QGLBuffer::map returns NULL? - c++

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).

Answer 2

create will return false if the GL implementation does not support buffers, or there is no current QGLContext.
bind returns false if binding was not possible, usually because type() is not supported on this GL implementation.
You are not checking if these two functions passed.

Answer 3

I got the same thing, map returns NULL. When I used the following order it is solved.
bool success = mPixelBuffer->create();
mPixelBuffer->setUsagePattern(QGLBuffer::DynamicDraw);
success = mPixelBuffer->bind();
mPixelBuffer->allocate(sizeof(imageData));
void* ptr =mPixelBuffer->map(QGLBuffer::ReadOnly);