OpenGL: Render Framebuffer into Framebuffer

Question

I am currently working on a c++ program using OpenXR with OpenGL. I managed to render everything that I wanted into VR.

I have one framebuffer per eye, and then 3 more for layers (crosshair/menu) that are displayed with different button presses (which is done in VR with xrEndFrame).

The problem is that I want to mirror everything also onto the computer screen.

Most of it was simple because I just used glBlitNamedFramebuffer to mirror one of the eye framebuffers onto the 0/default framebuffer so it's displayed.

But I can't use this method for the layers, as it is just in 2D on the screen, and I need to position the layer for example to where I am looking in VR for the crosshair.

I can draw a quad to the position i want it to be, but i don't know how i can copy the content of the framebuffer onto the quad. I thought maybe glReadPixels might help, but that seemed rather inefficient.

I don't understand the statement "But i can't use this method for the layers, as it is just in 2d" Framebuffers are also just 2d.. — Botje
– Botje, Commented Jun 21, 2024 at 8:43
Sorry, I meant that i have to position the content of the framebuffer into the 3D world. so that i have a 2d layer visible, but for example it has to be where i am looking in VR, not just in the middle of the screen. — dings
– dings, Commented Jun 21, 2024 at 8:50
Can you make your "layer" framebuffers be backed by a texture instead? Then you can just render that to a quad. — Botje
– Botje, Commented Jun 21, 2024 at 8:58
I might have a texture. I have a XrSwapchainImageOpenGLKHR, where "image is the OpenGL texture handle associated with this swapchain image" — dings
– dings, Commented Jun 21, 2024 at 9:44
Thanks, I managed to get the texure (it looked wrong at first), and rendered them onto quads. It looks a bit different then before, but i can work with that. — dings
– dings, Commented Jun 28, 2024 at 9:23

dings · Accepted Answer · 2024-06-28 10:23:04Z

0

So I wasn't able to render a framebuffer into another framebuffer, but at least for the OpenXR case, there is a solution.

I created the layers as usual, but instead of giving the layers to OpenXr via xrEndFrame, I used the color texture from the swapchain

colorTexture = reinterpret_cast<const XrSwapchainImageOpenGLKHR*>(hudLayer.m_swapchainImages[hudLayer.swapchain_][swapchainImageIndex])->image;

and rendered them onto quads within the first framebuffer. (I could use the positions/orientations of the layers for the quads as well)

answered Jun 28, 2024 at 10:23

dings

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