When something spins fast, it blurs. It has to otherwise it looks as if it’s rotating by arbitrary angles. Originally I had this effect done by rendering the same image multiple times, each time slightly rotated and more transparently than the previous time. Yesterday I tried doing it the GLSL way, which looks way better (and is faster). Here’s the result:

And here’s the shader:

uniform float angle; #define NUM_ROTATIONS 30 vec4 effect( vec4 color, Image texture, vec2 texture_coords, vec2 screen_coords ) { vec2 center = vec2( 0.5, 0.5 ); vec2 relPos = texture_coords - center; vec4 finalColor = vec4( 0.0, 0.0, 0.0, 0.0 ); float progression = 0.0; for( int i=0; i<NUM_ROTATIONS; i++ ) { float angleFract = - angle*float(i)/float(NUM_ROTATIONS); float cs = cos( angleFract ); float sn = sin( angleFract ); vec2 newPos = vec2( relPos.x*cs - relPos.y*sn, relPos.x*sn + relPos.y*cs ); newPos += center; vec4 tex = texture2D( texture, newPos ); float rate = pow(float(i),1.2) * (tex.a +0.4); progression = progression + rate; finalColor += tex * rate; } return finalColor/progression; }