I'm trying to retrieve a Pixel-value from a Renderbuffer attached to an unbound Framebuffer.
Since I'm trying to avoid changing global state when reading said value I'm unable to use glReadPixels([...]) which would normally be used to read Pixels from the currently bound Framebuffer.
DirectStateAccess methods usually solve this problem but I'm unable to locate a method that allows for directly specifying the source of the read operation.
Is it possible to read the pixel in a different way, without having to bind the associated Framebuffer? Why did this method not get included in the DSA extension?
If the framebuffer attachment is a Texture and not a Rrenderbuffer, you can read the pixel data from the texture object instead of the framebuffer with glGetTextureImage.
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In my application I have a module that manages rendering using OpenGL, and renders the results into QOpenGLFrameBufferObjects. I know this much works, because I'm able to save their contents using QOpenGLFrameBufferObject::toImage() and see the render.
I am now attempting to take the texture ID returned from QOpenGLFrameBufferObject::texture(), bind it as a normal OpenGL texture and render it into a QOpenGLWidget viewport using a fullscreen quad. (I'm using this two-step method because I'm not aware of a way to get around the fact that QOpenGLWidgets each work in their own context, but that's a different story.)
The problem here is that glBindTexture() returns InvalidOperation when I call it. According to the OpenGL documentation, this is because "[The] texture was previously created with a target that doesn't match that of [the input]." However, I created the frame buffer object by passing GL_TEXTURE_2D into the constructor, and am passing the same in as the target to glBindTexture(), so I'm not sure where I'm going wrong. There isn't much documentation online about how to correctly use QOpenGLFrameBufferObject::texture().
Other supplementary information, in case it helps:
The creation of the frame buffer object doesn't set any special formats. They're left at whatever defaults Qt uses. As far as I know, this means it also has no depth or stencil attachments as of yet, though once I've got the basics working this will probably change.
Binding the FBO before binding its texture doesn't seem to make a difference.
QOpenGLFrameBufferObject::isValid() returns true;
Calling glIsTexture() on the texture handle returns false, but I'm not sure why this would be given that it's a value provided to me by Qt for the purposes of binding an OpenGL texture. The OpenGL documentation does mention that "a name returned by glGenTextures, but not yet associated with a texture by calling glBindTexture, is not the name of a texture", but here I can't bind it anyway.
I'm attempting to bind the texture in a different context to the one the FBO was created in (ie. the QOpenGLWidget's context instead of the render module's context).
I'll provide some code, but a lot of what I have is specific to the systems that exist in the rendering module, so there's only a small amount of relevant OpenGL code.
In the render module context:
QOpenGLFrameBufferObject fbo = new QOpenGLFrameBufferObject(QSize(...), GL_TEXTURE_2D);
// Do some rendering in this context later
In the QOpenGLWidget context, after having rendered to the frame buffer in the rendering module:
GLuint textureId = fbo->texture();
glBindTexture(GL_TEXTURE_2D, textureId)) // Invalid operation
EDIT: It turns out the culprit was that my contexts weren't actually being shared, as I'd misinterpreted what the Qt::AA_ShareOpenGLContexts application attribute did. Once I made them properly shared the issue was fixed.
I want to read the depth component of a scene rendered to a framebuffer object. I initially used glReadPixels() but found that it could only read pixels from the default framebuffer.
The answers to some relevant questions on this website suggest using PBO, but I haven't tried it yet. It seems that the PBO reading is asynchronous, therefore, using which command can synchronize the reading at the end?
A PBO won't help you here, because those are just a different kind of buffer to read into (instead of memory on the host into memory of the OpenGL implementation).
The usual way to go about making a depth component back-readable in OpenGL is to use a depth texture, attached to the depth attachment and after rendering using glGetTexImage to retrieve the date.
In the case of a normal color attachment you could use glReadPixels with a previous call to glReadBuffer to select the GL_COLOR_ATTACHMENT<i> of the bound FBO.
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.
I'm writing data into a 3D texture from within a fragment shader, and I need to asynchronously read back said data into system memory. The only means of asynchronously initiating the packing operation into the buffer object seems to be calling glReadPixels() with a NULL pointer. But this function insists on getting passed a rectangle defining the region to read back. Now I don't know if these parameters are ignored when using PBOs, but I assume not. In this case, I have no idea what to pass to this function in order to obtain the whole 3D texture.
Even if have to read back individual slices (which would be kind of stupid IMO), I still have no idea how to communicate to OpenGL which slice to read from. Am I missing something?
BTW, I could use individual 2D textures for every slice, but that would screw up (3D-)mipmapping if I'm not mistaken. I wanted to use the 3D mipmaps in order to efficiently find regions of interest in the resulting 3D texture.
P.S. Sorry for the sub-optimal tags, apparently no one ever asked about 3d textures before and since I'm not allowed to create new tags...
Who says that glReadPixels is the only way to read image data? Maybe in OpenGL ES it is, but if you're using ES, you should say so. The rest of this answer will be assuming you're talking about desktop GL.
If you have a texture, and you want to read its contents, you should use glGetTexImage. The switch that controls whether it reads into a buffer object or not is the same switch that controls it for glReadPixels: whether a buffer is bound to GL_PIXEL_PACK_BUFFER.
Note that glGetTexImage will retrieve the entire texture (for a given mipmap level).
I'm considering refactoring a large part of my rendering code and one question popped to mind:
Is it possible to render to both the screen and to a texture using multiple color attachments in a Frame Buffer Object? I cannot find any information if this should be possible or not even though it has many useful applications. I guess it should be enough to bind my texture as color attachment0 and renderbuffer 0 to attachment1?
For example I want to make an interactive application where you can "draw" on a 3D model. I resolve where the user draws by rendering the UV-coordinates to a texture so I can look up at the mouse-coordinates where to modify the texture. In my case it would be fastest to have a shader that both draws the UV's to the texture and the actual texture to the screen in one pass.
Are there better ways to do this or am I on the right track?
There is no such thing as "default renderbuffer" in OpenGL. There is the window system provided default frame buffer with reserved name zero, but that basically means "no FBO enabled". So no, unfortunately normal OpenGL provides no method to somehow use its color buffer as a color attachment to any other FBO. I'm not aware of any extensions that could possible provide this feature.
With render buffers there is also the reserved name zero, but it's only a special "none" variable and allows unbinding render buffers.