#version 450

// One instance per Surface image attachment. The CPU has already converted
// the pane's cell rectangle and the aspect-preserving contain fit to NDC.
layout(location = 0) in vec4 a_rect;    // x0, y0, x1, y1
layout(location = 1) in vec4 a_uv_rect; // u0, v0, u1, v1

layout(location = 0) out vec2 v_uv;

void main() {
    vec2 pos;
    int corner = gl_VertexIndex % 6;
    if (corner == 0) {
        pos = vec2(a_rect.x, a_rect.y);
        v_uv = vec2(a_uv_rect.x, a_uv_rect.y);
    } else if (corner == 1) {
        pos = vec2(a_rect.z, a_rect.y);
        v_uv = vec2(a_uv_rect.z, a_uv_rect.y);
    } else if (corner == 2) {
        pos = vec2(a_rect.z, a_rect.w);
        v_uv = vec2(a_uv_rect.z, a_uv_rect.w);
    } else if (corner == 3) {
        pos = vec2(a_rect.x, a_rect.y);
        v_uv = vec2(a_uv_rect.x, a_uv_rect.y);
    } else if (corner == 4) {
        pos = vec2(a_rect.z, a_rect.w);
        v_uv = vec2(a_uv_rect.z, a_uv_rect.w);
    } else {
        pos = vec2(a_rect.x, a_rect.w);
        v_uv = vec2(a_uv_rect.x, a_uv_rect.w);
    }
    gl_Position = vec4(pos, 0.0, 1.0);
}