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OpenGL Which is Faster?

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I hear that it is really expensive to switch textures in an OpenGL program which got me thinking about optimizing my tile engine. Lets say I have 300 tiles in a level and each tile can have one of 30 textures binded to it. Would this way be faster than binding the same textures 300 times? I have a for loop that binds one of the 30 textures, looks through all the tiles and only draws the ones that are supposed to have that texture bound to them, and then moves on to the next texture and repeats the process. My writing sucks so I'll write some code too:

for(int tex = 0; tex < 30; tex ++)
{
glBindTexture(GL_TEXTURE_2D , textures[tex]);

for(int tile = 0; tile < 300; tile ++)
{
if(Tiles[tile]->getTexture() == tex)
Tiles[tile]->RenderTile();
}


}




Would this even make a measurable effect on the speed of my game if I'm only doing this 300 times anyway?

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Quote:
Original post by Sir Sapo
I hear that it is really expensive to switch textures in an OpenGL program which got me thinking about optimizing my tile engine.

Lets say I have 300 tiles in a level and each tile can have one of 30 textures binded to it. Would this way be faster than binding the same textures 300 times? I have a for loop that binds one of the 30 textures, looks through all the tiles and only draws the ones that are supposed to have that texture bound to them, and then moves on to the next texture and repeats the process.

My writing sucks so I'll write some code too:

*** Source Snippet Removed ***

Would this even make a measurable effect on the speed of my game if I'm only doing this 300 times anyway?



Sorting by texture and/or minimizing texture switching usually does help. Just how much in your case, you'll have to profile and find out [smile].

btw your loop would be more efficient if you sorted your tiles by texture instead of the nested loop like that. Something like the following then.


tex = 0;
for(int tile = 0; tile < 300; tile ++)
{
if(Tiles[tile]->getTexture() != tex)
{
glBindTexture(GL_TEXTURE_2D , Tiles[tile]->getTexture());
tex = Tiles[tile]->getTexture();
}
Tiles[tile]->RenderTile();
}



executes 300 times instead of 9000 :) but for maximum efficiency sort the tiles before hand by their textures (above will work without sorting aswell but not as efficiently).

HTH

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Quote:
I hear that it is really expensive to switch textures in an OpenGL program


It is only really expensive if you have modified the texture since you last drew it, or if you're binding more textures during a frame than fits in your graphics cards' VRAM (after framebuffer, Z buffer, double buffer etc are subtracted).

In OpenGL version 1.0, about ten years ago, there were no texture objects, and the texture had to be uploaded to the card each time you wanted to use it -- really quite expensive. Display lists were usually used to "hint" to the driver that it could cache that texture. However, as of texture objects, in OpenGL 1.1, that hasn't been an issue. It's long dead and buried.

Binding a new vertex program, or even worse, fragment program, is likely to cause a far bigger stall in your pipeline than switching textures. Pipe stalls == fewer pixels processed == lower frame rate, if that's your limiting factor.

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Even if the texture is sitting in the graphics cards VRAM the GPU has to do a fair amount of work when the texture is changed. Caches are flushed, data has to be shifted about, states setup etc.

So, yes, while a shader change is more expensive texture changes can hurt as well which is why its advised to minmise them.

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Quote:
Original post by hplus0603
Binding a new vertex program, or even worse, fragment program, is likely to cause a far bigger stall in your pipeline than switching textures.


Eh in OpenGL, you can only bind Program Objects which consist of both Vertex and Pixel Shaders, and from the fact that after linking a program object, modifying the original vertex/pixel shaders does not affect the already linked Program Object, am i right to conclude that for OpenGL atleast, sorting by vertex/pixel shaders is of no use and you should instead sort by Program Objects? Because it appears to me that the driver makes a copy of the program object upon linking which is why changes made to the original vertex/pixel shaders do not affect it.

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Quote:
Original post by GamerSg
Quote:
Original post by hplus0603
Binding a new vertex program, or even worse, fragment program, is likely to cause a far bigger stall in your pipeline than switching textures.


