How to read back data from fragment shader

Question

I am trying to find out how i can read back data from fragment shader to application but specifically fragment shader in built variables

Here is an example of what i am trying to do, in this fragment shader below i am trying to read back to the application the fragment shader variable gl_FragCoord.y

#version 330
out vec4 outputColor;
void main()
{
  gl_FragCoord.y??????
outputColor = vec4(0.2f, 0.2f, 0.2f, 1.0f);
}

How would i pass this value back to the application, is it done through uniform variable? buffer object? or some other way? or can it be done at all?

i am specifically trying to get the gl_FragCoord.y value for each time the fragment shader runs

Answer 1

Okay i have found a few ways to do this in case anybody needs to know and since nobody provided the answer i searched for it myself and found it(besides the one that was provided by the person who commented)

First way was mentioned by one of the commentators to the question, which is to create a framebuffer/renderbuffer object as an image, write data to it directly from the shader and read back the data as an image or to read back in general from framebuffer using glReadPixels() to currently bound framebuffer object

Second way is to create buffer object/array in fragment shader to write from the variable mentioned above and to map the buffer data using glMapBuffer*() and get a pointer to the buffer and read back the value in application

Another was to also create a buffer object and then use glGetBufferSubData to read back everything that is written to it from shader (obviously you declare the buffer object as read/write also)

And finally you can use image units texture data to also store from shader to these buffer objects and read back from them , if anyone needs exact details leave a comment and i will provide

See it IS possible and more then one way to do it, however it wont be run for each time the fragment shader runs, but rather the fragment shader will store the information to buffer object and those aforementioned and read back to application after execution , however the result will still be the same, in that the gl_FragCoord.y for each fragment will be stored and accessed as a chunk of memory