How to handle a dynamic number of draw calls in vulkan

Question

I'm currently trying to get into Vulkan, and I've mostly followed this well-know Vulkan tutorial, all the while trying to integrate it into a framework I built around OpenGL. I'm at the point where I can successfully render an object on the screen, and have the object move around by passing a transformation matrix to a uniform buffer linked to my shader code.

In this tutorial the author is focusing on drawing one object to the screen, which is a good starting point, but I would like to have end code that would look like this:

drawRect(position1, size1, color1);
drawRect(position2, size2, color2);
...

My first try to implement something like this ended up with me submitting the command buffer, which is created an recorded only once at the beginning, once for each object I wanted to render, and making sure to update the uniform data in-between each command buffer submission. This didn't work however, and after some debugging with renderdoc, I realized it was because starting a render pass clears the screen.

If I understand my situation correctly, the only way to achieve what I want would involve re-creating the command buffers every frame:

• Record n, the number of time we want to draw something on the screen;
• At the end of a frame, allocate n uniform buffers, and fill them with the corresponding data;
• Create n descriptor sets to be able to link these uniform buffers with my shader;
• Record the command buffer by repeating n times the process of binding a descriptor set using vkCmdBindDescriptorSets and drawing the requested data using vkCmdDrawIndexed.

This seems like a lot of work to do every frame. Is this how I should handle a dynamic number of draw calls ? Or is there some concept I don't know about/got wrong ?

Answer 1

1. Generally command buffers are actually re-recorded every frame, and Vulkan allows to multithread recording with command pools.
2. Indirect draws exist: you store data about draw commands (indeces count, instances count, etc.) into a separate buffer, and then the driver reads the data from the buffer when you submit the commands; vkCmdDraw*Indirect requires you to specify number of draw commands at recording time; vkCmdDraw*IndirectCount allows you to store number of draw commands in a buffer as well.

Also i dont see a reason why would you have to re-create uniform buffers, descriptor sets each frame; In fact, as far as I know, Vulkan encourages you to pre-bake things that you can, and descriptor sets are a tool for that.