So I'm considering two different approaches to render objects in OpenGL.
First approach:
Use the TRIANGLE_STRIP setting with the idea of compressing the amount of vertex data written into the buffer every render. The drawback here is that normals have to be calculated on the shaders, instead of being provided at initialization.
Second approach:
Use the regular old triangle draw method, which allows most if not all vertex data, including normals, to be provided at initialization. The drawback here is that vertex data can get repetitive when writing to the buffer, especially when multiple triangles share a single vertex.
So my question is, which method is preferable? And is there another way that improves on both that I'm not thinking of? I'm new to the OpenGL scene, so take it easy on me. Thanks!
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Nowadays I'm hearing from different places about the so called GPU driven rendering which is a new paradigm of rendering which doesn't require draw calls at all, and that it is supported by the new versions of OpenGL and Vulkan APIs. Can someone explain how it actually works on conceptual level and what are the main differences with the traditional approach?
Overview
In order to render a scene, a number of things have to happen. You need to walk your scene graph to figure out which objects exist. For each object which exists, you now need to determine if it is visible. For each object which is visible, you need to figure out where its geometry is stored, which textures and buffers will be used to render that object, which shaders to use to render the object, and so forth. Then you render that object.
The "traditional" method handling this is for the CPU to handle this process. The scene graph lives in CPU-accessible memory. The CPU does visibility culling on that scene graph. The CPU takes the visible objects and access some CPU data about the geometry (OpenGL buffer object and texture names, Vulkan descriptor sets and VkBuffers, etc), shaders, etc, transferring this as state data to the GPU. Then the CPU issues a GPU command to render that object with that state.
Now, if we go back farther, the most "traditional" method doesn't involve a GPU at all. The CPU would just take this mesh and texture data, do vertex transformations, rasterizatization, and so forth, producing an image in CPU memory. However, we started off-loading some of this to a separate processor. We started with the rasterization stuff (the earliest graphics chips were just rasterizers; the CPU did all the vertex T&L). Then we incorporated the vertex transformations into the GPU. When we did that, we started having to store vertex data in GPU accessible memory so the GPU could read it on its own time.
We did all of that, off-loading these things to a separate processor for two reasons: the GPU was (much) faster at it, and the CPU can now spend its time doing something else.
GPU driven rendering is just the next stage in that process. We went from no GPU, to rasterization GPU, to vertex GPU, and now to scene-graph-level GPU. The "traditional" method offloads how to render to the GPU; GPU driven rendering offloads the decision of what to render.
Mechanism
Now, the reason we haven't been doing this all along is because the basic rendering commands all take data that comes from the CPU. glDrawArrays/Elements takes a number of parameters from the CPU. So even if we used the GPU to generate that data, we would need a full GPU/CPU synchronization so that the CPU could read the data... and give it right back to the GPU.
That's not helpful.
OpenGL 4 gave us indirect rendering of various forms. The basic idea is that, instead of taking those parameters from a function call, they're just data stored in GPU memory. The CPU still has to make a function call to start the rendering operation, but the actual parameters to that call are just data stored in GPU memory.
The other half of that requires the ability of the GPU to write data to GPU memory in a format that indirect rendering can read. Historically, data on GPUs goes in one direction: data gets read for the purpose of being converted into pixels in a render target. We need a way to generate semi-arbitrary data from other arbitrary data, all on the GPU.
The older mechanism for this was to (ab)use transform feedback for this purpose, but nowadays we just use SSBOs or failing that, image load/store. Compute shaders help here as well, since they are designed to be outside of the standard rendering pipeline and therefore are not bound to its limitations.
The ideal form of GPU-driven rendering makes the scene-graph part of the rendering operation. There are lesser forms, such as having the GPU do nothing more than per-object viewport culling. But let's look at the most ideal process. From the perspective of the CPU, this looks like:
Update the scene graph in GPU memory.
Issue one or more compute shaders that generate multi-draw indirect rendering commands.
