chip than how much time it takes to change a register from one value to the next.
Most platform vendors do not document at all what the costs of various state changes are. They don't document how OpenGL state maps to their hardware state, for that matter.
Last, state changes like matrix state (glLoadIdentity and glOrtho) are a remnant of the past. In modern graphics cards, they are simply helper (CPU) functions to set up uniforms (and this is why they are deprecated in core GL 3.1). And all the math they require (usually not much) is done on the CPU, inside the driver.