/usr/share/sludge/scale.frag is in sludge-engine 2.2.1-2build2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57  | /*
   Inspired by a shader by guest(r) - guest.r@gmail.com - that was found at
   http://www.razyboard.com/system/morethread-smart-texture-mag-filter-for-ogl2-and-dosbox-pete_bernert-266904-5689051-0.html
*/
uniform sampler2D Texture;
uniform sampler2D lightTexture;
uniform bool antialias;
uniform bool useLightTexture;
uniform float scale;
varying vec2 varCoord0;
varying vec2 varCoord1;
varying vec4 color;
varying vec4 secondaryColor;
void main()
{
	vec4 c11 = texture2D(Texture, varCoord0.xy);
	if (antialias) {
		vec2 fw = fwidth(varCoord0.xy)*scale;
		vec2 sd1 = vec2( fw.x,fw.y);
		vec2 sd2 = vec2(-fw.x,fw.y);
		vec4 s00 = texture2D(Texture, varCoord0.xy-sd1);
		vec4 s20 = texture2D(Texture, varCoord0.xy-sd2);
		vec4 s22 = texture2D(Texture, varCoord0.xy+sd1);
		vec4 s02 = texture2D(Texture, varCoord0.xy+sd2);   
		vec4 dt = vec4(1.0,1.0,1.0,1.0);
		
		float m1=dot(abs(s00-s22),dt)+0.0001;
		float m2=dot(abs(s02-s20),dt)+0.0001;
		vec4 temp1 = m2*(s00 + s22) + m1*(s02 + s20);
	//	gl_FragColor = (temp1/(m1+m2)) * 0.5;
		gl_FragColor = c11*0.333333 + (temp1/(m1+m2)) * 0.333333;
	} else {
		gl_FragColor = c11;
	}
	if (gl_FragColor.a<0.001) discard;
	
	vec3 col;
	if (useLightTexture) {
		vec4 texture1 = texture2D (lightTexture, varCoord1.xy);
		col = texture1.rgb * gl_FragColor.rgb;
	} else {
		col = color.rgb * gl_FragColor.rgb;
	}
	col += vec3(secondaryColor);
	gl_FragColor = vec4 (col, color.a * gl_FragColor.a);
}
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