bsnes/Shaders/SmoothBilinear.fsh

vec4 scale(sampler2D image)
{
    vec2 texCoord = vec2(gl_FragCoord.x, uResolution.y - gl_FragCoord.y) / uResolution;

    vec2 pixel = texCoord * textureDimensions - vec2(0.5, 0.5);

    vec4 q11 = texture(image, (floor(pixel) + 0.5) / textureDimensions);
    vec4 q12 = texture(image, (vec2(floor(pixel.x), ceil(pixel.y)) + 0.5) / textureDimensions);
    vec4 q21 = texture(image, (vec2(ceil(pixel.x), floor(pixel.y)) + 0.5) / textureDimensions);
    vec4 q22 = texture(image, (ceil(pixel) + 0.5) / textureDimensions);

    vec2 s = smoothstep(0., 1., fract(pixel));

    vec4 r1 = mix(q11, q21, s.x);
    vec4 r2 = mix(q12, q22, s.x);

    return mix (r1, r2, s.y);
}