Capture off screen frame buffer?

[ryantrem]

Bgfx enables frame capture by setting the BGFX_RESET_CAPTURE reset flag and handling the various capture callbacks. This captures the default (on screen) frame buffer, but I'd like to capture an off screen frame buffer instead. Since capture doesn't really have any parameters (you just set a reset flag), I don't see any way to capture an off screen frame buffer. Is this possible?

[bkaradzic]

It's not possible.

[pezcode]

You can use getTexture and readTexture to read offscreen framebuffer attachments.

[pezcode]

Important to note: blit happens before all rendering in a view, so you'll have to call it in a second view after you've done all your rendering.

[ryantrem]

Thanks for the info. Is this expected to work on Android? I tried it but ran into a few problems:

1. m_readBackSupported (in renderer_gl.cpp) evaluates to false (I've set BGFX_CONFIG_RENDERER_OPENGLES rather than BGFX_CONFIG_RENDERER_OPENGL).
2. I tried to forcing m_readBackSupported to true in the debugger just for testing purposes. But then later glGetTexImage fails with a seg fault.

Any ideas where I might be going wrong?

[pezcode]

GLES doesn't have glGetTexImage. You can define BGFX_GL_CONFIG_TEXTURE_READ_BACK_EMULATION which then uses glReadPixels. Performance with that may or may not be terrible. Alternatively, try the Vulkan renderer, if that's an option for you.

[ryantrem]

Thanks, that seems functional, and performance is not terrible in my simple test. I tried running the same code on iOS though and it dies with a SIGABRT in the call to copyFromTexture in renderer_mtl.h. I will debug a bit more and see if I can understand what might be going on, but if you have ideas I'd love to hear them!

[ryantrem]

The reason it wasn't working on iOS is that when setting up my frame buffer, I created the color attachment texture as RGBA8, which succeeds and BGFX thinks the underlying texture is RBGA8, but the actual underlying metal texture is BGRA8, which causes the blit to fail. This seems like a BGFX bug, but if I change the texture format for iOS to BGRA8 then it works.

So in summary, the (seemingly) workable solution is to create an extra texture with the BGFX_TEXTURE_BLIT_DST | BGFX_TEXTURE_READ_BACK flags, then use bgfx::blit to copy the render texture to that temporary texture, then use bgfx::readTexture to read the temporary texture.

One thing that is a little challenging about this is when you get the texture handle from the frame buffer via bgfx::getTexture, you don't get enough info to know how to create the temporary texture or the buffer passed to bgfx::readTexture - you need to track all that texture info yourself. I will start a different discussion to propose a solution to make this easier.