Open GLES 3.0中的布局限定符

Question

I'm experimenting with using the Open GLES 3.0 API (with the accompanying core 330 GLSL shader language) in a Swift Playground. I have the following declaration at the top of my vertex shader:

layout(location = 0) in vec4 a_Position;

My understanding of the layout qualifier is that is specifies where a_Position is in the Vertex Buffer Object's attribute indices. (loose as my understanding of VAO's is..).

But my triangle doesn't render unless I also include the following code:

glEnableVertexAttribArray(0)
glVertexAttribPointer(
        0,
        3,
        GLenum(GL_FLOAT),
        GLboolean(GL_FALSE),
        GLsizei(MemoryLayout<Vertex>.size),
        BUFFER_OFFSET(0))

But my understanding of the layout qualifier would suggest to me that this is not necessary as the it is already specifying where a_Position lies in the vertex attribute array. (at index 0).

So either I'm missing some minor detail, or my understanding of how VAO data is accessed in shaders on the GPU is flawed. It is probably both. So why isn't the shader declaration with the layout qualifier sufficient to get my vertex positions into a_Position ?

Answer 1

layout(location=0) means that you don't have to call glGetVertexAttribLocation to get the attribute index for glEnableVertexAttribArray and glVertexAttribPointer calls. Those calls are still necessary.

Shader declaration is not sufficient because you still need to provide the information that is in the parameters of glVertexAttribPointer , like normalisation, data type etc.