rotate.glsl

uniform sampler2D texture;
uniform vec2 texSize;
varying vec2 texCoord;
uniform float degrees;
const float PI = 3.1415926535897932384626433832795;
const vec2 HALF = vec2(0.5);

float deg2rad(float deg){ return deg * PI / 180.0; }
void main() {
  vec2 coord = texCoord.xy;
  float aspect = texSize.x / texSize.y;

  float sin_factor = sin(deg2rad(degrees));
  float cos_factor = cos(deg2rad(degrees));

  mat2 rotMat      = mat2(cos_factor, sin_factor, -sin_factor, cos_factor);
  mat2 scaleMat    = mat2(aspect, 0.0, 0.0, 1.0);
  mat2 scaleMatInv = mat2(1.0 / aspect, 0.0, 0.0, 1.0);

  vec2 tc = coord;
  coord -= HALF.xy;
  coord = scaleMatInv * rotMat * scaleMat * coord;
  coord += HALF.xy;  

  vec4 color = texture2D(texture, coord);
  float grey = 0.299 * color.r + 0.587 * color.g + 0.114 * color.b;
  gl_FragColor = vec4(color);