OpenGL 着色器的显式与自动属性位置绑定

当为 OpenGL 着色器程序设置属性位置时，您将面临两种选择:

在链接到显式定义属性位置之前，glBindAttribLocation () 。

或者

GlGetAttribLocation () 链接后获得自动分配的属性位置。

使用其中一个而不使用另一个的实用程序是什么？

如果有的话，哪一个在实践中更受欢迎呢？

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最佳答案

I know one good reason to prefer explicit location definition.

Consider that you hold your geometry data in Vertex Array Objects. For a given object, you create a VAO in such way that the indices correspond to, for example:

  • index 0: positions,
  • index 1: normals,
  • index 2: texcoords

Now consider that you want to draw one object with two different shaders. One shader requires position and normal data as input, the other - positions and texture coords.

If you compile those shaders, you will notice that the first shader will expect the positions at attribute index 0 and normals at 1. The other would expect positions at 0 but texture coords at 1.

Quoting https://www.opengl.org/wiki/Vertex_Shader:

Automatic assignment

If neither of the prior two methods assign an input to an attribute index, then the index is automatically assigned by OpenGL when the program is linked. The index assigned is completely arbitrary and may be different for different programs that are linked, even if they use the exact same vertex shader code.

This means that you wouldn't be able to use your VAO with both shaders. Instead of having one VAO per, say, object, you'd need - in the worst case - a separate VAO per object per shader.

Forcing the shaders to use your own attribute numbering convention via glBindAttribLocation can solve this problem easily - all you need to do is to keep a consistent relation between attributes and their estabilished IDs, and force the shaders to use that convention when linking.

(That's not really a big issue if you don't use separate VAOs, but still might make your code clearer.)

BTW:

When setting up attribute locations for an OpenGL shader program, you are faced with two options

There's a third option in OpenGL/GLSL 3.3: Specify the location directly in shader code. It looks like this:

layout(location=0) in vec4 position;

But this is not present in GLSL ES shader language.

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Another answer here is that glGetAttribLocation returns data to the caller, which means that it implicitly requires a pipeline flush. If you call it right after you compile your program, you're essentially forcing asynchronous compilation to occur synchronously.

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The third option, ie layout(location=0) in vec4 position; in the shader code, is now available in OpenGL ES 3.0/GLSL 300 es. Only for vertex shader input variables though.


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