Read the first two bytes of mini box directly

The first two bytes of the mini box can be read directly without checking any condition. Parse the fields in the first two bytes using elementary operations (bitwise AND and right shift). No need to use avifROStreamReadBits() yet.
AOMediaCodec · Aug 23, 2024 · b2cac33 · b2cac33
1 parent 7176120
commit b2cac33
Showing 1 changed file with 43 additions and 18 deletions.
diff --git a/src/read.c b/src/read.c
@@ -3585,26 +3585,51 @@ static avifResult avifParseMinimizedImageBox(avifMeta * meta,
 
     meta->fromMiniBox = AVIF_TRUE;
 
-    uint32_t version;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &version, 2), AVIF_RESULT_BMFF_PARSE_FAILED); // bit(2) version = 0;
+    // Read and parse the first two bytes, which contain no conditional constructs.
+    uint8_t data[2];
+    AVIF_CHECKERR(avifROStreamRead(&s, data, 2), AVIF_RESULT_BMFF_PARSE_FAILED);
+
+    // bit(2) version = 0;
+    //
+    // // flags
+    // bit(1) explicit_codec_types_flag;
+    // bit(1) float_flag;
+    // bit(1) full_range_flag;
+    // bit(1) alpha_flag;
+    // bit(1) explicit_cicp_flag;
+    // bit(1) hdr_flag;
+    int byte = data[0];
+    const avifBool hasHdr = byte & 0x1;
+    byte >>= 1;
+    const avifBool hasExplicitCicp = byte & 0x1;
+    byte >>= 1;
+    const avifBool hasAlpha = byte & 0x1;
+    byte >>= 1;
+    const avifBool fullRange = byte & 0x1;
+    byte >>= 1;
+    const avifBool floatFlag = byte & 0x1;
+    byte >>= 1;
+    const avifBool hasExplicitCodecTypes = byte & 0x1;
+    byte >>= 1;
+    const uint32_t version = byte;
     AVIF_CHECKERR(version == 0, AVIF_RESULT_BMFF_PARSE_FAILED);
 
-    // flags
-    uint32_t hasExplicitCodecTypes, floatFlag, fullRange, hasAlpha, hasExplicitCicp, hasHdr, hasIcc, hasExif, hasXmp;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &hasExplicitCodecTypes, 1), AVIF_RESULT_BMFF_PARSE_FAILED); // bit(1) explicit_codec_types_flag;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &floatFlag, 1), AVIF_RESULT_BMFF_PARSE_FAILED);             // bit(1) float_flag;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &fullRange, 1), AVIF_RESULT_BMFF_PARSE_FAILED);             // bit(1) full_range_flag;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &hasAlpha, 1), AVIF_RESULT_BMFF_PARSE_FAILED);              // bit(1) alpha_flag;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &hasExplicitCicp, 1), AVIF_RESULT_BMFF_PARSE_FAILED); // bit(1) explicit_cicp_flag;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &hasHdr, 1), AVIF_RESULT_BMFF_PARSE_FAILED);          // bit(1) hdr_flag;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &hasIcc, 1), AVIF_RESULT_BMFF_PARSE_FAILED);          // bit(1) icc_flag;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &hasExif, 1), AVIF_RESULT_BMFF_PARSE_FAILED);         // bit(1) exif_flag;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &hasXmp, 1), AVIF_RESULT_BMFF_PARSE_FAILED);          // bit(1) xmp_flag;
-
-    uint32_t chromaSubsampling, orientation;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &chromaSubsampling, 2), AVIF_RESULT_BMFF_PARSE_FAILED); // bit(2) chroma_subsampling;
-    AVIF_CHECKERR(avifROStreamReadBits(&s, &orientation, 3), AVIF_RESULT_BMFF_PARSE_FAILED);       // bit(3) orientation_minus1;
-    ++orientation;
+    // bit(1) icc_flag;
+    // bit(1) exif_flag;
+    // bit(1) xmp_flag;
+    //
+    // bit(2) chroma_subsampling;
+    // bit(3) orientation_minus1;
+    byte = data[1];
+    const uint32_t orientation = (byte & 0x7) + 1;
+    byte >>= 3;
+    const uint32_t chromaSubsampling = byte & 0x3;
+    byte >>= 2;
+    const avifBool hasXmp = byte & 0x1;
+    byte >>= 1;
+    const avifBool hasExif = byte & 0x1;
+    byte >>= 1;
+    const avifBool hasIcc = byte;
 
     // Spatial extents
     uint32_t smallDimensionsFlag, width, height;