Avoid NaN generation in compute_angular_offsets

Docs/ChangeLog-5x.md

@@ -17,6 +17,8 @@ The 5.5.0 release is a minor maintenance release.
   * **Bug fix:** Add missing low-clamp when storing decompressed HDR data into
     a UNORM8 image. Prior to this fix, output colors would be incorrect if a
     HDR void-extent block contained a negative FP16 color value.
+  * **Bug fix:** Avoid undefined NaN to int conversion behavior that is
+    triggered by NaNs generated by atan2() calls where both inputs are zero.
 
 <!-- ---------------------------------------------------------------------- -->
 ## 5.4.0

Source/astcenc_weight_align.cpp

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 // ----------------------------------------------------------------------------
-// Copyright 2011-2025 Arm Limited
+// Copyright 2011-2026 Arm Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
 // use this file except in compliance with the License. You may obtain a copy
@@ -130,6 +130,10 @@ static void compute_angular_offsets(
 		}
 
 		vfloat angle = atan2(anglesum_y, anglesum_x);
+
+		// Suppress NaNs generated if anglesums are both zero
+		angle = select(vfloat::zero(), angle, angle == angle);
+
 		vfloat ofs = angle * mult;
 		storea(ofs, offsets + i);
 	}