This is a feature request for an extension to allow for a fragment shader to read the current value of the framebuffer when it is running as outlined in the current GL/GLES extension. GL_EXT_shader_framebuffer_fetch

This can be achieved functionality can be achieved with fragment shader interlock (#395), but for hardware that has direct support for this (for example Intel Gen9 or higher GPU's) framebuffer fetch is better because a GPU can more easily use its render cache which in turn means it can more easily maintain lossless color compression. Naturally tiled based renderers can then read directly from the tile which is always better than a read/write to an image.