#version 450 core

// Fullscreen-triangle vertex shader for the backdrop downsample and H-blur sub-passes.
// Emits a single triangle covering NDC [-1,1]^2; the rasterizer clips edges outside.
// No vertex buffer; uses gl_VertexIndex to pick corners.
//
// The CPU sets the viewport (and matching scissor) per layer-bracket to limit work to
// the union AABB of the layer's backdrop primitives, expanded by 3*max_sigma and
// clamped to swapchain bounds. The fragment shader uses gl_FragCoord (absolute pixel
// space in the bound target) plus an inv-size uniform to compute its own UVs — see
// each fragment shader for the per-pass sampling math.

void main() {
    // gl_VertexIndex 0 -> ( -1, -1)
    // gl_VertexIndex 1 -> (  3, -1)
    // gl_VertexIndex 2 -> ( -1,  3)
    vec2 ndc = vec2(
            (gl_VertexIndex == 1) ? 3.0 : -1.0,
            (gl_VertexIndex == 2) ? 3.0 : -1.0);
    gl_Position = vec4(ndc, 0.0, 1.0);
}