Is there any way to attach a texture buffer object (ARB_texture_buffer_object) to a framebuffer (EXT_framebuffer_object), so that I can directly render into the texture buffer object?
I need this to make an exact, bit-wise copy of a multisample framebuffer (color buffer, depth buffer and stencil buffer), and have this copy reside in main memory rather than VRAM.
UPDATE:
The problem is that I cannot directly call glReadPixels on a multi sampled frame buffer, to copy its contents. Instead, I have to blit the multi sampled frame buffer to an intermediate frame buffer and then call glReadPixels on that. During this process, multiple samples are averaged and written to the intermediate buffer. There is now, of course, a loss in precision if I restore this buffer with glWritePixels.
I realize that I can use a multi sample texture as the backing storage for a frame buffer object, but this texture will reside in VRAM and there appears to be no way of copying it to main memory without the same loss of precision. Specifically, I am worried about a loss of precision pertinent to the multi sampled depth buffer attachment, rather than the color buffer.
Is there another way to make an exact copy (and restore this copy) of a multi sampled frame buffer in OpenGL?
TL;DR: How to copy the exact contents of a multi sample frame buffer (specifically, depth buffer) to main memory and restore those contents later, without a loss of precision.
OpenGL does not allow you to bind a buffer texture as a render target. However, I don't see what is stopping you from making "an exact, bit-wise copy of a multisample framebuffer". What problem are you encountering that you believe buffer textures can solve?
How to copy the exact contents of a multi sample frame buffer (specifically, depth buffer) to main memory and restore those contents later, without a loss of precision.
No.
And you don't need to copy the contents of an image to main memory to be able to save and restore it later. If you need to preserve the contents of a multisample image, simply blit it to another multisample image. You can blit it back to restore it. Or better yet, render to a multisample texture that you don't erase until you're done with it. That way, there's no need for any copying.
Related
Well i have a texture that is generated every frame and I was wondering the best way to render it in opengl. It is simply pixel data that is generated on the cpu in rgba8 (32-bit, 8 bit for each component) format, I simply need to transfer it to the gpu and draw it onto the screen. I remember there being some sort of pixel buffer or frame buffer that does this without having to generate a new texture every frame in association with glTexImage2d?
Pixel Buffer Objects do not change the fact that you need to call glTexImage2D (...) to (re-)allocate texture storage and copy your image. PBOs provide a means of asynchronous pixel transfer - basically making it so that a call to glTexImage2D (...) does not have to block until it finishes copying your memory from the client (CPU) to the server (GPU).
The only way this is really going to improve performance for you is if you map the memory in a PBO (Pixel Unpack Buffer) and write to that mapped memory every frame while you are computing the image on the CPU.
While that buffer is bound to GL_PIXEL_UNPACK_BUFFER, call glTexImage2D (...) with NULL for the data parameter and this will upload your texture using memory that is already owned by the server, so it avoids an immediate client->server copy. You might get a marginal improvement in performance by doing this, but do not expect anything huge. It depends on how much work you do between the time you map/unmap the buffer's memory and when you upload the buffer to your texture and use said texture.
Moreover, if you call glTexSubImage2D (...) every frame instead of allocating new texture image storage by calling glTexImage2D (...) (do not worry -- the old storage is reclaimed when no pending command is using it anymore) you may introduce a new source of synchronization overhead that could reduce your performance. What you are looking for here is known as buffer object streaming.
You are more likely to improve performance by using a pixel format that requires no conversion. Newer versions of GL (4.2+) let you query the optimal pixel transfer format using glGetInternalFormativ (...).
On a final, mostly pedantic note, glTexImage2D (...) does not generate textures. It allocates storage for their images and optionally transfers pixel data. Texture Objects (and OpenGL objects in general) are actually generated the first time they are bound (e.g. glBindTexture (...)). From that point on, glTexImage2D (...) merely manages the memory belonging to said texture object.
I have a texture that was created by another part of my code (with QT5's bindTexture, but this isn't relevant).
How can I set an OpenGL hint that this texture will be frequently updated?
glBindTexture(GL_TEXTURE_2D, textures[0]);
//Tell opengl that I plan on streaming this texture
glBindTexture(GL_TEXTURE_2D, 0);
There is no mechanism to indicating that a texture will be updated repeatedly; that is only related to buffers (e.g., VBOs, etc.) through the usage parameter. However, there are two possibilities:
Attache your texture as a framebuffer object and update it that way. That's probably the most efficient method to do what you're asking. The memory associated with the texture remains resident on the GPU, and you can update it at rendering speeds.
Try using a pixel buffer object (commonly called a PBO, and has an OpenGL buffer type of GL_PIXEL_UNPACK_BUFFER) as the buffer that Qt writes its generated texture into, and mark that buffer as GL_DYNAMIC_DRAW. You'll still need to call glTexImage*D() with the buffer offset of the PBO (i.e., probably zero) for each update, but that approach may afford some efficiency over just blasting texels to the pipe directly through glTexImage*D().
