blender-archive/source/blender/draw/modes/shaders/paint_texture_frag.glsl

in vec2 uv_interp;
#ifdef TEXTURE_PAINT_MASK
in vec2 masking_uv_interp;
#endif

out vec4 fragColor;

uniform sampler2D image;
uniform float alpha = 1.0;
uniform bool nearestInterp;

#ifdef TEXTURE_PAINT_MASK
uniform sampler2D maskingImage;
uniform vec3 maskingColor;
uniform bool maskingInvertStencil;
#endif

void main()
{
	vec2 uv = uv_interp;
	if (nearestInterp) {
		vec2 tex_size = vec2(textureSize(image, 0).xy);
		uv = (floor(uv_interp * tex_size) + 0.5) / tex_size;
	}

	vec4 color = texture(image, uv);
	color.a *= alpha;

#ifdef TEXTURE_PAINT_MASK
	vec4 mask = vec4(texture(maskingImage, masking_uv_interp).rgb, 1.0);
	if (maskingInvertStencil) {
		mask.rgb = 1.0 - mask.rgb;
	}
	float mask_step = smoothstep(0, 3.0, mask.r+mask.g+mask.b);
	mask.rgb *= maskingColor;
	color = mix(color, mask, mask_step);
#endif

	fragColor = color;
}