#version 450

// Fullscreen triangle. The scene image is drawn as a single textured quad; the
// push constant carries the destination rectangle (in window-normalized
// coordinates, y down), so the map is letterboxed rather than stretched.
layout(push_constant) uniform Push {
    vec4 rect; // xy = top-left (0..1), zw = size (0..1)
} pc;

layout(location = 0) out vec2 out_uv;

void main() {
    // Vulkan clip space has +Y pointing DOWN, so p already runs top->bottom and
    // matches the image's top-left origin; do NOT flip it (OpenGL would).
    vec2 p = vec2((gl_VertexIndex << 1) & 2, gl_VertexIndex & 2);
    out_uv = (p - pc.rect.xy) / pc.rect.zw;
    gl_Position = vec4(p * 2.0 - 1.0, 0.0, 1.0);
}