some parts of this are less than pretty

potation.pde

/*
rotating a 2D image of a potato
 as suggested by Mother
 april 11, 2014
 */

PImage potato;
float[][] rotationMat;
float theta;
float pw, ph;

void setup() {
  size(300, 300);
  //frameRate(6);
  background(200);
  potato = loadImage("potato.png");
  pw = potato.width;
  ph = potato.height;
  theta = 0;
}

void draw() {
  background(200);
  changePixels();
  theta = (theta+0.05)%TWO_PI;
}

void changePixels() {
  rotationMat = getRMat(theta);
  loadPixels();
  potato.loadPixels();
  for (int y = 0; y < ph; y++) {
    for (int x = 0; x < pw; x++) {
      color oldCoor = color(potato.pixels[y*potato.width+x]);
      // this is here because apparently transparent images still have weirdly colored backgrounds?
      // or maybe i'm doing something wrong?
      if (oldCoor != color(6573620)) {
        PVector coors = new PVector(x-pw/2, y-ph/2);
        PVector newCoors = matMult(rotationMat, coors);
        int newCoor = ((height/2)+(int)newCoors.y)*width+((width/2)+(int)newCoors.x);
        if (newCoor < width*height && newCoor > 0) { 
          pixels[newCoor] = oldCoor;
        }
      }
    }
  }
  updatePixels();
}

PVector matMult(float[][] mat, PVector p) {
  float x = p.x*mat[0][0] + p.y*mat[0][1];
  float y = p.x*mat[1][0] + p.y*mat[1][1];
  return new PVector(x, y);
}

float[][] getRMat(float a) {
  rotationMat = new float[2][2];
  // column-major matrix
  rotationMat[0][0] = cos(a);
  rotationMat[0][1] = sin(a);
  rotationMat[1][0] = -sin(a);
  rotationMat[1][1] = cos(a);
  return rotationMat;
}