BLT-With-Graphics-Template/include/blt/gfx/renderer/shaders/pp_outline_step.frag

#ifdef __cplusplus
#include <string>
const std::string shader_pp_outline_step_frag = R"("
#version 300 es
precision mediump float;

out vec4 FragColor;
in vec2 uv;
in vec2 pos;

uniform vec2 viewportSize;

uniform sampler2D albedo;
uniform sampler2D mask;

vec4 outline_color = vec4(1.0f, 0.0f, 0.0f, 1.0f);

float longest(vec3 v){
    return max(v.r, max(v.b, v.g));
}

#define LINE_WEIGHT 3.0

void main() {
    float dx = (1.0 / viewportSize.x) * LINE_WEIGHT;
    float dy = (1.0 / viewportSize.y) * LINE_WEIGHT;

    vec2 uvCenter   = uv;
    vec2 uvRight    = vec2(uvCenter.x + dx, uvCenter.y);
    vec2 uvTop      = vec2(uvCenter.x,      uvCenter.y - dx);
    vec2 uvTopRight = vec2(uvCenter.x + dx, uvCenter.y - dx);

    float mCenter   = longest(texture(mask, uvCenter).rgb);
    float mTop      = longest(texture(mask, uvTop).rgb);
    float mRight    = longest(texture(mask, uvRight).rgb);
    float mTopRight = longest(texture(mask, uvTopRight).rgb);

    float dT  = abs(mCenter - mTop);
    float dR  = abs(mCenter - mRight);
    float dTR = abs(mCenter - mTopRight);

    float delta = 0.0f;
    delta = max(delta, dT);
    delta = max(delta, dR);
    delta = max(delta, dTR);

    vec4 outline = vec4(delta, delta, delta, 1.0);
    vec4 albedo  = texture(albedo, uv);

    if (delta <= 0.001)
        FragColor = albedo;
    else
        FragColor = outline_color;
}

")";
#endif