OpenGL ES 2.0 - GL_POINTS and Stencil Buffer

Question

I am trying to draw a texture in OpenGL ES 2.0 using GL_POINTS by applying a stencil buffer. The stencil buffer should come from a texture. I am rendering the results to another texture and then presenting the texture to screen. Here is my code for rendering to texture:

//Initialize buffers, initialize texture, bind frameBuffer
.....
glClearStencil(0);
glClear (GL_STENCIL_BUFFER_BIT);

glColorMask( GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE );
glEnable(GL_STENCIL_TEST);
glStencilFunc(GL_ALWAYS, 1, 1);
glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
glBindTexture(GL_TEXTURE_2D, stencil);

glUseProgram(program[PROGRAM_POINT].id);
glDrawArrays(GL_POINTS, 0, (int)vertexCount);

glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
glStencilFunc(GL_NEVER, 0, 1);
glStencilOp(GL_REPLACE, GL_KEEP, GL_KEEP);

glBindTexture(GL_TEXTURE_2D, texture);
glUseProgram(program[PROGRAM_POINT].id);
glDrawArrays(GL_POINTS, 0, (int)vertexCount);
glDisable(GL_STENCIL_TEST);
....
//Render texture to screen

The result I am getting is just my texture being drawn without any masking applied from the stencil. I had a few questions regarding this issue:

1. Is is possible to use a stencil buffer with GL_POINTS?
2. Is is possible to use a stencil buffer when rendering to a texture?
3. Does the stencil texture have to have any special properties (solid colour, internal format...etc)?
4. Are there any apparent mistakes with my code?

This is the result I am looking for:

UPDATE:

My problem, as pointed out by the selected answer, was primarily that I did not attach the stencil to the stencil attachment of the FBO:

glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_STENCIL_ATTACHMENT,
                      GL_RENDERBUFFER, stencilBufferId);

I did not know that it was required when rendering to a texture. Secondly I was not using the proper stencil test.

glStencilFunc(GL_EQUAL, 1, 1);
glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);

Did the job.

Answer 1

Addressing your questions in order:

1. Is is possible to use a stencil buffer with GL_POINTS?

Yes. The stencil test is applied to all fragments, no matter of the primitive type rendered. The only case where you write to the framebuffer without applying the stencil test is with glClear().

2. Is is possible to use a stencil buffer when rendering to a texture?

Yes. However, when you render to a texture using an FBO, the stencil buffer of your default framebuffer will not be used. You have to create a stencil renderbuffer, and attach it to the stencil attachment of the FBO:

GLuint stencilBufferId = 0;
glGenRenderbuffers(1, &stencilBufferId);
glBindRenderbuffer(GL_RENDERBUFFER, stencilBufferId);
glRenderbufferStorage(GL_RENDERBUFFER, GL_STENCIL_INDEX8, width, height);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_STENCIL_ATTACHMENT,
                          GL_RENDERBUFFER, stencilBufferId);

3. Does the stencil texture have to have any special properties (solid colour, internal format...etc)?

OpenGL ES 2.0 does not have stencil textures. You have to use a renderbuffer as the stencil attachment, as shown in the code fragment above. GL_STENCIL_INDEX8 is the only format supported for renderbuffers that can be used as stencil attachment. ES 3.0 supports depth/stencil textures.

4. Are there any apparent mistakes with my code?

Maybe. One thing that looks slightly odd is that you never really apply a stencil test in the code that is shown. While you do enable the stencil test, you only use GL_ALWAYS and GL_NEVER for the stencil function. As the names suggest, these functions either always or never pass the stencil test. So you don't let fragments pass/fail depending on the stencil value. I would have expected something like this before the second draw call:

glStencilFunc(GL_EQUAL, 1, 1);
glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);

This would only render the fragments where the current stencil buffer value is 1, which corresponds to the fragments you rendered with the previous draw call.