Glass tile distortion shader

Occassionally, I will post tiny shaders, useful for effects in 4k intros (eg kaleidoscope). I discovered recently how to do glass tiles as a post process overlay in very little code. Distortion shaders require you to render to copy to texture first. I have done other blogs about this. However I would guess most 4ks already do this for blur, AO, rgb distortion or whatever already, so the extra bytes of this shader are absolutely tiny.

Here is the shader pair:

varying vec4 v;
void main(){ v = gl_Position = ftransform(); }

The vertex shader assumes something is drawn on the screen (a quad say) and covers the screen as a surface for the distortion shader. This means v will hold co-ordinates -1.0..1.0

uniform sampler2D s;
varying vec4 v;
void main(){
vec2 x = v.xy/2.0 + 0.5;
gl_FragColor = texture2D( s, x + 0.1*tan(x*5.0) );

Firstly v is mapped to 0.0..1.0 range of normal texture co-ordinates. The glass tile effect is then simply that tan distortion! Here is an image of what it can produce - ignore the background, thats just something to be distorted.

If you want to "waste" bytes, its a good idea to do something like:

gl_FragColor = texture2D( s, x+min( 0.1*tan(x*5.0), 0.2 ) );

This helps to prevent "sparklies" in the distortion by keeping that tan under control.