Eh in OpenGL, you can only bind Program Objects which consist of both Vertex and Pixel Shaders, and from the fact that after linking a program object, modifying the original vertex/pixel shaders does not affect the already linked Program Object, am i right to conclude that for OpenGL atleast, sorting by vertex/pixel shaders is of no use and you should instead sort by Program Objects? Because it appears to me that the driver makes a copy of the program object upon linking which is why changes made to the original vertex/pixel shaders do not affect it.

You're probably right if you're talking about GLSL Program Objects, but IIRC with the previous shader extensions you could bind just a fragment program or just a vertex program.

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Quote:
Original post by OrangyTang
IIRC with the previous shader extensions you could bind just a fragment program or just a vertex program.


You are also able to not include either vertex-, or fragment-shader in GLSL program-object. Not to be picky about it, but I couldn't resist ;)

ch.

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What if you were to compile your textures into one file?
I suppose having 30 textures in one file is quite large, but what if you had 10 textures in 3 files? It'd probably be even better to logically spearate the tiles with other like tiles. Say, have all the grass textures in one "master grass texture file"?

Then, you could do 3 passes, one with each of the "master texture files" bound. Depending on the texture that needs to be drawn, you could simply alter the texture coordinates of the quad you draw.

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Quote:

What if you were to compile your textures into one file?
I suppose having 30 textures in one file is quite large, but what if you had 10 textures in 3 files? It'd probably be even better to logically spearate the tiles with other like tiles. Say, have all the grass textures in one "master grass texture file"?

Then, you could do 3 passes, one with each of the "master texture files" bound. Depending on the texture that needs to be drawn, you could simply alter the texture coordinates of the quad you draw.


The only problem I see with that is that there will be ugly-looking grid lines at the edges of the quads, at least that's what I've experienced when using multiple parts of one image as multiple textures. If anyone can tell me how to fix that, please let me know, its handy.

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You'd have to add a bit of a pixel border around each tile in a texture. Or you might be able to use some OGL extension to clamp your UVs to a region of the texture (same thing but no need to manually create borders).

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Thanks Puppet, I'll look into that, but while we're on the subject, why do the lines even appear? Is it just from imprecision when binding textures or is it something else?

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a couple of days ago i wrote a piece of code (~200lines) that takes textures and packs them into an ubertexture, not to hard to write.
the borders happen cause of interpolation eg GL_LINEAR will use the average pixels and its neighbours when it draws something, this gives better results visually (unless the neightbour pixels belong to a completely different texture)
the problem is even worse with mipmapping

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So thats why a larger pixel border fixes the problem, it has nothing to interpolate so there is no border?

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Quote:
Original post by Funkapotamus
What if you were to compile your textures into one file?
I suppose having 30 textures in one file is quite large, but what if you had 10 textures in 3 files? It'd probably be even better to logically spearate the tiles with other like tiles. Say, have all the grass textures in one "master grass texture file"?

Then, you could do 3 passes, one with each of the "master texture files" bound. Depending on the texture that needs to be drawn, you could simply alter the texture coordinates of the quad you draw.


This is the most common way. You just need to implement a formula which calculates the right u and v coords, given a tile No. that you input. And there u go. :)
If you need to set a texture each time you draw a tile.. oh my. 0_o.

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never ever ever optimize stuff like this without profiling. EVER.

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What do you mean profiling? It seems like a pretty risk free/easy to read optimization.

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      Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
      Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
      Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
      Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
      m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
      m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
      Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
      m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
      The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
      m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
      Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
      m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains tutorials, sample applications, asteroids performance benchmark and an example Unity project that uses Diligent Engine in native plugin.
      Atmospheric scattering sample demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

      Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

      Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

      Future Work
      The engine is under active development. It currently supports Windows desktop, Universal Windows, Linux, Android, MacOS, and iOS platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and Metal backend is in the plan.
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