There is no such hint. OpenGL defines functionality, not performance. Just upload to it whenever you need to.
This is a question about syncronization in OpenGL. And the question is:
At which point in the following (pseudo) code sample does syncronization happen.
// 1.
try to map buffer object (write only and invalidate buffer)
copy new data to mapped buffer
unmap buffer
// 2.
bind buffer
call subteximage to fill texture from buffer
unbind buffer
// 3.
render with texture
As far as i know syncronization happens as soon as 'an object is used'. Now it's questionable if the texture is used if it is filled from the buffer or if it is used in rendering.
If glSubTexImage doesn't block it would be possible to generally stream texture data by using buffer updates in texture update calls.
Florian
Your code can block anywhere between copy and glFlush after render with texture (or frame buffer swap). It's up to the implementation.
glDrawPixels(GLsizei width, GLsizei height, GLenum format, GLenum type, const ovid *pixels);
Is there a function like this, except instead of accessing CPU memory, it accesses GPU memory? [Either a texture of a frame buffer object]
Let's cover all the bases here.
First, a direct answer: yes, there is such a function. It's called glDrawPixels. I'm not kidding.
glDrawPixels can certainly read from GPU memory, provided that you are using buffer objects as their source data (commonly called "pixel buffer objects"). glDrawPixels can use pixel buffer objects as the source for pixel data. Buffer objects are (theoretically, at least) in GPU memory, so they qualify.
However, you add onto this "Either a texture of a frame buffer object". Under this qualification, you're asking, "is there a way to copy pixel data from one texture/framebuffer to the current framebuffer?"
Yes. glBlitFramebuffer can do that. It blits from the GL_READ_FRAMEBUFFER to the GL_DRAW_FRAMEBUFFER. And since you can add images from textures to FBOs, you can copy from images just fine. You can even copy from the default framebuffer to some renderbuffer or texture.
You can also employ glCopyImageSubData, which copies pixel rectangles from one image to another. It's a lot more convenient than glBlitFramebuffer if all you're doing is copying pixel data. This is quite new at present (GL 4.3, or ARB_copy_image). It cannot be used to copy data to the default framebuffer.
If it is in a texture:
set up orthographic frustum
disable blending, depth test, etc.
bind texture
draw screen-aligned textured quad with correct texture coordinates
I use this in for example in Compositor::_drawPixels
glDrawPixels can read from a Buffer Object. Just do a
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, XXX)
before calling glDrawPixels.
Caveat: glDrawPixels is deprecated...
Use glBlitFramebuffer, which operates on frambuffer objects (Link). Ans this is not deprecated.
You can take advantage of format conversion, scaling and multisampling.
What is the difference between FBO and PBO? Which one should I use for off-screen rendering?
What is the difference between FBO and PBO?
A better question is how are they similar. The only thing that is similar about them is their names.
A Framebuffer Object (note the capitalization: framebuffer is one word, not two) is an object that contains multiple images which can be used as render targets.
A Pixel Buffer Object is:
A Buffer Object. FBOs are not buffer objects. Again: framebuffer is one word.
A buffer object that is used for asynchronous uploading/downloading of pixel data to/from images.
If you want to render to a texture or just a non-screen framebuffer, then you use FBOs. If you're trying to read pixel data back to your application asynchronously, or you're trying to transfer pixel data to OpenGL images asynchronously, then you use PBOs.
They're nothing alike.
A FBO (Framebuffer object) is a target where you can render images other than the default frame buffer or screen.
A PBO (Pixel Buffer Object) allows asynchronous transfers of pixel data to and from the device. This can be helpful to improve overall performance when rendering if you have other things that can be done while waiting for the pixel transfer.
I would read VBOs, PBOs and FBOs:
Apple has posted two very nice bits of
sample code demonstrating PBOs and
FBOs. Even though these are
Mac-specific, as sample code they're
good on any platoform because PBOs and
FBOs are OpenGL extensions, not
windowing system extensions.
So what are all these objects? Here's
the situation:
I want to highlight something.
FBO it not memory block. I think it look like struct of pointer. You Must attach Texture to FBO to use it. After attach Texture you now can draw to it for offscreen render or for second pass effect.
struct FBO{
AttachColor0 *ptr0;
AttachColor1 *ptr1;
AttachColor2 *ptr2;
AttachDepth *ptr3;
};
In the other hand, PBO is memory block "block to hold type of memory. "Try to think of it as malloc of x size, then you can use memcpy to copy data from it to texture/FBO or to it".
Why to use PBO?
Create intermediate memory buffer to interface with Host memory and not stop OpenGL drawing will upload texture to or from host.