openGL Texture masking

I am attempting to fill a circle with a series of other images and have those images masked off by the circle. I can see why this isn't working, but I can't come up with a solution as to how to fix it.

My drawing code (using processing) is as follows:

  PGraphicsOpenGL pgl = (PGraphicsOpenGL) g;  // g may change
  // This fixes the overlap issue
  gl.glDisable(GL.GL_DEPTH_TEST);
  // Turn on the blend mode
  gl.glEnable(GL.G开发者_运维技巧L_BLEND);

  // Define the blend mode
  gl.glBlendFunc(GL.GL_SRC_ALPHA, GL.GL_ONE_MINUS_SRC_ALPHA);

  // draw the backgroud
  fill(200,200,200);
  rect(0, 0, width, height);

  // cut out the circle
  gl.glBlendFunc(GL.GL_ZERO, GL.GL_ONE_MINUS_SRC_ALPHA);
  tint(0,0,0,255);
  image(circle, 0, 0);

  // draw the circle
  gl.glBlendFunc(GL.GL_ONE_MINUS_DST_ALPHA, GL.GL_ONE);
  tint(140,0,0,255);
  image(circle, 0, 100);

  gl.glBlendFunc(GL.GL_ONE_MINUS_DST_ALPHA, GL.GL_ONE);
  tint(140,0,140,255);
  image(circle, 0, 0);

I have been following the directions at http://bigbucketsoftware.com/2010/10/04/how-to-blend-an-8-bit-slide-to-unlock-widget/ which seem to describe the effect that I want. I have also tried this on iphone with similar results.

Here is what I was expecting to happen, and what happened:

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The problem must be with how you treat the transparent region. You could enable GL_ALPHA_TEST.

Or if your pictures stay that simple you can just draw them with triangles.

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I can't really help you with the blending code but I have another suggestion that might simplify your drawing logic.

I have used the stencil buffer for something like that. I wanted to draw a disk textured with a linear grating. I didn't want to bother with texture coordinates because an important function was to be able to exactly step through the phases of the grating.

I drew the texture in a big rectangle and afterwards I drew in the stencil a white disk.

http://www.swiftless.com/tutorials/opengl/basic_reflection.html

(let ((cnt 0d0))
  (defmethod display ((w window))
    ;; the complex number z represents amplitude and direction
    ;; of the grating constant
    ;; r and psi addresses different points in the back focal plane
    ;; r=0 will result in z=w0. the system is aligned to illuminate
    ;; the center of the back focal plane for z=w0.
    (let* ((w0 (* 540d0 (exp (complex 0d0 (/ pi 4d0)))))
           (r 260d0)
           (psi 270d0)
           (w (* r (exp (complex 0d0 (* psi (/ pi 180d0))))))
           (z (+ w w0)))
      (clear-stencil 0)
      (clear :color-buffer-bit :stencil-buffer-bit)
      (load-identity)
      ;; http://www.swiftless.com/tutorials/
      ;;    opengl/basic_reflection.html
      ;; use stencil buffer to cut a disk out of the grating
      (color-mask :false :false :false :false)
      (depth-mask :false)
      (enable :stencil-test)
      (stencil-func :always 1 #xffffff)
      (stencil-op :replace :replace :replace)

      (draw-disk 100d0 (* .5d0 1920) (*  .5d0 1080))
      ;; center on camera 549,365
      ;; 400 pixels on lcos = 276 pixels on camera (with binning 2)
      (color-mask :true :true :true :true)
      (depth-mask :false)
      (stencil-func :equal 1 #xffffff)
      (stencil-op :keep :keep :keep)

      ;; draw the grating
      (disable :depth-test)
      (with-pushed-matrix 
          (translate (* .5 1920) (*  .5 1080) 0)
        (rotate (* (phase z) 180d0 (/ pi)) 0 0 1)
        (translate (* -.5 1920) (* -.5 1080) 0)
        (draw *bild*))
      (disable :stencil-test)
      (enable :depth-test)

      (fill-grating *grating* (abs z))
      (format t "~a~%" cnt)
      (if (< cnt 360d0)
          (incf cnt 30d0)
          (setf cnt 0d0))
      (update *bild*)
      (swap-buffers)
      (sleep (/ 1d0)) ;; 1 frame per second
      (post-redisplay))))