obs-studio/libobs/data/lanczos_scale.effect

/*
 * lanczos sharper
 * 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
{
	AddressU  = Clamp;
	AddressV  = Clamp;
	Filter    = Linear;
};

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

struct FragData {
	float4 pos : POSITION;
	float2 uv  : TEXCOORD0;
	float2 scale  : TEXCOORD1;
};

FragData VSDefault(VertData v_in)
{
	FragData vert_out;
	vert_out.pos = mul(float4(v_in.pos.xyz, 1.0), ViewProj);
	vert_out.uv  = v_in.uv;
	vert_out.scale = min(0.25 + abs(0.75 / mul(float4(1.0 / base_dimension_i.xy, 1.0, 1.0), ViewProj).xy), 1.0);

	return vert_out;
}

float sinc(float x)
{
	const float PIval = 3.1415926535897932384626433832795;
	return sin(x * PIval) / (x * PIval);
}

float weight(float x, float radius)
{
	float ax = abs(x);
	if (x == 0.0)
		return 1.0;
	else if (ax < radius)
		return sinc(x) * sinc(x / radius);
	else
		return 0.0;
}

float3 weight3(float x, float scale)
{
	return float3(
		weight((x * 2.0 + 0.0 * 2.0 - 3.0) * scale, 3.0),
		weight((x * 2.0 + 1.0 * 2.0 - 3.0) * scale, 3.0),
		weight((x * 2.0 + 2.0 * 2.0 - 3.0) * scale, 3.0));
}

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, float3 xpos1, float3 xpos2, float3 rowtap1,
		float3 rowtap2, bool undistort)
{
	return
		pixel(xpos1.r, ypos, undistort) * rowtap1.r +
		pixel(xpos1.g, ypos, undistort) * rowtap2.r +
		pixel(xpos1.b, ypos, undistort) * rowtap1.g +
		pixel(xpos2.r, ypos, undistort) * rowtap2.g +
		pixel(xpos2.g, ypos, undistort) * rowtap1.b +
		pixel(xpos2.b, ypos, undistort) * rowtap2.b;
}

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

	float3 rowtap1 = weight3((1.0 - f.x) / 2.0,       v_in.scale.x);
	float3 rowtap2 = weight3((1.0 - f.x) / 2.0 + 0.5, v_in.scale.x);
	float3 coltap1 = weight3((1.0 - f.y) / 2.0,       v_in.scale.y);
	float3 coltap2 = weight3((1.0 - f.y) / 2.0 + 0.5, v_in.scale.y);

	/* make sure all taps added together is exactly 1.0, otherwise some
         * (very small) distortion can occur */
	float suml = rowtap1.r + rowtap1.g + rowtap1.b + rowtap2.r + rowtap2.g + rowtap2.b;
	float sumc = coltap1.r + coltap1.g + coltap1.b + coltap2.r + coltap2.g + coltap2.b;
	rowtap1 /= suml;
	rowtap2 /= suml;
	coltap1 /= sumc;
	coltap2 /= sumc;

	float2 xystart = (-2.5 - f) * stepxy + pos;
	float3 xpos1 = float3(xystart.x                 , xystart.x + stepxy.x      , xystart.x + stepxy.x * 2.0);
	float3 xpos2 = float3(xystart.x + stepxy.x * 3.0, xystart.x + stepxy.x * 4.0, xystart.x + stepxy.x * 5.0);

	return
		get_line(xystart.y                 , xpos1, xpos2, rowtap1, rowtap2, undistort) * coltap1.r +
		get_line(xystart.y + stepxy.y      , xpos1, xpos2, rowtap1, rowtap2, undistort) * coltap2.r +
		get_line(xystart.y + stepxy.y * 2.0, xpos1, xpos2, rowtap1, rowtap2, undistort) * coltap1.g +
		get_line(xystart.y + stepxy.y * 3.0, xpos1, xpos2, rowtap1, rowtap2, undistort) * coltap2.g +
		get_line(xystart.y + stepxy.y * 4.0, xpos1, xpos2, rowtap1, rowtap2, undistort) * coltap1.b +
		get_line(xystart.y + stepxy.y * 5.0, xpos1, xpos2, rowtap1, rowtap2, undistort) * coltap2.b;
}

float4 PSDrawLanczosRGBA(FragData v_in, bool undistort) : TARGET
{
	return DrawLanczos(v_in, undistort);
}

float4 PSDrawLanczosRGBADivide(FragData v_in) : TARGET
{
	float4 rgba = DrawLanczos(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  = PSDrawLanczosRGBA(v_in, false);
	}
}

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

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