Question

I’m trying to use QGLbuffer to display an image.
Sequence is something like:

initializeGL() {
  glbuffer= QGLBuffer(QGLBuffer::PixelUnpackBuffer);        
  glbuffer.create();
  glbuffer.bind();
  glbuffer.allocate(image_width*image_height*4); // RGBA
  glbuffer.release();
}

// Attempting to write an image directly the graphics memory.
// map() should map the texture into the address space and give me an address in the   
// to write directly to  but always returns NULL
unsigned char* dest = glbuffer.map(QGLBuffer::WriteOnly);  FAILS
MyGetImageFunction( dest );         
glbuffer.unmap();

paint() {
  glbuffer.bind();
    glBegin(GL_QUADS);
    glTexCoord2i(0,0); glVertex2i(0,height());
    glTexCoord2i(0,1); glVertex2i(0,0);
    glTexCoord2i(1,1); glVertex2i(width(),0);
    glTexCoord2i(1,0); glVertex2i(width(),height());
    glEnd();             
  glbuffer.release();
}

There aren’t any examples of using GLBuffer in this way, it’s pretty new

Edit — for search here is the working solution ——-

 // Where glbuffer is defined as  
 glbuffer= QGLBuffer(QGLBuffer::PixelUnpackBuffer); 

// sequence to get a pointer into a PBO, write data to it and copy it to a texture
glbuffer.bind(); // bind before doing anything
unsigned char *dest = (unsigned char*)glbuffer.map(QGLBuffer::WriteOnly);
MyGetImageFunction(dest);                           
glbuffer.unmap(); // need to unbind before the rest of openGL can access the PBO

glBindTexture(GL_TEXTURE_2D,texture);                       
// Note 'NULL' because memory is now onboard the card                                                                                   
glTexSubImage2D(GL_TEXTURE_2D, 0, 0,0, image_width, image_height, glFormatExt, glType, NULL);  
glbuffer.release();  // but don't release until finished the copy


// PaintGL function
glBindTexture(GL_TEXTURE_2D,textures);
glBegin(GL_QUADS);
    glTexCoord2i(0,0); glVertex2i(0,height());
    glTexCoord2i(0,1); glVertex2i(0,0);
    glTexCoord2i(1,1); glVertex2i(width(),0);
    glTexCoord2i(1,0); glVertex2i(width(),height());
glEnd();             

Answer 1

You should bind the buffer before mapping it!

In the documentation for QGLBuffer::map:

It is assumed that create() has been called on this buffer and that it has been bound to the current context.

In addition to VJovic’s comments, I think you are missing a few points about PBOs:

A pixel unpack buffer does not give you a pointer to the graphics texture. It is a separate piece of memory allocated on the graphics card to which you can write to directly from the CPU.

The buffer can be copied into a texture by a glTexSubImage2D(….., 0) call, with the texture being bound as well, which you do not do. (0 is the offset into the pixel buffer). The copy is needed partly because textures have a different layout than linear pixel buffers.

See this page for a good explanation of PBO usages (I used it a few weeks ago to do async texture upload).