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.
@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.
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.
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.