See mask(). If you search the forums, you will find examples of usage.

hmm that's not what I am looking for..

No simple way, but mask(), which you reject lightly, is the way to do it in Processing.

Just came across this thread because i had the same problem.
simplest solution I could think of is to make a circle using vertices and texturing those.

textureMode(NORMAL);
beginShape();
texture(texPImg);
for (int i = 0; i<20; i++) {
      float stepFac = 2*PI*(float(i)/20);
      vertex(x+cRadius/2*sin(stepFac),y+cRadius/2*cos(stepFac),(sin(stepFac)+1)/2,(cos(stepFac)+1)/2);
}
endShape(CLOSE);