OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Texture Fetch with Offsets

Some applications require a number of texels around a region of interest, or may need to offset the texture coordinates slightly during sampling. GLSL includes functions for doing this that will likely be more efficient than physically offsetting the texture coordinates in the shader. This functionality is exposed through an overloaded set of texture lookup functions called textureOffset with some example prototypes as follows: (A full list is in Appendix C, “Built-in GLSL Variables and Functions”.)

gvec4 textureOffset(gsampler1D tex, float P, int offset,                         [float bias]);gvec4 textureOffset(gsampler2D tex, vec2 P, ivec2 offset,                         [float bias]);gvec4 textureOffset(gsampler3D ...