Access different Fragment in Fragmentshader OpenGL

Question

Can I access and change output values of another Fragment at a certain location in the Fragmentshader?

For example in the main() loop I process everything just like usualy and output the color with some value. But in adition to that I also want the fragment at position vec3(5,3,6) (in world coordinates) to have the same colour.

Now I already did some researche on the web on that. The OpenGL site says, the fragmentshader has one fragment as input and has one fragment as output, which doesnt sound very promising. Also I know that all fragments are being processed in parallel. But maybe it is posible to say, if the fragment at this position has not been processed yet, write this color to it and take this fragment as already processed.

My be someone can explain if this is posible somehow and if not, why this is not a good idea. The best guess I would have is, to build this logic into the shader, it would have a very bad effect on the general performance.

Answer 1

My be someone can explain if this is posible somehow and if not, why this is not a good idea.

It's not a question of bad idea vs. good idea. It's simply not possible.

The closest you can get to this functionality is ARB_fragment_shader_interlock. Through its interlock and ordering guarantees, it allows limited interoperation. And that limitation is... it only allows interoperation for fragments that cover the same pixel/sample.

So even this functionality does not allow you to write to some other pixel.

The absolute best you can do is use SSBOs and atomic counters to have fragment shaders write what color values and "world coordinates" they would like to write to, then have a second process execute that buffer as either a rendering command or a compute shader to actually write that data.

Answer 2

As already pointed out in Nicol's answer, you can't write to additional fragments of a framebuffer surface in the fragment shader.

The description of your use case is not clear enough to tell what might work best. In the interest of brainstorming, the most direct approach that comes to mind is that you don't use a framebuffer draw surface at all, but output to an image instead.

If you bind a texture as an image, you can write to it in the fragment shader using the imageStore() built-in function. This function takes coordinates as one of the argument, so you can write to any pixel you want, as well as write multiple pixels from the same shader invocation.

Depending on what exactly you want to achieve, I could also imagine a hybrid approach, where your primary rendering still goes to a framebuffer, but you write additional pixel values to an image at the desired positions. Then, in a second rendering pass, you can combine the content of the image with the primary rendering. The combination could be done with blending if the math/logic is simple enough. If you need a more complex combination, you can use a texture as the framebuffer attachment of the initial pass, and then use the result of the rendering and the extra image as two inputs for the fragment shader of the combination pass.