How to create Stencil buffer with texture (Image) in OpenGL-ES 2.0

Question

Can I have Stencil prepared with a texture (Image) in OpenGL 2.0 So that some part of the Image will be transparent and as a result it will be transfered as is
to Sten

Answer 1

Finally I have managed to do it using conditional passing of colors using discard keyword in shader. I also used 2 textures in same shader and combined them accordingly.

Thanks all.

Answer 2

Yes, you can use alpha testing (glEnable(GL_ALPHA_TEST); glAlphaFunc(COMPARISION, VALUE);) to discard fragments; discarded fragments will not be stencil tested, so you can use the passing fragments to build the stencil mask.

EDIT code example

/* the order in which orthogonal OpenGL state is set doesn't matter */
glEnable(GL_STENCIL_TEST);
glEnable(GL_ALPHA_TEST);

/* don't write to color or depth buffer */
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
glDepshMask(GL_FALSE);

glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);

/* first set alpha test so that fragments greater than the threshold
   will pass thus will set nonzero bits masked by 1 in stencil */
glStencilFunc(GL_ALWAYS, 1, 1);
glAlphaFunc(GL_GREATER, threshold);
draw_textured_primitive();


/* second pass of the fragments less or equal than the threshold
   will pass thus will set zero bits masked by 1 in stencil */
glStencilFunc(GL_ALWAYS, 0, 1);
glAlphaFunc(GL_LEQUAL, threshold);
draw_textured_primitive();

Don't forget to revert OpenGL state for drawing afterwards.

EDIT to account for updated question:

Since you're actually using iOS, thus OpenGL-ES 2.0 things get a bit different. There's no alpha test in OpenGL-ES2. Instead you're expected to discard the fragment in the fragment shader, if it's below/above a chosen threshold, by using the discard keyword/statement.