'How to efficiently initialize texture with zeroes?

I need to initialize an OpenGL 2D texture with zeroes.

Why? I render to this texture with OpenCL. Roughly, rendering stages are:

  1. Create RGBA texture, initialize with zeros (so it contains transparent black pixels)
  2. OpenCL: compute color of pixels where some objects are visible and write them to this texture
  3. OpenGL: add environment mapping to the transparent pixels and display final image

I therefore do not need to pass any actual texture data to glTexImage2d. But according to the spec, when I pass data=NULL, the texture memory is allocated but not initialized.

Allocating a client-side memory filled with zeroes and passing to glTexImage2D is not a very good option although only one I can think of.

EDIT: I actually want to init only alpha component (RGB will get overriden anyway), but I doubt this make the situation any easier.

opengl


Solution 1:[1]

Just a heads up: The new GL_ARB_clear_texture (core in 4.4) extension specifically addresses this problem.

Solution 2:[2]

  1. glTexSubImage is filled with the whole image:
        GLfloat* pData = new GLfloat[1024*768*4];
        memset(pData, 0x00, 1024*768*4*sizeof(GLfloat));
        glBindTexture(GL_TEXTURE_2D, m_ImageTex);
        glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 1024, 768, GL_RGBA, GL_FLOAT, pData);
        glBindTexture(GL_TEXTURE_2D, 0);
        delete pData;

  1. use Pixel Buffer to copy data from gpu, this method is faster and better

    i. init PBO

        glGenBuffers(1, &m_PixelBuffer);
        glBindBuffer(GL_PIXEL_UNPACK_BUFFER, m_PixelBuffer);
        glBufferData(GL_PIXEL_UNPACK_BUFFER, SCREEN_WIDTH*SCREEN_HEIGHT*4*sizeof(GLfloat), NULL, GL_STATIC_DRAW);
        GLfloat* pData = nullptr;
        pData = (GLfloat*)glMapBuffer(GL_PIXEL_UNPACK_BUFFER, GL_WRITE_ONLY);
        memset(pData, 0x00, 1024*768*4*sizeof(GLfloat));
        glUnmapBuffer(GL_PIXEL_UNPACK_BUFFER);

ii.     

        glBindBuffer(GL_PIXEL_UNPACK_BUFFER, m_PixelBuffer);
        glBindTexture(GL_TEXTURE_2D, m_ImageTex);
        glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, GL_RGBA, GL_FLOAT, NULL);
        glBindTexture(GL_TEXTURE_2D, 0);
        glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);

Solution 3:[3]

Maybe you can set the texture as the rendering target for the first pass and draw a black primitive. It may be faster since there are no memory transfert between the ram and graphic card.

Sources

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Source: Stack Overflow

Solution Source
Solution 1
Solution 2 jeyko
Solution 3 neodelphi

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