https://docs.godotengine.org/en/3.1/tutorials/viewports/custom_postprocessing.html (https://en.wikipedia.org/wiki/Sobel_operator)

Sobel_operator.shader

shader_type canvas_item;

void fragment() {
    vec3 col = -8.0 * texture(SCREEN_TEXTURE, SCREEN_UV).xyz;
    col += texture(SCREEN_TEXTURE, SCREEN_UV + vec2(0.0, SCREEN_PIXEL_SIZE.y)).xyz;
    col += texture(SCREEN_TEXTURE, SCREEN_UV + vec2(0.0, -SCREEN_PIXEL_SIZE.y)).xyz;
    col += texture(SCREEN_TEXTURE, SCREEN_UV + vec2(SCREEN_PIXEL_SIZE.x, 0.0)).xyz;
    col += texture(SCREEN_TEXTURE, SCREEN_UV + vec2(-SCREEN_PIXEL_SIZE.x, 0.0)).xyz;
    col += texture(SCREEN_TEXTURE, SCREEN_UV + SCREEN_PIXEL_SIZE.xy).xyz;
    col += texture(SCREEN_TEXTURE, SCREEN_UV - SCREEN_PIXEL_SIZE.xy).xyz;
    col += texture(SCREEN_TEXTURE, SCREEN_UV + vec2(-SCREEN_PIXEL_SIZE.x, SCREEN_PIXEL_SIZE.y)).xyz;
    col += texture(SCREEN_TEXTURE, SCREEN_UV + vec2(SCREEN_PIXEL_SIZE.x, -SCREEN_PIXEL_SIZE.y)).xyz;
    COLOR.xyz = col;
}