builtin-programs/draw/line.folk

Wish the GPU compiles pipeline "line" {
    {vec2 viewport mat3 surfaceToClip
     vec2 from vec2 to float thickness vec4 color} {
        vec2 dir = normalize(to - from);
        vec2 perp = vec2(-dir.y, dir.x) * thickness/2.0;

        vec2 vertices[6] = vec2[6](
            from + perp,
            from - perp,
            to - perp,

            from + perp,
            to - perp,
            to + perp
        );
        vec3 v = surfaceToClip * vec3(vertices[gl_VertexIndex], 1.0);
        return vec4(v.xy/v.z, 0.0, 1.0);
    } {
        vec2 clipXy = (gl_FragCoord.xy / viewport) * 2.0 - 1.0;
        vec3 surfaceXy = inverse(surfaceToClip) * vec3(clipXy, 1.0);
        surfaceXy /= surfaceXy.z;

        float l = length(to - from);
        vec2 d = (to - from) / l;
        vec2 q = (surfaceXy.xy - (from + to)*0.5);
        q = mat2(d.x, -d.y, d.y, d.x) * q;
        q = abs(q) - vec2(l, thickness)*0.5;
        float dist = length(max(q, 0.0)) + min(max(q.x, q.y), 0.0);

        return (dist < 0.0) ? color : vec4(0.0);
    }
}

When the color map is /colorMap/ &\
     /p/ has canvas /id/ with /...wiOptions/ &\
     /p/ has canvas projection /surfaceToClip/ &\
     /someone/ wishes to draw a line onto /p/ with /...options/ {

    set layer [dict getdef $options layer 0]
    set points [dict get $options points]
    set width [dict get $options width]
    set color [dict get $options color]
    set color [dict getdef $colorMap $color $color]

    set wiResolution [list [dict get $wiOptions width] [dict get $wiOptions height]]
    set instances [list]
    for {set i 0} {$i < [llength $points] - 1} {incr i} {
        set from [lindex $points $i]
        set to [lindex $points [+ $i 1]]
        lappend instances [list $wiResolution $surfaceToClip $from $to $width $color]
    }

    Wish the GPU draws pipeline "line" onto canvas $id \
        with instances $instances layer $layer
}