The Do-Nothing Effect

If the contents of the texture are copied directly to the fragment color, then the output will be indistinguishable from an un-processed rendering.

precision mediump float
uniform sampler2D colorTexture
varying vec2 fragmentTexCoord

function main()
gl_FragColor ← texture2d(colorTexture, fragmentTexCoord)

This shader's sampler uniform and texture coordinate varying are common to all following post-process shaders.

While this shader does the bare minimum, it does enable a circumstance where the off-screen rendering may happen at a different resolution than the on-screen rendering, giving a quality-speed trade-off.

If the off-screen buffer has lower resolution, then performance may improve due to a drop in the number of scene pixels, though with a decrease in image quality.
If the off-screen buffer has higher resolution, then image quality may improve due to an oversampling of the scene, though with a potential decrease in performance.