Question

I’ve written a simple GL fragment shader which performs an RGB gamma adjustment on an image:

uniform sampler2D tex;
uniform vec3 gamma;

void main()
{
    vec3 texel = texture2D(tex, gl_TexCoord[0].st).rgb;
    texel = pow(texel, gamma);
    gl_FragColor.rgb = texel;
}

The texture paints most of the screen and it’s occurred to me that this is applying the adjustment per output pixel on the screen, instead of per input pixel on the texture. Although this doesn’t change its appearance, this texture is small compared to the screen.

For efficiency, how can I make the shader process the texture pixels instead of the screen pixels? If it helps, I am changing/reloading this texture’s data on every frame anyway, so I don’t mind if the texture gets permanently altered.

Answer 1

and it’s occurred to me that this is applying the adjustment per output pixel on the screen

Almost. Fragment shaders are executed per output fragment (hence the name). A fragment is a the smallest unit of rasterization, before it’s written into a pixel. Every pixel that’s covered by a piece of visible rendered geometry is turned into one or more fragments (yes, there may be even more fragments than covered pixels, for example when drawing to an antialiased framebuffer).

For efficiency,

Modern GPUs won’t even “notice” the slightly reduced load. This is a kind of microoptimization, that’s on the brink of non-measureability. My advice: Don’ worry about it.

how can I make the shader process the texture pixels instead of the screen pixels?

You could preprocess the texture, by first rendering it through a texture sized, not antialiased framebuffer object to a intermediate texture. However if your change is nonlinear, and a gamma adjustment is exactly that, then you should not do this. You want to process images in a linear color space and apply nonlinear transformation only as late as possible.