fragment shader wipe effect,
Members - Reputation: 127
Posted 01 July 2012 - 03:05 PM
I'm back in shader since ten years ... so
I looking for a starting code to make a sort of wipe effect in 2D:
I think its a classical effect but i can't find any example
I need to make cursor as a moving shape on the screen that make appear a texture behind. If the cursor move again over the same path it reveals another texture behind. Every time the shape pass over a texture it reveals another texture behind it . etc.
Is someone can give me a snipet as a starting point, I'm going in many direction but still stuck
Thanks a lot
Members - Reputation: 132
Posted 06 August 2012 - 06:36 AM
You have two full frame textures you want to composite in some fashion, TLeaving and TArriving. You've setup those with Texture2D samplers so that your fragment shader can sample each of them. At the beginning of the transition, you want 100% TLeaving and 0% TArriving. At the end of the transition, you want 0% TLeaving and 100% of TArriving. In between you want a uniform (set on each frame by your driving CPU code) that is varying from 0% to 100% to drive your transition. If it's a wipe, then you can imagine the transformed X coordinate in clip space to be what is driven as the boundary between sampling TLeaving and TArriving in your shader.
(By 'frame' I mean, render cycle during this transtion. You decide how fast you want to drive the transition, how many render cycles it is going to take to complete. You drive that externally, your shader responds to the current render cycle/frame. If it is 60 frames, then frame 0 is 0%, frame 59 is 100%, etc.
You could use an if but don't. Instead, calculate the clip X(gl_PointCoord.X, 0 to 1) for this Transition%, divide every clip X by that value, and assign to an integer to get 0 or 1.(the index of the sampler to use, not to be confused with the normalized range of the clip space, 0 to 1) Use that as an index to sample either sampler  or sampler . The alternative, using an if(pos.x > transClipX), will have unique paths in your shader, causing a stall. A stall isn't fatal but your shader will execute faster if every fragment instance takes the same conditional path. (ie. if you can arrange it, only use conditionals when each path will the same for every instance in a workset.)
Make it a vertical wipe by using ClipY(gl_PointCoord.Y)
Make it a fade by alpha blending the two samples, etc.