Low readback performance with PBO , help !!!!!

[PixIn]

Hi

i use a PBO approach to grab a bitmap from my 3d scene . steps are the following :

glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pbo1);

glReadPixels(0, 0,bWIDTH,bHEIGHT,GL_BGRA, GL_UNSIGNED_BYTE, 0);

copymem(glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB,GL _READ_ONLY_ARB)^, buffer,BWidth * BHeigh * 4);

glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pbo2);

glReadPixels(0, 0,pWIDTH,pHEIGHT ,GL_BGRA, GL_UNSIGNED_BYTE, 0);

copymem(glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY_ARB)^,bitmapbuf, bWidth * bHeigh * 4);
glUnmapBufferARB(GL_PIXEL_PACK_BUFFER_ARB);

swap(pbo1,pbo2)

with methode i get 15~25 fps low than a direct :

glReadPixels(0, 0,pWIDTH,pHEIGHT ,GL_BGRA, GL_UNSIGNED_BYTE, bitmapbuf);

as i see in many forum the PBO should be more Faster than the glReadPixels one


my card is : nVidia Geforce 7600 GS
forceware version : 169.21
Bus PCI Express x16
CPU : P4 3.0Ghz


are there any bad implementation in my PBO code, how can i boost it ?

help please

Thanks in Advance

[Brolingstanz]

Is this for a screenshot?

You want to use PBOs when you have something else to do while the transfer is taking place behind the scenes, not when you need the results straight away.

Check out the PBO spec for some common usage scenarios and example code.

[mfort]

1) Why are you reading the data twice?
2) Why are you coping data out of PBO? the PBO memory is just fine.
3) Do not call MapBuffer just after read pixels. There is no benefit of PBO then.

[PixIn]

Is this for a screenshot?

You want to use PBOs when you have something else to do while the transfer is taking place behind the scenes, not when you need the results straight away.

Check out the PBO spec for some common usage scenarios and example code.

yes it is for a screenshot.

just now i change it to this to acheive a Asynchron readback

glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[0]);
glReadPixels(0, 0, imagewidth, imageheight/2, GL_BGRA, GL_UNSIGNED_BYTE, BUFFER_OFFSET(0));

glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[1]);
glReadPixels(0,imageheight/2,imagewidth,imageheight/2,GL_BGRA,GL_UNSIGNED_BYTE, BUFFER_OFFSET(0));

// Process partial images. Mapping the buffer waits for
// outstanding DMA transfers into the buffer to finish.
glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[0]);
pboMemory1 = glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY);
processImage(pboMemory1);

glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, imageBuffers[1]);
pboMemory2 = glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY);
processImage(pboMemory2);

but still get low performance instead the glreadpixels

can someone please guide me to the correct steps

Thanks

[yooyo]

In current frame bind pbo and do readback.
In next frame map pbo and copy it's content to sysmem.

Why this? When app call glReadPixels while pbo is bind, then glReadPixels is nonblocking call. But if you try to map pbo buffer soon after glReadPixels then this glMapBuffers will be blocked until glReadPixels is finished.
When to call map buffer is hard to tell, because it depends on underlaying hardware, driver, screen size, chipset, ... So the best will be to do that operation (glMapBuffer and memcpy) in next frame.

Also.. this pbo memory is not cacheable so do not try to do some weird access pattern. Plain memcpy in sysmem buffer is best approach.

[PixIn]

Thanks for reply

ok, i am not sure that i get it well but i change my code a bit to :


index = (index + 1) % 2;
nextIndex = (index + 1) % 2;

glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[index]);
glReadPixels(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, PIXEL_FORMAT, GL_UNSIGNED_BYTE, 0);

GLubyte* src = (GLubyte*)glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY_ARB);

glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[nextIndex]);
GLubyte* src = (GLubyte*)glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY_ARB);

glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0);

but still same problem ... low performance

any idea, perhaps some code will help me better

Thanks

[yooyo]

No.. thats wrong.. see this

// At end of frame before SwapBuffers call
// to use this... just set bDoScreenShot to true.
if (bCopyToSysMem)
{
 bCopyToSysMem = false;
 glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pbo); 
 GLubyte* src = (GLubyte*)glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY_ARB);
 memcpy(sysme, src, imgsize);
 glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0); 
}
 
if (bDoScreenShot)
{
 glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pbo); 
 glReadPixels(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, PIXEL_FORMAT, GL_UNSIGNED_BYTE, 0);
 glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0); 
 bCopyToSysMem = true;
 bDoScreenShot = false;
}
 
SwapBuffers(...);

Above code snippet is just for single screenshot!

[PixIn]

Thanks yooyo for the hints

your code give a faster result (10~15 fps faster).
but :s i get a black bitmap it seems that the src is empy :S

can u tell me what the sysme is ?

Thanks

[Jackis]

yooyo wants to say, that in order to use befits from PBO, you should make asynchronous readbacks. In the code above yooyo advice you to make ReadPixels with PBO on the first frame, but you can use this memory only next frame (or some frames later, maybe 2-3), and only way like that may get you PBO benefit.

[yooyo]

Just insert my code before you call SwapBuffers, at end of render frame. Something like...

// this is a very basic render loop
while (bQuit == false)
{
 UpdateGame();
 RenderGame();
 // insert my code here
 SwapBuffers(); // present frame
}

can u tell me what the sysme is ?

sysme is a typo... it should be sysmem
sysmem is pointer to system memory buffer. Applicatio should allocate this buffer. Size should be SCR_WIDTH * SCR_HEIGHT * BYTES_PER_PIXEL.