I managed to figure something out. it doesn't smooth things exactly like blender does but it is good enough for me.

here is a comparison

it uses the default filtering and a bicubic filtering (made by theagentd)

https://jvm-gaming.org/t/glsl-simple-fast-bicubic-filtering-shader-function/52549

I also made my implementation a bit more "understandable" and easier to set the colors

```
shader_type spatial;
render_mode unshaded;
uniform sampler2D tex : hint_albedo;
uniform vec4 white : hint_color;
uniform vec4 black : hint_color;
// Bicubic filter
// made by theagentd https://jvm-gaming.org/t/glsl-simple-fast-bicubic-filtering-shader-function/52549
vec4 cubic(float v){
vec4 n = vec4(1.0, 2.0, 3.0, 4.0) - v;
vec4 s = n * n * n;
float x = s.x;
float y = s.y - 4.0 * s.x;
float z = s.z - 4.0 * s.y + 6.0 * s.x;
float w = 6.0 - x - y - z;
return vec4(x, y, z, w) * (1.0/6.0);
}
vec4 textureBicubic(sampler2D sampler, vec2 texCoords){
ivec2 texSize = textureSize(tex, 0);
vec2 invTexSize = 1.0 / vec2(texSize);
texCoords = texCoords * vec2(texSize) - 0.5;
vec2 fxy = fract(texCoords);
texCoords -= fxy;
vec4 xcubic = cubic(fxy.x);
vec4 ycubic = cubic(fxy.y);
vec4 c = texCoords.xxyy + vec2(-0.5, +1.5).xyxy;
vec4 s = vec4(xcubic.xz + xcubic.yw, ycubic.xz + ycubic.yw);
vec4 offset = c + vec4(xcubic.yw, ycubic.yw) / s;
offset *= invTexSize.xxyy;
vec4 sample0 = texture(sampler, offset.xz);
vec4 sample1 = texture(sampler, offset.yz);
vec4 sample2 = texture(sampler, offset.xw);
vec4 sample3 = texture(sampler, offset.yw);
float sx = s.x / (s.x + s.y);
float sy = s.z / (s.z + s.w);
return mix(
mix(sample3, sample2, sx), mix(sample1, sample0, sx)
, sy);
}
void fragment()
{
//Bicubic
vec4 cubicTex = textureBicubic(tex,UV);
float value = cubicTex.r;
//Default
// float value = texture(tex,UV).r;
if (value > .5) {
ALBEDO = vec3(white.r,white.g,white.b);
}
else{
ALBEDO = vec3(black.r,black.g,black.b);
}
}
```

also thanks to aXu_AP for the helpful comment