Instead of using multiple framebuffer objects, can I also create only one and achieve the same results by switching it's target texture when needed?
Is this a bad idea in all cases? If yes, why?
I've been implementing a function render.SetTargetTexture() in my program's API, and logically it wouldn't work if there were more framebuffers used behind the scenes. I'd have to fully expose framebuffers then.
A FBO itself is just some logical construct, maintained by the implementation and it will consume only the little memory that its parameters require. The main purpose of a FBO is to have an object that maintains a consistent state.
Whenever you do changes to a FBO's structure, the implementation must check for validity and completeness. The OpenGL specification doesn't say anything about the complexity of this operation, but it's safe to assume that changes to a FBO's structure is about the most time consuming operation (maybe by a large margin).
Since the FBO itself does not consume noteworthy memory, only the attachments take memory, which are independent objects, allocating multiple FBOs and swithing those is what I recommend.
Switching framebuffers is usually faster for the reasons people have given. In my experience, there are also problems when attaching different texture types to fbos (2D texture to fbo which previously had a 1D texture attached, so the first texture attached to the fbo set some kind of target state which could not be altered later). A solution I used was to have an fbo for each target type. This error occured on NVidia Quadro devices, not sure whether it's a driver issue.
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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 am very new to the OpenGL API and just discovered textures and how to use them. The generated texture buffers are not bound in the same way as regular uniforms are bound and instead use glActiveTexture, followed by a bind rather than just supplying the texture to the shaders via glUniform as we do with other constants.
What is the reasoning behind this convention?
The only reason I can think of is to utilize the graphics card's full potential and texture processing capabilities instead of just binding buffers directly. Is this correct reasoning, or is it simply the way the API was implemented?
No reasoning is given on the official wiki, just says that it's weird: "Binding textures for use in OpenGL is a little weird" https://www.khronos.org/opengl/wiki/Texture
Your question can be interpreted in two ways.
"Why do we bind textures to the context rather than to the shader?"
Because that would make it needlessly difficult to have multiple shaders use the same textures. Note that pretty much no graphics API directly attaches the texture to the program. Not D3D of any version, not Metal, nor even Vulkan.
Textures are resources used by shaders. But they are not part of the shader.
"Why do we treat textures differently from a general array of values?"
In modern OpenGL, shaders have access to several different kinds of resources: UBOs, SSBOs, textures, and images. Each of these kinds of resources ultimately represents a potentially distinct part of the graphics hardware.
A storage block is not just a uniform block that can be bigger. Storage buffers represent the shader doing global memory accesses, while uniform blocks are often copied directly into the shader execution units. In the latter case, accessing their data is much faster, but that also means that you're limited by how much storage those execution units can have.
Now, this is not true for all hardware (AMD's GCN hardware treats the two almost identically, which is why their UBO limits are so large). But it is true of much hardware.
Textures are even more complicated, because implementations need to be able to store their data in an optimal way for performance reasons. As such, texture storage formats are opaque. They're even opaque in ostensibly low-level APIs like Vulkan. Oh sure, linear formats exist, but implementations aren't required to let you read from them at all.
So textures are not just constant arrays.
You are comparing two completely different things
A texture object can (somehow) be compared to a buffer object. The texture is bound by a combination of glActiveTexture and glBindTexture to a texture unit, whereas a buffer is bound by glBindBuffer which is kind-off similar.
A texture sampler uniform is a uniform in the shader and should thus be compared with other uniforms. This sampler is set by a glUniform1i call.
I'm designing the sorting part of my rendering engine. I know that changing the render target, shader program, texture bindings, and more are expensive and therefore one should sort the draw order based on them to reduce state changes. However, what about sorting based on what index buffer is bound, and which vertex buffers are used for attributes?
I'm confused about these because VAOs are mandatory and they encapsulate all of that state. So should I peek behind the scenes of vertex array objects (VAOs), see what state they set and sort based on it? Or should I just not care in what order VAOs are called?
This is what confuses me so much about vertex array objects. It makes sense to me to not be switching which buffers are in use over and over and yet VAOs just seem to force one to not care about that.
Is there a general vague or not agreed on order on which to sort stuff for rendering/game engines?
I know that binding a buffer simply changes some global state but surely it must be beneficial to the hardware to draw from the same buffer multiple times, maybe some small cache coherency?
While VAOs are mandated in GL 3.1 without GL_ARB_compatibility or core 3.2+, you do not have to use them the way they are intended... that is to say, you can bind a single VAO for the duration of your application's lifetime and continue to bind and unbind VBOs, etc. the traditional way if this somehow makes your life easier. Valve is famous for advocating doing this in their presentation on porting the Source engine from D3D to GL... I tend to disagree with them on some points though. A lot of things that they mention in their presentation make me cringe as someone who has years of experience with both D3D and OpenGL; they are making suggestions on how to port something to an API they have a minimal working knowledge of.
Getting back to your performance concern though, there can be validation overhead for changing bound resources frequently, so it is actually more than just "simply changing a global state." All GL commands have to do validation in order to determine if they need to set an error state. They will validate your input parameters (which is pretty trivial), as well as the state of any resource the command needs to use (this can be complicated).
Other types of GL objects like FBOs, textures and GLSL programs have more rigorous validation and more complicated memory dependencies than buffer objects and vertex arrays do. Swapping a vertex pointer should be cheaper in the grand scheme of things than most other kinds of object bindings, especially since a lot of stuff can be deferred by an implementation until you actually issue a glDrawElements (...) command.
