Getting the OpenGL-ES Stencil Buffer Working

StackOverflow https://stackoverflow.com/questions/3138724

Question

With the stencil buffer in opengl-es for Android, I'm simply trying to mask out drawing part of the screen. I think I have it set up right, but it is not masking out the non-stenciled parts. Below is an extraction of code for what I'm doing.

gl.glEnable(GL10.GL_STENCIL_TEST);
gl.glClearStencil(0);
gl.glClear(GL10.GL_STENCIL_BUFFER_BIT);
gl.glColorMask(false, false, false, false);
gl.glDepthMask(false);
gl.glStencilFunc(GL10.GL_ALWAYS, 1, 1);
gl.glStencilOp(GL10.GL_REPLACE, GL10.GL_REPLACE, GL10.GL_REPLACE);

drawMask(); //I have confirmed this draws to the correct location by allowing colour to show. Trust that this draws the mask to the correct location.

gl.glColorMask(true, true, true, true);
gl.glDepthMask(true);
gl.glStencilFunc(GL10.GL_EQUAL, 1, 1);
gl.glStencilOp(GL10.GL_KEEP, GL10.GL_KEEP, GL10.GL_KEEP);

drawItems(); //Draw everything. Only items inside the masked area should be drawn. But everything is drawn with this code...

gl.glDisable(GL10.GL_STENCIL_TEST);

Anyone spot anything wrong with this? What it does basically is draw a box for say, half the screen (this works if I had colour enabled) which is setting the stencil buffer to 1 for that area. And at the end I draw to the whole screen. I want it to draw to the top half only, but it draws everything.

Thanks in advance.

Was it helpful?

Solution 2

The reason ended up being that I had not set my EGLConfig properly to support stencil buffer.

OTHER TIPS

You have to set the stencilSize using setEGLConfigChooser. Note that different phones have different surfaces which may or may not support this.

For example:

    // void android.opengl.GLSurfaceView.setEGLConfigChooser(int redSize, int greenSize, int blueSize, int alphaSize, int depthSize, int stencilSize)
    mGLSurfaceView.setEGLConfigChooser(8, 8, 8, 8, 16, 8);

You need to explicitly request the stencil buffer with GLSurfaceView.setEGLConfigChooser:

public class MyActivity extends Activity {
    GLSurfaceView view;
    ...
    onCreate(...
        view.setEGLConfigChooser(5,6,5,0,16,8);
        view.setRenderer(...

The numbers are red, green, blue, alpha, depth, stencil bits. RGB565 with 16 bit depth and 8 bit stencil is the minimum which is supported by each EGL capable android device.

This answer is reposted from the link shown below. This fixed an OpenGL ES2.0 bug in a sample for me.

"It is necessary to set stencil mask glStencilMask(0xff) before calling glClear(GL_STENCIL_BUFFER_BIT) to clear all bits of stencil buffer. "

android opengl 2.0 stencil buffer not working

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