From 84898dfc222b8969fa65e2685b91a51b086d67ba Mon Sep 17 00:00:00 2001
From: kleonc <9283098+kleonc@users.noreply.github.com>
Date: Tue, 31 May 2022 13:50:21 +0200
Subject: [PATCH] Make `Mesh::generate_triangle_mesh()` handle
 `PRIMITIVE_TRIANGLE_STRIP` and `PRIMITIVE_TRIANGLE_FAN`

---
 doc/classes/Mesh.xml     |  2 +-
 scene/resources/mesh.cpp | 64 +++++++++++++++++++++++++++++-----------
 2 files changed, 48 insertions(+), 18 deletions(-)

diff --git a/doc/classes/Mesh.xml b/doc/classes/Mesh.xml
index 0dfe08c5cb7..e96c5d0b487 100644
--- a/doc/classes/Mesh.xml
+++ b/doc/classes/Mesh.xml
@@ -40,7 +40,7 @@
 		<method name="generate_triangle_mesh" qualifiers="const">
 			<return type="TriangleMesh" />
 			<description>
-				Generate a [TriangleMesh] from the mesh.
+				Generate a [TriangleMesh] from the mesh. Considers only surfaces using one of these primitive types: [constant PRIMITIVE_TRIANGLES], [constant PRIMITIVE_TRIANGLE_STRIP], or [constant PRIMITIVE_TRIANGLE_FAN].
 			</description>
 		</method>
 		<method name="get_aabb" qualifiers="const">
diff --git a/scene/resources/mesh.cpp b/scene/resources/mesh.cpp
index 0edc6c05eb3..b6fe1f98c2a 100644
--- a/scene/resources/mesh.cpp
+++ b/scene/resources/mesh.cpp
@@ -47,32 +47,46 @@ Ref<TriangleMesh> Mesh::generate_triangle_mesh() const {
 		return triangle_mesh;
 	}
 
-	int facecount = 0;
+	int faces_size = 0;
 
 	for (int i = 0; i < get_surface_count(); i++) {
-		if (surface_get_primitive_type(i) != PRIMITIVE_TRIANGLES) {
-			continue;
-		}
-
-		if (surface_get_format(i) & ARRAY_FORMAT_INDEX) {
-			facecount += surface_get_array_index_len(i);
-		} else {
-			facecount += surface_get_array_len(i);
+		switch (surface_get_primitive_type(i)) {
+			case PRIMITIVE_TRIANGLES: {
+				int len = (surface_get_format(i) & ARRAY_FORMAT_INDEX) ? surface_get_array_index_len(i) : surface_get_array_len(i);
+				// Don't error if zero, it's valid (we'll just skip it later).
+				ERR_CONTINUE_MSG((len % 3) != 0, vformat("Ignoring surface %d, incorrect %s count: %d (for PRIMITIVE_TRIANGLES).", i, (surface_get_format(i) & ARRAY_FORMAT_INDEX) ? "index" : "vertex", len));
+				faces_size += len;
+			} break;
+			case PRIMITIVE_TRIANGLE_FAN:
+			case PRIMITIVE_TRIANGLE_STRIP: {
+				int len = (surface_get_format(i) & ARRAY_FORMAT_INDEX) ? surface_get_array_index_len(i) : surface_get_array_len(i);
+				// Don't error if zero, it's valid (we'll just skip it later).
+				ERR_CONTINUE_MSG(len != 0 && len < 3, vformat("Ignoring surface %d, incorrect %s count: %d (for %s).", i, (surface_get_format(i) & ARRAY_FORMAT_INDEX) ? "index" : "vertex", len, (surface_get_primitive_type(i) == PRIMITIVE_TRIANGLE_FAN) ? "PRIMITIVE_TRIANGLE_FAN" : "PRIMITIVE_TRIANGLE_STRIP"));
+				faces_size += (len == 0) ? 0 : (len - 2) * 3;
+			} break;
+			default: {
+			} break;
 		}
 	}
 
-	if (facecount == 0 || (facecount % 3) != 0) {
+	if (faces_size == 0) {
 		return triangle_mesh;
 	}
 
 	PoolVector<Vector3> faces;
-	faces.resize(facecount);
+	faces.resize(faces_size);
 	PoolVector<Vector3>::Write facesw = faces.write();
 
 	int widx = 0;
 
 	for (int i = 0; i < get_surface_count(); i++) {
-		if (surface_get_primitive_type(i) != PRIMITIVE_TRIANGLES) {
+		Mesh::PrimitiveType primitive = surface_get_primitive_type(i);
+		if (primitive != PRIMITIVE_TRIANGLES && primitive != PRIMITIVE_TRIANGLE_FAN && primitive != PRIMITIVE_TRIANGLE_STRIP) {
+			continue;
+		}
+		int len = (surface_get_format(i) & ARRAY_FORMAT_INDEX) ? surface_get_array_index_len(i) : surface_get_array_len(i);
+		if ((primitive == PRIMITIVE_TRIANGLES && (len <= 0 || (len % 3) != 0)) || ((primitive == PRIMITIVE_TRIANGLE_FAN || primitive == PRIMITIVE_TRIANGLE_STRIP) && len < 3)) {
+			// Error was already shown, just skip (including zero).
 			continue;
 		}
 
@@ -88,14 +102,30 @@ Ref<TriangleMesh> Mesh::generate_triangle_mesh() const {
 			PoolVector<int> indices = a[ARRAY_INDEX];
 			PoolVector<int>::Read ir = indices.read();
 
-			for (int j = 0; j < ic; j++) {
-				int index = ir[j];
-				facesw[widx++] = vr[index];
+			if (primitive == PRIMITIVE_TRIANGLES) {
+				for (int j = 0; j < ic; j++) {
+					int index = ir[j];
+					facesw[widx++] = vr[index];
+				}
+			} else { // PRIMITIVE_TRIANGLE_FAN, PRIMITIVE_TRIANGLE_STRIP
+				for (int j = 2; j < ic; j++) {
+					facesw[widx++] = vr[ir[(primitive == PRIMITIVE_TRIANGLE_FAN) ? 0 : j - 2]];
+					facesw[widx++] = vr[ir[j - 1]];
+					facesw[widx++] = vr[ir[j]];
+				}
 			}
 
 		} else {
-			for (int j = 0; j < vc; j++) {
-				facesw[widx++] = vr[j];
+			if (primitive == PRIMITIVE_TRIANGLES) {
+				for (int j = 0; j < vc; j++) {
+					facesw[widx++] = vr[j];
+				}
+			} else { // PRIMITIVE_TRIANGLE_FAN, PRIMITIVE_TRIANGLE_STRIP
+				for (int j = 2; j < vc; j++) {
+					facesw[widx++] = vr[(primitive == PRIMITIVE_TRIANGLE_FAN) ? 0 : j - 2];
+					facesw[widx++] = vr[j - 1];
+					facesw[widx++] = vr[j];
+				}
 			}
 		}
 	}