cover.glsl

/**
 * @param {vec2} coord - base texture coord
 * @param {vec2} view - resolution of viewport
 * @param {vec2} resource - resolution of resource
 * @return {vec2} like a `background-size: cover` on css
 */
vec2 cover(vec2 coord, vec2 view, vec2 resource){
    vec2 ratio = vec2(
        min((view.x / view.y) / (resource.x / resource.y), 1.0),
        min((view.y / view.x) / (resource.y / resource.x), 1.0)
    );
    return vec2(
        coord.x * ratio.x + (1.0 - ratio.x) * 0.5,
        coord.y * ratio.y + (1.0 - ratio.y) * 0.5
    );
}