Comment by raphlinus
9 hours ago
The output of this renderer is a bitmap, so you have to do an upload to GPU if that's what your environment is. As part of the larger work, we also have Vello Hybrid which does the geometry on CPU but the pixel painting on GPU.
We have definitely thought about having the CPU renderer while the shaders are being compiled (shader compilation is a problem) but haven't implemented it.
In any interactive environment you have to upload to the GPU on each frame to output to a display, right? Or maybe integrated SoCs can skip that? Of course you only need to upload the dirty rects, but in the worst case the full image.
>geometry on CPU but the pixel painting on GPU
Wow. Is this akin to running just the vertex shader on the CPU?
It just depends on what architecture your computer has.
On a PC, the CPU typically has exclusive access to system RAM, while the GPU has its own dedicated VRAM. The graphics driver runs code on both the CPU and the GPU since the GPU has its own embedded processor so data is constantly being copied back and forth between the two memory pools.
Mobile platforms like the iPhone or macOS laptops are different: they use unified memory, meaning the CPU and GPU share the same physical RAM. That makes it possible to allocate a Metal surface that both can access, so the CPU can modify it and the GPU can display it directly.
However, you won’t get good frame rates on a MacBook if you try to draw a full-screen, pixel-perfect surface entirely on the CPU it just can’t push pixels that fast. But you can write a software renderer where the CPU updates pixels and the GPU displays them, without copying the surface around.
Surely not if the CPU and video output device share common RAM?
Or with old VGA, the display RAM was mapped to known system RAM addresses and the CPU would write directly to it. (you could write to an off-screen buffer and flip for double/triple buffering)
I regularly do remote VNC and X11 access on stuff like raspberry pi zero and in these cases GPU does not work, you won't be able to open a GL context at all. Also whenever i upadte my kernel on archlinux i'm not able to open a gl context until i reboot, so I really need apps that don't need a gpu context just to show stuff
For the Pi Zero you can force a headless HDMI output in the config and then use that instead of a virtual display to get working GPU with VNC.
It's analogous, but vertex shaders are just triangles, and in 2D graphics you have a lot of other stuff going on.
The actual process of fine rasterization happens in quads, so there's a simple vertex shader that runs on GPU, sampling from the geometry buffers that are produced on CPU and uploaded.