Command Buffer Synchronization in Vulkan
Entry posted by Josh ·
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The Vulkan graphics API is unbelievably complex. To create a render context, you must create a series of images for the front and back buffers (you can create three for triple-buffering). This is called a swap chain. Now, Vulkan operates on the principle of command buffers, which are a list of commands that get sent to the GPU. Guess what? The target image is part of the command buffer! So for each image in your swap chain, you need to maintain a separate command buffer If anything changes in your program like the camera clearscreen color, you have to recreate the command buffers...all of them! But some of them will still be in use at the time your frame begins, so you need to store a flag that says "recreate this command buffer when it is time to start rendering with this image / command buffer".
The whole thing is really bad, but admitting that there is any practical limit to the how complex an APi should be opens a developer to ridicule. I make complex technologies easy to use for a living, so I'm just calling it out, this is garbage design. Vulkan is actually good for me because it means fewer people can figure out how to make a game engine, but it's really ridiculous. Khronos has stated that they expect semi-standard open-source code to arise to address these complexities, but I don't see that happening. I'm not going to touch something like AMD's V-EZ because it's just another layer of code that might stop being supported at any time. As a result of the terrible design, Vulkan is going to continue to struggle to gain adoption, and we are now entering an era where the platform holders are in a fight with the application developers about who is responsible for writing graphics drivers.
I really like some aspects of Vulkan. SPIR-V shaders are great, and I am very glad to be rid of OpenGL's implicit global states, FBO creation, strange resource sharing, and so on. But nobody needs detailed access to the swap chain. Nobody needs to manage their own synchronization. That's what we have graphics drivers for.
Anyways, here is my test application. The screen will change color when you press the space key, which involves re-creation of the command buffers. The Vulkan stuff is 1300 lines of code.
The good thing is that although the initial setup is prohibitive, this stuff tends to get compartmentalized away as I add more capabilities, so it gets easier as time goes on. This is very difficult stuff but we will be better off once I get through this.
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