Merge pull request #68673 from clayjohn/RD-alpha-hash

Fix alpha hash by correcting typos and doing calculations in object space

servers/rendering/renderer_rd/shaders/forward_clustered/scene_forward_clustered.glsl

@@ -826,7 +826,8 @@ void fragment_shader(in SceneData scene_data) {
 
 // alpha hash can be used in unison with alpha antialiasing
 #ifdef ALPHA_HASH_USED
-	if (alpha < compute_alpha_hash_threshold(vertex, alpha_hash_scale)) {
+	vec3 object_pos = (inverse(read_model_matrix) * inv_view_matrix * vec4(vertex, 1.0)).xyz;
+	if (alpha < compute_alpha_hash_threshold(object_pos, alpha_hash_scale)) {
 		discard;
 	}
 #endif // ALPHA_HASH_USED

servers/rendering/renderer_rd/shaders/forward_mobile/scene_forward_mobile.glsl

@@ -779,7 +779,8 @@ void main() {
 
 // alpha hash can be used in unison with alpha antialiasing
 #ifdef ALPHA_HASH_USED
-	if (alpha < compute_alpha_hash_threshold(vertex, alpha_hash_scale)) {
+	vec3 object_pos = (inverse(read_model_matrix) * inv_view_matrix * vec4(vertex, 1.0)).xyz;
+	if (alpha < compute_alpha_hash_threshold(object_pos, alpha_hash_scale)) {
 		discard;
 	}
 #endif // ALPHA_HASH_USED

servers/rendering/renderer_rd/shaders/scene_forward_aa_inc.glsl

@@ -11,7 +11,8 @@ float hash_3d(vec3 p) {
 
 float compute_alpha_hash_threshold(vec3 pos, float hash_scale) {
 	vec3 dx = dFdx(pos);
-	vec3 dy = dFdx(pos);
+	vec3 dy = dFdy(pos);
+
 	float delta_max_sqr = max(length(dx), length(dy));
 	float pix_scale = 1.0 / (hash_scale * delta_max_sqr);
 
@@ -32,9 +33,9 @@ float compute_alpha_hash_threshold(vec3 pos, float hash_scale) {
 			1.0 - ((1.0 - a_interp) * (1.0 - a_interp) / (2.0 * min_lerp * (1.0 - min_lerp))));
 
 	float alpha_hash_threshold =
-			(lerp_factor < (1.0 - min_lerp)) ? ((lerp_factor < min_lerp) ? cases.x : cases.y) : cases.z;
+			(a_interp < (1.0 - min_lerp)) ? ((a_interp < min_lerp) ? cases.x : cases.y) : cases.z;
 
-	return clamp(alpha_hash_threshold, 0.0, 1.0);
+	return clamp(alpha_hash_threshold, 0.00001, 1.0);
 }
 
 #endif // ALPHA_HASH_USED