Keep motion filter mirror() in bounds for tiny frames

libvmaf/src/feature/arm64/motion_neon.c

@@ -7,8 +7,18 @@
 
 static inline int mirror(int idx, int size)
 {
-    if (idx < 0) return -idx;
-    if (idx >= size) return 2 * size - idx - 2;
+    // Reflect out of range indices back into [0, size) using the same
+    // reflect 101 boundary the five tap motion filter expects (the edge
+    // sample is never repeated). One reflection is not enough when size
+    // is below 3, so fold repeatedly until the index lands in range.
+    if (size == 1)
+        return 0;
+    while (idx < 0 || idx >= size) {
+        if (idx < 0)
+            idx = -idx;
+        if (idx >= size)
+            idx = 2 * size - idx - 2;
+    }
     return idx;
 }
 

libvmaf/src/feature/integer_motion.c

@@ -148,8 +148,18 @@ static const VmafOption options[] = {
 
 static inline int mirror(int idx, int size)
 {
-    if (idx < 0) return -idx;
-    if (idx >= size) return 2 * size - idx - 2;
+    // Reflect out of range indices back into [0, size) using the same
+    // reflect 101 boundary the five tap motion filter expects (the edge
+    // sample is never repeated). One reflection is not enough when size
+    // is below 3, so fold repeatedly until the index lands in range.
+    if (size == 1)
+        return 0;
+    while (idx < 0 || idx >= size) {
+        if (idx < 0)
+            idx = -idx;
+        if (idx >= size)
+            idx = 2 * size - idx - 2;
+    }
     return idx;
 }
 

libvmaf/src/feature/x86/motion_avx2.c

@@ -25,8 +25,18 @@
 
 static inline int mirror(int idx, int size)
 {
-    if (idx < 0) return -idx;
-    if (idx >= size) return 2 * size - idx - 2;
+    // Reflect out of range indices back into [0, size) using the same
+    // reflect 101 boundary the five tap motion filter expects (the edge
+    // sample is never repeated). One reflection is not enough when size
+    // is below 3, so fold repeatedly until the index lands in range.
+    if (size == 1)
+        return 0;
+    while (idx < 0 || idx >= size) {
+        if (idx < 0)
+            idx = -idx;
+        if (idx >= size)
+            idx = 2 * size - idx - 2;
+    }
     return idx;
 }
 

libvmaf/src/feature/x86/motion_avx512.c

@@ -25,8 +25,18 @@
 
 static inline int mirror(int idx, int size)
 {
-    if (idx < 0) return -idx;
-    if (idx >= size) return 2 * size - idx - 2;
+    // Reflect out of range indices back into [0, size) using the same
+    // reflect 101 boundary the five tap motion filter expects (the edge
+    // sample is never repeated). One reflection is not enough when size
+    // is below 3, so fold repeatedly until the index lands in range.
+    if (size == 1)
+        return 0;
+    while (idx < 0 || idx >= size) {
+        if (idx < 0)
+            idx = -idx;
+        if (idx >= size)
+            idx = 2 * size - idx - 2;
+    }
     return idx;
 }
 

libvmaf/test/test_motion_neon.c

@@ -74,9 +74,11 @@ static int compute_motion_sad(unsigned w, unsigned h,
 
 static char *test_motion_neon_matches_scalar()
 {
-    // w/h < 3 excluded: mirror()'s radius-2 reflection goes out of bounds there
-    // (same bug in scalar/AVX2), so those sizes compare garbage against garbage.
+    // Sizes with width or height 1 or 2 are the regression case for issue
+    // 1580: mirror()'s reflection used to read out of bounds there. With the
+    // bounded reflection scalar and NEON must still agree bit exactly.
     static const struct { unsigned w, h; } sizes[] = {
+        {1, 1}, {2, 2}, {1, 2}, {2, 1}, {1, 5}, {5, 1}, {2, 5}, {5, 2},
         {3, 3}, {4, 4}, {5, 5}, {7, 7}, {9, 9},
         {15, 15}, {16, 16}, {17, 17}, {20, 4}, {33, 9}, {64, 48}, {65, 63},
     };