first commit

addons/zylann.hterrain/tools/brush/shaders/alpha.gdshader

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+shader_type canvas_item;
+render_mode blend_disabled;
+
+uniform sampler2D u_src_texture;
+uniform vec4 u_src_rect;
+uniform float u_opacity = 1.0;
+uniform float u_value = 1.0;
+
+vec2 get_src_uv(vec2 screen_uv) {
+	vec2 uv = u_src_rect.xy + screen_uv * u_src_rect.zw;
+	return uv;
+}
+
+void fragment() {
+	float brush_value = u_opacity * texture(TEXTURE, UV).r;
+	
+	vec4 src = texture(u_src_texture, get_src_uv(SCREEN_UV));
+	COLOR = vec4(src.rgb, mix(src.a, u_value, brush_value));
+}