Issue a single multi-draw indirect call that draws everything.
Now of course, there's no such thing as a free lunch. Doing full scene graph processing on the GPU requires building your scene graph in a way that is efficient for GPU processing. Even more importantly, visibility culling mechanisms have to be engineered with efficient GPU processing in mind. That's complexity I'm not going to address here.
Implementation
Instead, let's look at the nuts-and-bolts of making the drawing part work. We have to sort out a lot of things here.
See, the indirect rendering command is still a regular old rendering command. While the multi-draw form draws multiple distinct "objects", it's still one CPU rendering command. This means that, for the duration of this command, all rendering state is fixed.
So everything under the purview of this multi-draw operation must use the same shader, bound buffers&textures, blending parameters, stencil state, and so forth. This makes implementing a GPU-driven rendering operation a bit complicated.
State and Shaders
If you need blending, or similar state-based differences in rendering operations, then you are going to have to issue another rendering command. So in the blending case, your scene-graph processing is going to have to compute multiple sets of rendering commands, with each set being for a specific set of blending modes. You may also need to have this system sort transparent objects (unless you're rendering them with an OIT mechanism). So instead of having just one rendering command, you have a small number of them.
But the point of this exercise however isn't to have only one rendering command; the point is that the number of CPU rendering commands does not change with regard to how much stuff you're rendering. It shouldn't matter how many objects are in the scene; the CPU will be issuing the same number of rendering commands.
When it comes to shaders, this technique requires some degree of "ubershader" style: where you have a very few number of rather flexible shaders. You want to parameterize your shader rather than having dozens or hundreds of them.
However things were probably going to fall out that way anyway, particularly with regard to deferred rendering. The geometry pass of deferred renderers tends to use the same kind of processing, since they're just doing vertex transformation and extracting material parameters. The biggest difference usually is with regard to doing skinned vs. non-skinned rendering, but that's really only 2 shader variations. Which you can handle similarly to the blending case.
Speaking of deferred rendering, the GPU driven processes can also walk the graph of lights, thus generating the draw calls and rendering data for the lighting passes. So while the lighting pass will need a separate draw call, it will still only need a single multidraw call regardless of the number of lights.
Buffers
Here's where things start to get interesting. See, if the GPU is processing the scene graph, that means that the GPU needs to somehow associate a particular draw within the multi-draw command with the resources that particular draw needs. It may also need to put the data into those resources, like the matrix transforms for a given object and so forth.
Oh, and you also somehow need to tie the vertex input data to a particular sub-draw.
That last part is probably the most complicated. The buffers which OpenGL/Vulkan's standard vertex input method pull from are state data; they cannot change between sub-draws of a multi-draw operation.
Your best bet is to try to put every object's data in the same buffer object, using the same vertex format. Essentially, you have one gigantic array of vertex data. You can then use the drawing parameters for the sub-draw to select which parts of the buffer(s) to use.
But what do we do about per-object data (matrices, etc), things you would typically use a UBO or global uniform for? How do you effectively change the buffer binding state within a CPU rendering command?
Well... you can't. So you cheat.
First, you realize that SSBOs can be arbitrarily large. So you don't really need to change buffer binding state. What you need is a single SSBO that contains everyone's per-object data. For each vertex, the VS simply needs to pick out the correct data for that sub-draw from the giant list of data.
This is done via a special vertex shader input: gl_DrawID. When you issue a multi-draw command, the VS gets an input value that represents the index of this sub-draw operation within the multidraw command. So you can use gl_DrawID to index into a table of per-object data to fetch the appropriate data for that particular object.
This also means that the compute shader which generates this sub-draw also needs use the index of that sub-draw to define where in the array to put the per-object data for that sub-draw. So the CS that writes a sub-draw also needs to be responsible for setting up the per-object data that matches the sub-draw.
Textures
OpenGL and Vulkan have pretty strict limits on the number of textures that can be bound. Well actually those limits are quite large relative to traditional rendering, but in GPU driven rendering land, we need a single CPU rendering call to potentially access any texture. That's harder.