Nevertheless, the best way to tackle this problem is just to increase reuse of vertex buffers. Object reuse is pretty high to begin with for vertex buffers, if you have 200 instances of the same opaque model in a scene you can potentially draw all 200 of them back-to-back and never have to change a vertex pointer. Materials tend to change far more frequently than actual vertex buffers, and so you would generally sort your drawing first and foremost by material (sub-sorted by associated states like opaque/translucent, texture(s), shader(s), etc.). You can add another level to batch sorting to draw all batches that share the same vertex data after they have been sorted by material. The ultimate goal is usually to minimize the number of draw commands necessary to complete your frame, and using priority/hierarchy-based sorting with emphasis on material often delivers the best results.
Furthermore, if you can fit multiple LODs of your model into a single vertex buffer, instead of swapping between different vertex buffers sometimes you can just draw different sets of indices or even just a different range of indices from a single index buffer. In a very similar way, texture swapping pressure can be alleviated by using packed texture atlases / sprite sheets instead of a single texture object for each texture.
You can definitely squeeze out some performance by reducing the number of changes to vertex array state, but the takeaway message here is that vertex array state is pretty cheap compared to a lot of other states that change frequently. If you can quickly implement a secondary sort to reduce vertex state changes then go for it, but I would not invest a lot of time in anything more sophisticated unless you know it is a bottleneck. Prioritize texture, shader and framebuffer state first as a general rule.
ARB_texture_storage was introduced into OpenGL 4.2 core.
Can you explain what immutability for texture objects means?
Why it is better from the previous texture usage and what are disadvantages of this feature?
I know I can read the spec of this extension (which I did :)), but I would like to see some examples or other explanation.
Just read the introduction from the extension itself:
The texture image specification commands in OpenGL allow each level
to be separately specified with different sizes, formats, types and
so on, and only imposes consistency checks at draw time. This adds
overhead for implementations.
This extension provides a mechanism for specifying the entire
structure of a texture in a single call, allowing certain
consistency checks and memory allocations to be done up front. Once
specified, the format and dimensions of the image array become
immutable, to simplify completeness checks in the implementation.
When using this extension, it is no longer possible to supply texture
data using TexImage*. Instead, data can be uploaded using TexSubImage*,
or produced by other means (such as render-to-texture, mipmap generation,
or rendering to a sibling EGLImage).
This extension has complicated interactions with other extensions.
The goal of most of these interactions is to ensure that a texture
is always mipmap complete (and cube complete for cubemap textures).
The obvious advantages are that the implementation can remove completeness / consistency checks at runtime, and your code is more robust because you can't accidentally create a wrong texture.
To elaborate: "immutable" here means that the texture storage (one of the three components of a texture: storage, sampling, parameters) gets allocated once and it's already complete. Note that storage doesn't mean the storage contents -- they can change at any time; it refers to the logical process of acquiring resources for those contents (like, a malloc).
With non-immutable textures, you can change the storage at any time, by means of glTexImage<N>D calls. There are many many ways of shooting yourself in the foot this way:
you may create mipmap-incomplete textures (probably the most common newbie error with textures, as textures by default have 1000 mipmap levels, and people upload only one image)
you may create textures with different formats in different mipmap levels (illegal)
you may create cubemap-incomplete cubemaps (illegal)
you may create cubemaps with different formats in different faces (illegal)
Since you're allowed to call glTexImage<N>D at any time, the implementation must always check, at draw time, that your texture is legal. Immutable storage always does the right thing for you by allocating everything in one go (all mipmap levels, all cubemap faces, etc.) with the right format. So you can't screw up a texture (easily) any more, and the implementation can remove some checks, which speeds things up. And everybody is happy :)
So I'm rendering my scene in batches, to try and minimize state changes.
Since my shaders usually need multiple textures, I have a few of them bound at the same time to different texture units. Some of these textures are used in multiple batches, so could even stay bound.
Now my question is, is it fine to just re-bind all of the textures that I need during a batch, even though some of them may already be bound? Or should I check which ones are bound, and only bind new ones? How expensive is glBindTexture? I'm using shaders, is it bad to have unused textures bound to texture units that the shader won't sample from, or should I unbind them?
I'm mostly asking "how to make it fast", not "how to".
EDIT: Would it help if I supplied code?
The holy rule of GPU programming is that less you talk to GPU, the more she loves you.
glBindTexture is much more expensive than single condition check on CPU, so for frequent state changes, I think you should see if there is a need to transfer new texture index to GPU.
The answer is difficult, because any strong GL implementation should make binding a already bound texture a no-op as an optimization. You should try benchmarking to see if putting a condition makes a difference or not.
The same applies to unused textures in texture units, since the GL implementation does know what texture units are finally used, it should not affect performance (as an optimization) to have unused texture units.
Track which textures you've bound (don't use glGet(GL_TEXTURE_BINDING_2D) ) and only rebind if it changes. Some drivers will do this for you, some won't.
If you know what platform you're targeting in advance, it's pretty easy to do a simple benchmark to see how much difference it makes.
You can also sort your batches by what textures they are using to minimize texture changes.
I'd say the best way is to bind all the textures at once using glBindTextures https://www.khronos.org/opengl/wiki/GLAPI/glBindTextures . Or use bindless textures :)