#version 450

// Terminal cell renderer. The CPU uploads one instance per visible cell; this
// shader expands each instance into a two-triangle quad with gl_VertexIndex.
// The fragment shader mixes fg/bg by the atlas's alpha (a glyph mask in the R
// channel), so background fills and glyph foregrounds draw in a single pass.

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 = 2) in vec3 a_fg;
layout(location = 3) in vec3 a_bg;

layout(location = 0) out vec2 v_uv;
layout(location = 1) out vec3 v_fg;
layout(location = 2) out vec3 v_bg;

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