Now, we do have gl_DrawID; coupled with the table mentioned above, we can retrieve per-object data. So: how do we convert this to a texture?
There are multiple ways. We could put a bunch of our 2D textures into an array texture. We can then use gl_DrawID to fetch an array index from our SSBO of per-object data; that array index becomes the array layer we use to fetch "our" texture. Note that we don't use gl_DrawID directly because multiple different sub-draws could use the same texture, and because the GPU code that sets up the array of draw calls does not control the order in which textures appear in our array.
Array textures have obvious downsides, the most notable of which is that we must respect the limitations of an array texture. All elements in the array must use the same image format. They must all be of the same size. Also, there are limits on the number of array layers in an array texture, so you might encounter them.
The alternatives to array textures differ along API lines, though they basically boil down to the same thing: convert a number into a texture.
In OpenGL land, you can employ bindless texturing (for hardware that supports it). This system provides a mechanism that allows one to generate a 64-bit integer handle which represents a particular texture, pass this handle to the GPU (since it is just an integer, use whatever mechanism you want), and then convert this 64-bit handle into a sampler type. So you use gl_DrawID to fetch a 64-bit handle from the per-object data, then convert that into a sampler of the appropriate type and use it.
In Vulkan land, you can employ sampler arrays (for hardware that supports it). Note that these are not array textures; in GLSL, these are sampler types which are arrayed: uniform sampler2D my_2D_textures[6000];. In OpenGL, this would be a compile error because each array element represents a distinct bind point for a texture, and you cannot have 6000 distinct bind points. In Vulkan, an arrayed sampler only represents a single descriptor, no matter how many elements are in that array. Vulkan implementations have limits on how many elements there can be in such arrays, but hardware that supports the feature you need to employ this (shaderSampledImageArrayDynamicIndexing) will typically offer a generous limit.
So your shader uses gl_DrawID to get an index from the per-object data. The index is turned into a sampler by just fetching the value from the sampler array. The only limitation for textures in that arrayed descriptor is that they must all be of the same type and basic data format (floating-point 2D for sampler2D, unsigned integer cubemap for usamplerCube, etc). The specifics of formats, texture sizes, mipmap counts, and the like are all irrelevant.
And if you're concerned about the cost difference of Vulkan's array of samplers compared to OpenGL's bindless, don't be; implementations of bindless are just doing this behind your back anyway.
I want to draw a mass of branches with different shapes, each of which consisting of 4 triangle strips. (Using OpenGL)
So now I'm considering using one of those method calls (multiDrawArrays, using primitive restart and multiDrawElements).
I was wondering which one is more efficient. Is the method multiDrawArrays() equivalent to several drawArrays() in terms of speed?
Does VAO store the vertex info in the RAM while the SSBO store those in the VRAM? If so, is it better to use SSBO rather than VAO considering the performance?
As #derhass already pointed out in a comment, some of your terminology is mixed up. A VAO (Vertex Array Object) contains state that defines how vertex data is associated with vertex attributes. It's the VBO (Vertex Buffer Object) that contains the actual vertex data.
I doubt that using a SSBO for vertex data would be beneficial. In general, the buffer types primarily define how the data is used. The graphics pipeline is tailored towards fetching vertex data from VBOs, and many GPUs have dedicated fixed function hardware to pull data from VBOs and feed it into the vertex shader. I can't see how using explicit code in the vertex shader to pull the vertex data from a SSBO instead would be more efficient.
Whether the data is stored in VRAM or SRAM is a different consideration. The only control you have over that is with the last argument to glBufferData(). It provides a hint on how you plan to use the data. For example, if you specify GL_STATIC_DRAW, you're telling the driver that you're not planning to modify the data, which suggests that placing it in VRAM might be a good idea. Whether it will actually be in VRAM is then up to the driver, and it may decide that based on various criteria.
