stroke_eraser.f.glsl

// Copyright (c) 2015 Sergio Gonzalez. All rights reserved.
// License: https://github.com/serge-rgb/milton#license

in vec3 v_pointa;
in vec3 v_pointb;

uniform sampler2D u_canvas;

void
main()
{
    vec2 screen_point = vec2(gl_FragCoord.x, u_screen_size.y - gl_FragCoord.y);

    vec2 canvas_point = raster_to_canvas_gl(screen_point);
    vec2 a = v_pointa.xy;
    vec2 b = v_pointb.xy;

    vec2 ab = b - a;
    float len_ab = length(ab);

    float t = clamp(dot((canvas_point - a)/len_ab, ab / len_ab), 0.0, 1.0);

    vec2 stroke_point = mix(a, b, t);

    float pressure = mix(v_pointa.z, v_pointb.z, t);

    // Distance between fragment and stroke
    float dist = distance(stroke_point, canvas_point) - u_radius*pressure;

    if ( dist < 0 ) {
        vec2 coord = gl_FragCoord.xy / u_screen_size;
        vec4 eraser_color = texture(u_canvas, coord);
        out_color = eraser_color;
    } else {
        discard;
    }
}//END