Why is accessing textures much slower when calculating the texture coordinate in the fragment shader?

Question

When using textures in GLSL, it is best to calculate the final texture coordinates in the vertex shader and hand them over to the fragment shader using varyings. Example with a simple flip in the y coordinate:

// Vertex shader
attribute vec2 texture;
varying highp vec2 texCoord;
// ...
void main() {
    texCoord = vec2(texture.x, 1.0-texture.y);
    // ...
}

// Fragment shader
varying highp vec2 textureCoordinates;
uniform sampler2D tex;
// ...
void main() {
    highp vec4 texColor = texture2D(tex, texCoord);
    // ...
}

If the flip in the y coordinate, or an even simpler operation like adding vec2(0.5) to the texture coordinate is performed in the fragment shader the texture access is much slower. Why?

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As a note, e.g. blending two textures, using a weighted sum of them, is much cheaper in terms of time and also needs to be done for each pixel, so the computation of the texture coordinate itself does not seem to be that costly.

Answer 1

What you're talking about is commonly called "dependent texture reads" in the mobile development community. It's an implementation detail of certain hardware, and therefore it really depends on the GPU as to whether or not it has any performance implications. Typically it's something you see brought up for PowerVR GPU's in Apple hardware, since it was explicitly mentioned in both Imagination and Apple documentation. If I recall correctly, the issue basically came from hardware in the GPU that would start to pre-fetch textures before the fragment shader even began running, so that it could do a better job of hiding the latency. The docs I linked mention that it's no longer an issue on Series6 hardware, so at least on newer Apple hardware it's not something that you have to worry about. I'm honestly not sure about other mobile GPU's, since that's not my area of expertise. You should try and consult their documentation to find out for sure.

If you decide to do some Google searches on this issue, be aware that you'll probably find some older material that's talking about dependent texture fetches on older desktop hardware. Basic in the early days of pixel/fragment shaders, the term "dependent texture fetch" referred to using a UV address that relied on a previous texture fetch. The classic example was bump-mapped environment map rendering, where you wanted to use a reflection vector based on the normal map in order to sample the environment map. On this older hardware there was some major performance implications, and I think it wasn't even supported on some very old GPU's. With modern GPU's the hardware and the shader ISA is much more generalized, and so the performance situation is much more complicated.