Functionally, glMultiDrawArrays() is equivalent to multiple calls to glDrawArrays(). But it can certainly be more efficient. If nothing else, it saves the overhead of making multiple API calls. Each API call has a certain amount of overhead, for example:
It might pass through a couple of software layers, resulting in additional function calls under the hood.
It needs to get the current context from thread local storage.
It needs to do error checking.
It may need some form of locking to deal with access from multiple contexts in multiple threads (might not be needed for a draw call).
It needs to check for pending state changes.
The MultiDraw calls were introduced to cut down on the number of API calls needed.
Now, whether glMultiDrawArrays() or glDrawElements() with primitive restart is more efficient, that's impossible to say in general. If you're not already using an index buffer, I would be a bit hesitant to introduce one just so that you can use primitive restart. So my instinct would be:
Use glDrawElements() with primitive restart if you're using an index buffer anyway.
Use glMultiDrawArrays() if you're not using an index buffer.
The real answer, as always, can only be obtained by benchmarking. And it can of course be platform/hardware dependent. My prediction is that you will not see a significant difference in most cases, since both of these allow you to draw a lot of your geometry with a single API call, which should avoid bottlenecks in this area.
I wanted to use a GLSL geometry shader to look at a line strip and determine the place to put a textured annotation, taking into account the current ModelView. It seems I'm limited to only getting 4 vertices per invokation (using GL_LINE_STRIP_ADJACENCY), but what I need is the entire line strip to evaluate.
I could use some other primitive type (such as a Multi-point, if there is an equivalent in GL), but the important point is I want to consider all the geometry, not just a portion.
Is there an extension of come kind that would provide additional vertices to the Geometry shader? Or is there a better way to do this other than using the Geometry shader?
There is no mechanism that will give you access to an entire rendered primitive stream. Primitives can be arbitrarily large, so they can easily blow past any reasonable internal buffer sizes that GPUs have. Thus implementing this would be impractical.
You could bind your array as a buffer texture and just read the data from there. But that's going to be exceedingly slow, since every GS invocation is going to have to process hundreds of vertices. That's not exactly taking advantage of GPU parallelism.
If you just want to put a text tag near something, you should designate a vertex or something as being where annotations should go.
So I understand how to use a Vertex Buffer Object, and that it offers a large performance increase over immediate mode drawing. I will be drawing a lot of 2D quads (sprites), and I am wanting to know if I am supposed to create a VBO for each one, or create one VBO to hold all of the data?
You shouldn't use a new VBO for each sprite/quad. So putting them all in a single VBO would be the better solution in your case.
But in general i don't think this can be answered in one sentence.
Creating a new VBO for each Quad won't give you a real performance increase. If you do so, a lot of time will be wasted with glBindBuffer calls for switching the VBOs. However if you create VBOs that hold too much data you can run into other problems.
Small VBOs:
Are often easier to handle in your program code. You can use a new VBO for each Object you render. This way you can manage your objects very easy in your world
If VBOs are too small (only a few triangles) you don't gain much benefit. A lot of time will be lost with switching buffers (and maybe switching shaders/textures) between buffers
Large VBOs:
You can render tons of objects with a single drawArrays() call for best performance.
Depending on your data its possible that you create overhead for managing a lot of objects in a single VBO (what if you want to move one of these objects and rotate another object?).
If your VBOs are too large its possible that they can't be moved into VRAM
The following links can help you:
Vertex Specification Best Practices
Use one (or a small number of) VBO(s) to hold all/most of your geometry.
Generally the fewer API calls it takes to render your scene, the better.
It also depends what d you want to do with those sprites?
Are they dynamic? Do you want to change only the centre of quad or maybe modify all four points?
This is important because if your data are dynamic then, in the simplest way, you will have to transfer from cpu to gpu each frame. Maybe you could perform all computation on the GPU - for instance by using geometry shaders?
Also for very simple quads/sprites one can use GL_POINT_SPRITE. With that one need to send only one vertex for whole quad. But the drawback is that it is hard to rotate it...