Refactor imir implementation to match HEIF Draft Amendment 2, which replaces...

Refactor imir implementation to match HEIF Draft Amendment 2, which replaces 'axis' with 'mode' and inverts the behavior

apps/avifenc.c

@@ -103,7 +103,7 @@ static void syntax(void)
     printf("    --crop CROPX,CROPY,CROPW,CROPH    : Add clap property (clean aperture), but calculated from a crop rectangle\n");
     printf("    --clap WN,WD,HN,HD,HON,HOD,VON,VOD: Add clap property (clean aperture). Width, Height, HOffset, VOffset (in num/denom pairs)\n");
     printf("    --irot ANGLE                      : Add irot property (rotation). [0-3], makes (90 * ANGLE) degree rotation anti-clockwise\n");
-    printf("    --imir AXIS                       : Add imir property (mirroring). 0=vertical axis (\"left-to-right\"), 1=horizontal axis (\"top-to-bottom\")\n");
+    printf("    --imir MODE                       : Add imir property (mirroring). 0=top-to-bottom, 1=left-to-right\n");
     printf("\n");
     if (avifCodecName(AVIF_CODEC_CHOICE_AOM, 0)) {
         printf("aom-specific advanced options:\n");
@@ -439,7 +439,7 @@ int main(int argc, char * argv[])
     uint32_t clapValues[8];
     avifBool cropConversionRequired = AVIF_FALSE;
     uint8_t irotAngle = 0xff; // sentinel value indicating "unused"
-    uint8_t imirAxis = 0xff;  // sentinel value indicating "unused"
+    uint8_t imirMode = 0xff;  // sentinel value indicating "unused"
     avifCodecChoice codecChoice = AVIF_CODEC_CHOICE_AUTO;
     avifRange requestedRange = AVIF_RANGE_FULL;
     avifBool lossless = AVIF_FALSE;
@@ -729,9 +729,9 @@ int main(int argc, char * argv[])
             }
         } else if (!strcmp(arg, "--imir")) {
             NEXTARG();
-            imirAxis = (uint8_t)atoi(arg);
-            if (imirAxis > 1) {
-                fprintf(stderr, "ERROR: Invalid imir axis: %s\n", arg);
+            imirMode = (uint8_t)atoi(arg);
+            if (imirMode > 1) {
+                fprintf(stderr, "ERROR: Invalid imir mode: %s\n", arg);
                 returnCode = 1;
                 goto cleanup;
             }
@@ -909,9 +909,9 @@ int main(int argc, char * argv[])
         image->transformFlags |= AVIF_TRANSFORM_IROT;
         image->irot.angle = irotAngle;
     }
-    if (imirAxis != 0xff) {
+    if (imirMode != 0xff) {
         image->transformFlags |= AVIF_TRANSFORM_IMIR;
-        image->imir.axis = imirAxis;
+        image->imir.mode = imirMode;
     }
 
     avifBool usingAOM = AVIF_FALSE;

apps/shared/avifutil.c

@@ -99,9 +99,7 @@ static void avifImageDumpInternal(avifImage * avif, uint32_t gridCols, uint32_t
             printf("    * irot (Rotation)      : %u\n", avif->irot.angle);
         }
         if (avif->transformFlags & AVIF_TRANSFORM_IMIR) {
-            printf("    * imir (Mirror)        : %u (%s)\n",
-                   avif->imir.axis,
-                   (avif->imir.axis == 0) ? "Vertical axis, \"left-to-right\"" : "Horizontal axis, \"top-to-bottom\"");
+            printf("    * imir (Mirror)        : Mode %u (%s)\n", avif->imir.mode, (avif->imir.mode == 0) ? "top-to-bottom" : "left-to-right");
         }
     }
 }

include/avif/avif.h

@@ -360,14 +360,24 @@ typedef struct avifImageRotation
 
 typedef struct avifImageMirror
 {
-    // 'imir' from ISO/IEC 23008-12:2017 6.5.12:
-    // "axis specifies a vertical (axis = 0) or horizontal (axis = 1) axis for the mirroring operation."
+    // 'imir' from ISO/IEC 23008-12:2017 6.5.12 (Draft Amendment 2):
+    //
+    //     'mode' specifies how the mirroring is performed:
+    //
+    //     0 indicates that the top and bottom parts of the image are exchanged;
+    //     1 specifies that the left and right parts are exchanged.
+    //
+    //     NOTE In Exif, orientation tag can be used to signal mirroring operations. Exif
+    //     orientation tag 4 corresponds to mode = 0 of ImageMirror, and Exif orientation tag 2
+    //     corresponds to mode = 1 accordingly.
     //
     // Legal values: [0, 1]
     //
-    // 0: Mirror about a vertical axis ("left-to-right")
-    // 1: Mirror about a horizontal axis ("top-to-bottom")
-    uint8_t axis;
+    // NOTE: As of HEIF Draft Amendment 2, the name of this variable has changed from 'axis' to 'mode' as
+    //       the logic behind it has been *inverted*. Please use the wording above describing the legal
+    //       values for 'mode' and update any code that previously may have used `axis` to use
+    //       the *opposite* value (0 now means top-to-bottom, where it used to mean left-to-right).
+    uint8_t mode;
 } avifImageMirror;
 
 // ---------------------------------------------------------------------------

src/read.c

@@ -1553,10 +1553,10 @@ static avifBool avifParseImageMirrorProperty(avifProperty * prop, const uint8_t
     BEGIN_STREAM(s, raw, rawLen, diag, "Box[imir]");
 
     avifImageMirror * imir = &prop->u.imir;
-    CHECK(avifROStreamRead(&s, &imir->axis, 1)); // unsigned int (7) reserved = 0; unsigned int (1) axis;
-    if ((imir->axis & 0xfe) != 0) {
+    CHECK(avifROStreamRead(&s, &imir->mode, 1)); // unsigned int (7) reserved = 0; unsigned int (1) mode;
+    if ((imir->mode & 0xfe) != 0) {
         // reserved bits must be 0
-        avifDiagnosticsPrintf(diag, "Box[imir] contains nonzero reserved bits [%u]", imir->axis);
+        avifDiagnosticsPrintf(diag, "Box[imir] contains nonzero reserved bits [%u]", imir->mode);
         return AVIF_FALSE;
     }
     return AVIF_TRUE;

src/write.c

@@ -275,8 +275,8 @@ static void avifEncoderWriteColorProperties(avifRWStream * s, const avifImage *
     }
     if (imageMetadata->transformFlags & AVIF_TRANSFORM_IMIR) {
         avifBoxMarker imir = avifRWStreamWriteBox(s, "imir", AVIF_BOX_SIZE_TBD);
-        uint8_t axis = imageMetadata->imir.axis & 0x1;
-        avifRWStreamWrite(s, &axis, 1); // unsigned int (7) reserved = 0; unsigned int (1) axis;
+        uint8_t mode = imageMetadata->imir.mode & 0x1;
+        avifRWStreamWrite(s, &mode, 1); // unsigned int (7) reserved = 0; unsigned int (1) mode;
         avifRWStreamFinishBox(s, imir);
         if (ipma && itemPropertyIndex) {
             ipmaPush(ipma, ++(*itemPropertyIndex), AVIF_TRUE);