#version 450

// Per instance; layout must match SpriteRenderSystem::Instance.
layout (location = 0) in mat4 model;  // Maps the unit quad to world space.
layout (location = 4) in vec4 color;
layout (location = 5) in vec4 uvRect;

layout (location = 0) out vec4 fragColor;
layout (location = 1) out vec2 fragUv;

layout (push_constant) uniform Push {
  mat4 viewProjection;
} push;

// Two triangles covering the unit quad (0,0)-(1,1).
const vec2 CORNERS[6] = vec2[](
  vec2(0.0, 0.0), vec2(1.0, 0.0), vec2(1.0, 1.0),
  vec2(1.0, 1.0), vec2(0.0, 1.0), vec2(0.0, 0.0)
);

void main() {
  vec2 corner = CORNERS[gl_VertexIndex];

  gl_Position = push.viewProjection * model * vec4(corner, 0.0, 1.0);
  // Draw order comes from the CPU-side sort, so flatten depth; sprites should
  // never be clipped by the camera's near/far planes.
  gl_Position.z = 0.0;

  fragColor = color;
  fragUv = mix(uvRect.xy, uvRect.zw, corner);
}