glBlitFramebuffer() from (or to) specific texture layer

Is it still true that the only way to blit from (or to) a specific layer in a layered texture is to bind that specific layer to the framebuffer and then call blit?

An extension of glBlitFramebuffer which took a source/dest layer index would be useful here.

Use case: Say you have a layered texture of say 2 or 6 layers. Each frame you render those layers with layered rendering and resolve them into another texture or the system framebuffer. So you have an FBO where entire textures (color, depth, and possibly stencil) are bound as FBO attachments for drawing. Then you want to resolve/blit each face into other textures or the system framebuffer. What do you do? Reconfigure the FBO 2-6 times before each Blit, and then put it back the way it was afterwards? FBO reconfigs are expensive. Or do you have 2 or 6 FBOs (one per layer) which you pre-configure, and then cycle through binding each “layer FBO” calling Blit for each? Neither of these seem like great options.

Yes, I realize I could just chuck glBlitFramebuffer and render quads with the layered texture bound to a sampler as input to resolve/blit, iteratively assigning different layer indices to each quad, but this seems like overkill.

Any simpler methods I’m missing? I’m open to any GL extensions as answers too.

For what it’s worth, Vulkan’s blit and resolve functions work with 3D coordinates.

The OpenGL issue here is likely an outgrowth of the fact that glBlitFramebuffer was written before layered framebuffers existed. The effective solution would probably be to use multiple FBOs, to avoid the overhead of re-validating the framebuffers.

I’m not sure if it helps in your use-case, but I’m using glCopyImageSubData() to copy a cube map into an cube map array.

		cubeMapFBO->cubeMapColorTexture, GL_TEXTURE_CUBE_MAP
		, 0, 0, 0, 0
		, cubeMapArrayTexture, GL_TEXTURE_CUBE_MAP_ARRAY
		, 0, 0, 0, index * 6
		, cubeMapFBO->framebufferWidth, cubeMapFBO->framebufferWidth, 6);