How to read variables in fragment shader?

[R@y]

For example, depending on the variable A, an program will continue or stop by the end of every pass.

float4 main(......)
{
......

IF Condition
Change A

......
}

[k_szczech]

Sorry, but your question is unclear. Are you asking how to pass variables from application to shader?

[R@y]

Sorry, it was my fault.

I am writing a seed filling algorithm on GPU, and need passing some variables from shader to application. These variables will be used to change the application running: to continue to filling, to generate new seed and start new round seed filling or to stop the application.

[k_szczech]

Ok, actually you can't pass variables from shader to application.
The only output of your shader are pixels (color, depth and indirectly stencil). So, your shader should draw a pixel, and application should read it using glReadPixels.
You could also use occlusion query to count pixels that were drawn if you need such information.

[k_szczech]

And one more thing - I don't think that using GPU for seed fill is a good idea. The nature of this algorithm is too far away from the way GPU's work. Seed fill is recursive/scatter and GPU's are parallel/gather.

[R@y]

many thanks

[Jedimaster]

maybe you should use pbuffer or fbo to process the texture data