// ┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
// ┃ ██████ ██████ ██████       █      █      █      █      █ █▄  ▀███ █       ┃
// ┃ ▄▄▄▄▄█ █▄▄▄▄▄ ▄▄▄▄▄█  ▀▀▀▀▀█▀▀▀▀▀ █ ▀▀▀▀▀█ ████████▌▐███ ███▄  ▀█ █ ▀▀▀▀▀ ┃
// ┃ █▀▀▀▀▀ █▀▀▀▀▀ █▀██▀▀ ▄▄▄▄▄ █ ▄▄▄▄▄█ ▄▄▄▄▄█ ████████▌▐███ █████▄   █ ▄▄▄▄▄ ┃
// ┃ █      ██████ █  ▀█▄       █ ██████      █      ███▌▐███ ███████▄ █       ┃
// ┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┫
// ┃ Copyright (c) 2017, the Perspective Authors.                              ┃
// ┃ ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ ┃
// ┃ This file is part of the Perspective library, distributed under the terms ┃
// ┃ of the [Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0). ┃
// ┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛

// Unit-quad vertex shader for one map tile. The same static [0..1]
// corner buffer is bound for every tile in a frame; per-tile uniforms
// stretch the corner into Mercator space and pick the UV sub-rect.
// Sub-rect support is for the parent-tile fallback path (sample the
// loaded parent at the child's quadrant while the child fetches).
attribute vec2 a_corner;

uniform mat4 u_projection;
uniform vec2 u_extent_min;
uniform vec2 u_extent_max;
uniform vec2 u_uv_min;
uniform vec2 u_uv_max;

varying vec2 v_uv;

void main() {
    vec2 pos = mix(u_extent_min, u_extent_max, a_corner);
    // Tiles ship north-up; the WebGL Y axis points up at clip
    // coordinates so flip the V channel before sampling.
    vec2 uv = mix(u_uv_min, u_uv_max, vec2(a_corner.x, 1.0 - a_corner.y));
    v_uv = uv;
    gl_Position = u_projection * vec4(pos, 0.0, 1.0);
}
