Simulate background-size contain in fragment shader

uvContain.glsl

vec2 uvContain(vec2 baseUv, vec2 size, vec2 resolution) {
    float tAspect = size.x / size.y;
    float pAspect = resolution.x / resolution.y;
    float pWidth = resolution.x;
    float pHeight = resolution.y;

    float width = 0.;
    float height = 0.;

    if ( tAspect < pAspect ) {
        height = pHeight;
        width = height * tAspect;
    } else {
        width = pWidth;
        height = width / tAspect;
    }

    float x = ( pWidth - width ) / 2.;
    float y = ( pHeight - height ) / 2.;

    vec2 uv = baseUv;
    uv -= vec2(x, y) / resolution;
    uv /= vec2(width, height) / resolution;

    return uv;
}