Push Constants

Command buffers may update a very small data cache which shaders may read during execution using push constants.

The Vulkan minimum is 128 bytes, but many devices expose a larger limit. Check PhysicalDeviceLimits::max_push_constants_size and keep the payload small.

API docs: ComputeCommandRef::push_constants, GraphicCommandRef::push_constants, RayTraceCommandRef::push_constants.

// render_mesh.glsl
#version 460 core

layout(push_constant) uniform PushConstants {
    layout(offset = 0) uint mesh_index;
};

...

#![allow(unused)]
fn main() {
let info = ComputePipelineInfo::default();
let code = include_bytes!("render_mesh.spv");
let shader = Shader::new_compute(code.as_slice());
let pipeline = ComputePipeline::create(device, info, shader)?;

let mut graph = Graph::default();
let data = 42u32.to_ne_bytes();

graph
    .begin_cmd()
    .bind_pipeline(&pipeline)
    .record_cmd(move |cmd| {
        cmd
            .push_constants(0, &data)
            .dispatch(1, 1, 1);
    });
}

Tip

A crate such as bytemuck is helpful for converting Rust structures to bytes suitable for push constant usage. See the example code for more.