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 Stencil buffer and then will use this use this stencil buffer for further drawing.

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EDIT by datenwolf to account for OPs question update in a answer:

By @InfiniteLoop:

@datenwolf thanks a lot for ur reply but no success :( here is my code

- (void)render 
{
    [EAGLContext setCurrentContext:context];
    glBindFramebuffer(GL_FRAMEBUFFER, defaultFramebuffer);
    glUseProgram(program);
    glClearColor(0, 104.0/255.0, 55.0/255.0, 1.0);
    glViewport(0, 0, backingWidth, backingHeight);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
    // Set the stencil clear value
    glClearStencil ( 0x1 );

    // Set the depth clear value
    glClearDepthf( 0.75f );



    ////////
    ////
    GLfloat threshold = 0.5f;//half transparency
    /* 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);
    glDepthMask(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);//set one
    glAlphaFunc(GL_GREATER, threshold);//greater than half transparency

    glVertexAttribPointer(positionLoc, 3, GL_FLOAT, GL_FALSE, 0, fullVertices);
    glVertexAttribPointer(colorLoc, 4, GL_FLOAT, GL_FALSE, 0, colorVertices);
    glVertexAttribPointer(textureLoc, 2, GL_FLOAT, GL_FALSE, 0, textureVertices);
    [self drawTexture:texture2DMaskImage];
    /* 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);//set zero
    glAlphaFunc(GL_LEQUAL, threshold);//Less than Half transparency
    glVertexAttribPointer(positionLoc, 3, GL_FLOAT, GL_FALSE, 0, fullVertices);
    glVertexAttribPointer(colorLoc, 4, GL_FLOAT, GL_FALSE, 0, colorVertices);
    glVertexAttribPointer(textureLoc, 2, GL_FLOAT, GL_FALSE, 0, textureVertices);
    [self drawTexture:texture2DMaskImage];


// till here as suggested by u after this I have added to draw
// the next Image which will use the created Stencil
    glDepthMask(GL_TRUE);
    glDisable(GL_ALPHA_TEST);
    glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
    //
    glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);//stop further write to stencil buffer
    glStencilFunc(GL_EQUAL, 0, 1);//pass if 0
    glVertexAttribPointer(positionLoc, 3, GL_FLOAT, GL_FALSE, 0, fullVertices);
    glVertexAttribPointer(colorLoc, 4, GL_FLOAT, GL_FALSE, 0, colorVertices);
    glVertexAttribPointer(textureLoc, 2, GL_FLOAT, GL_FALSE, 0, textureVertices);
    [self drawTexture:texture2D];
    ////
    ///////



    // Validate program before drawing. This is a good check, 
    // but only really necessary in a debug build.
    // DEBUG macro must be defined in your debug configurations
    //  if that's not already the case.
#if defined(DEBUG)
    if (![self validateProgram:program])
    {
        NSLog(@"Failed to validate program: %d", program);
        return;
    }
#endif
    // draw
    [context presentRenderbuffer:GL_RENDERBUFFER];//Present
}

- (void) drawTexture:(Texture2D*)texture
{
    // handle viewing matrices
    GLfloat proj[16], modelview[16], modelviewProj[16];
    // setup projection matrix (orthographic)
    mat4f_LoadOrtho(-1.0f, 1.0f, -1.0f, 1.0f, -1.0f, 1.0f, proj);
    //   glUniformMatrix4fv(uniforms[UNIFORM_PROJECTION], 1, 0, proj);
    // setup modelview matrix (rotate around z)
    mat4f_LoadIdentity(modelview);
    mat4f_LoadZRotation(0.0, modelview);
    mat4f_LoadTranslation3D(0.0, 0.0, 0.0, modelview);
    // projection matrix * modelview matrix
    mat4f_MultiplyMat4f(proj, modelview, modelviewProj);
    glUniformMatrix4fv(_modelViewUniform, 1, 0, modelviewProj);
    // update uniform values
    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, texture.name);
    glUniform1i(_textureUniform, 0);
    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, cubeIndices);
}

please shed some light I am in desperate need :( thanks once again.

Answer 1

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);
glDepthMask(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.

Answer 2

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.