Do it from ground up like normal

by copying a tutorial/demo

shaders/decay_cs.glsl

@@ -0,0 +1,46 @@
+#[compute]
+#version 450
+
+// Invocations in the (x, y, z) dimension
+layout(local_size_x = 8, local_size_y = 8, local_size_z = 1) in;
+
+// Our textures.
+layout(r32f, set = 0, binding = 0) uniform restrict readonly image2D current_image;
+layout(r32f, set = 1, binding = 0) uniform restrict readonly image2D previous_image;
+layout(r32f, set = 2, binding = 0) uniform restrict writeonly image2D output_image;
+
+// PushConstants
+layout(push_constant, std430) uniform Params {
+    vec2 texture_size;
+    float decay_factor;
+    float dunno_lol;
+} params;
+
+
+// The code we want to execute in each invocation
+void main() {
+
+    ivec2 size = ivec2(params.texture_size.x - 1, params.texture_size.y - 1);
+    ivec2 uv = ivec2(gl_GlobalInvocationID.xy);
+
+    // Just in case the texture size is not divisable by 8.
+    if ((uv.x > size.x) || (uv.y > size.y)) {
+        return;
+    }
+
+    ivec2 tl = ivec2(0, 0);
+    float current_v = imageLoad(current_image, uv).r;
+    float up_v = imageLoad(current_image, clamp(uv - ivec2(0, 1), tl, size)).r;
+    float down_v = imageLoad(current_image, clamp(uv + ivec2(0, 1), tl, size)).r;
+    float left_v = imageLoad(current_image, clamp(uv - ivec2(1, 0), tl, size)).r;
+    float right_v = imageLoad(current_image, clamp(uv + ivec2(1, 0), tl, size)).r;
+
+    float new_v = current_v * params.decay_factor;
+
+    if (new_v < 0.0) {
+        new_v = 0.0;
+    }
+    vec4 result = vec4(new_v, new_v, new_v, 1.0);
+
+    imageStore(output_image, uv, result);
+}