using whiteness for alpha

OpenGL OpenGL: Basic Coding
1

Hello,

I’m using a framebuffer object and I’d like this fbo to generate a transparent texture.

I’ve tried to render transparent 2D objects to the fbo and then render its texture. However, this doesn’t seem to work, only pixels with alpha=1 are rendered to the fbo texture. (unless I use an opaque background)

So my new plan is now to let the fbo generate a black and white texture(I don’t need the colors) and use this texture’s whiteness-intensity as an alpha value instead.

At the moment I use this blending function:

glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA)

This doesn’t use the white-intensity for transparency however.
So my question is: how can I make openGL use the white-intensity instead of the alpha channel for transparency during rendering?

I’m thinking about solutions like:

  • use a different blending function
  • use a different internal format for the fbo-texture
  • etc.
2

Does your fbo have an alpha channel ? Getting only alpha=1 can mean there is none, and default value is always returned.

3

Use a shader and set fragcolor.a equal to texture.r, then stick with your current blendfunc.

4

I build my fbo like this:


    glGenFramebuffers(1, &fbo);

    glGenTextures(1,&textTexture);



	glBindFramebuffer(GL_FRAMEBUFFER_EXT, fbo);



    glBindTexture(GL_TEXTURE_2D, textTexture);

    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA,

                GLsizei(x2-x1), GLsizei(total_textHeight),

                 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);

	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,		GL_LINEAR);

	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER,		GL_LINEAR);

	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S,			GL_CLAMP);

	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T,			GL_CLAMP);



	glFramebufferTexture2D(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, textTexture, 0);



    GLenum status=glCheckFramebufferStatus(GL_FRAMEBUFFER_EXT);



    glBindFramebuffer(GL_FRAMEBUFFER_EXT, 0);

	glBindTexture(GL_TEXTURE_2D, 0);

Also, it’s not like I only get alpha=1, it’s just that the pixels with alpha<1 are not rendered to the fbo.

5

Also, it’s not like I only get alpha=1, it’s just that the pixels with alpha<1 are not rendered to the fbo.

Alpha test might be enabled. Try glDisable(GL_ALPHA_TEST);

6

I already tried that. Sorry, I didn’t mention it!

7

Yes! This sounds like it can work. Thank you!