obs-studio/libobs/data/bicubic_scale.effect

/*
 * bicubic sharper (better for downscaling)
 * note - this shader is adapted from the GPL bsnes shader, very good stuff
 * there.
 */

uniform float4x4 ViewProj;
uniform texture2d image;
uniform float2 base_dimension_i;
uniform float undistort_factor = 1.0;

sampler_state textureSampler {
	Filter    = Linear;
	AddressU  = Clamp;
	AddressV  = Clamp;
};

struct VertData {
	float4 pos : POSITION;
	float2 uv  : TEXCOORD0;
};

VertData VSDefault(VertData v_in)
{
	VertData vert_out;
	vert_out.pos = mul(float4(v_in.pos.xyz, 1.0), ViewProj);
	vert_out.uv  = v_in.uv;
	return vert_out;
}

float weight(float x)
{
	float ax = abs(x);

	/* Sharper version.  May look better in some cases. */
	const float B = 0.0;
	const float C = 0.75;

	if (ax < 1.0)
		return (pow(x, 2.0) *
			((12.0 - 9.0 * B - 6.0 * C) * ax +
				(-18.0 + 12.0 * B + 6.0 * C)) +
				(6.0 - 2.0 * B))
			/ 6.0;
	else if ((ax >= 1.0) && (ax < 2.0))
		return (pow(x, 2.0) *
			((-B - 6.0 * C) * ax + (6.0 * B + 30.0 * C)) +
				(-12.0 * B - 48.0 * C) * ax +
				(8.0 * B + 24.0 * C))
			/ 6.0;
	else
		return 0.0;
}

float4 weight4(float x)
{
	return float4(
		weight(x - 2.0),
		weight(x - 1.0),
		weight(x),
		weight(x + 1.0));
}

float AspectUndistortX(float x, float a)
{
	// The higher the power, the longer the linear part will be.
	return (1.0 - a) * (x * x * x * x * x) + a * x;
}

float AspectUndistortU(float u)
{
	// Normalize texture coord to -1.0 to 1.0 range, and back.
	return AspectUndistortX((u - 0.5) * 2.0, undistort_factor) * 0.5 + 0.5;
}

float2 pixel_coord(float xpos, float ypos)
{
	return float2(AspectUndistortU(xpos), ypos);
}

float4 pixel(float xpos, float ypos, bool undistort)
{
	if (undistort)
		return image.Sample(textureSampler, pixel_coord(xpos, ypos));
	else
		return image.Sample(textureSampler, float2(xpos, ypos));
}

float4 get_line(float ypos, float4 xpos, float4 linetaps, bool undistort)
{
	return
		pixel(xpos.r, ypos, undistort) * linetaps.r +
		pixel(xpos.g, ypos, undistort) * linetaps.g +
		pixel(xpos.b, ypos, undistort) * linetaps.b +
		pixel(xpos.a, ypos, undistort) * linetaps.a;
}

float4 DrawBicubic(VertData v_in, bool undistort)
{
	float2 stepxy = base_dimension_i;
	float2 pos = v_in.uv + stepxy * 0.5;
	float2 f = frac(pos / stepxy);

	float4 rowtaps = weight4(1.0 - f.x);
	float4 coltaps = weight4(1.0 - f.y);

	/* make sure all taps added together is exactly 1.0, otherwise some
 	 * (very small) distortion can occur */
	rowtaps /= rowtaps.r + rowtaps.g + rowtaps.b + rowtaps.a;
	coltaps /= coltaps.r + coltaps.g + coltaps.b + coltaps.a;

	float2 xystart = (-1.5 - f) * stepxy + pos;
	float4 xpos = float4(
		xystart.x,
		xystart.x + stepxy.x,
		xystart.x + stepxy.x * 2.0,
		xystart.x + stepxy.x * 3.0
	);

	return
		get_line(xystart.y                 , xpos, rowtaps, undistort) * coltaps.r +
		get_line(xystart.y + stepxy.y      , xpos, rowtaps, undistort) * coltaps.g +
		get_line(xystart.y + stepxy.y * 2.0, xpos, rowtaps, undistort) * coltaps.b +
		get_line(xystart.y + stepxy.y * 3.0, xpos, rowtaps, undistort) * coltaps.a;
}

float4 PSDrawBicubicRGBA(VertData v_in, bool undistort) : TARGET
{
	return DrawBicubic(v_in, undistort);
}

float4 PSDrawBicubicRGBADivide(VertData v_in) : TARGET
{
	float4 rgba = DrawBicubic(v_in, false);
	float alpha = rgba.a;
	float multiplier = (alpha > 0.0) ? (1.0 / alpha) : 0.0;
	return float4(rgba.rgb * multiplier, alpha);
}

technique Draw
{
	pass
	{
		vertex_shader = VSDefault(v_in);
		pixel_shader  = PSDrawBicubicRGBA(v_in, false);
	}
}

technique DrawAlphaDivide
{
	pass
	{
		vertex_shader = VSDefault(v_in);
		pixel_shader  = PSDrawBicubicRGBADivide(v_in);
	}
}

technique DrawUndistort
{
	pass
	{
		vertex_shader = VSDefault(v_in);
		pixel_shader  = PSDrawBicubicRGBA(v_in, true);
	}
}