Experimental reduced HEIF header

Remove a few hundred bytes of container boilerplate for simple images.
This prototype uses a small box that expands in meaning to a full meta
box with all defined items and properties.

Warning: This feature is experimental and forbidden by the current
AVIF specification.

apps/avifenc.c

@@ -103,6 +103,7 @@ static void syntax(void)
     printf("    -s,--speed S                      : Encoder speed (%d-%d, slowest-fastest, 'default' or 'd' for codec internal defaults. default speed: 6)\n",
            AVIF_SPEED_SLOWEST,
            AVIF_SPEED_FASTEST);
+    printf("    --minimize                        : Minimize header if possible\n");
     printf("    -c,--codec C                      : AV1 codec to use (choose from versions list below)\n");
     printf("    --exif FILENAME                   : Provide an Exif metadata payload to be associated with the primary item (implies --ignore-exif)\n");
     printf("    --xmp FILENAME                    : Provide an XMP metadata payload to be associated with the primary item (implies --ignore-xmp)\n");
@@ -630,6 +631,7 @@ typedef struct
     avifBool autoTiling;
     avifBool progressive;
     int speed;
+    avifEncoderHeaderStrategy headerStrategy;
 
     int paspCount;
     uint32_t paspValues[8]; // only the first two are used
@@ -846,6 +848,7 @@ static avifBool avifEncodeImagesFixedQuality(const avifSettings * settings,
     encoder->autoTiling = settings->autoTiling;
     encoder->codecChoice = settings->codecChoice;
     encoder->speed = settings->speed;
+    encoder->headerStrategy = settings->headerStrategy;
     encoder->timescale = settings->outputTiming.timescale;
     encoder->keyframeInterval = settings->keyframeInterval;
     encoder->repetitionCount = settings->repetitionCount;
@@ -1064,6 +1067,7 @@ int main(int argc, char * argv[])
     settings.autoTiling = AVIF_FALSE;
     settings.progressive = AVIF_FALSE;
     settings.speed = 6;
+    settings.headerStrategy = AVIF_ENCODER_FULL_HEADER;
     settings.repetitionCount = AVIF_REPETITION_COUNT_INFINITE;
     settings.keyframeInterval = 0;
     settings.ignoreExif = AVIF_FALSE;
@@ -1291,6 +1295,8 @@ int main(int argc, char * argv[])
                     settings.speed = AVIF_SPEED_SLOWEST;
                 }
             }
+        } else if (!strcmp(arg, "--minimize")) {
+            settings.headerStrategy = AVIF_ENCODER_MINIMIZE_HEADER;
         } else if (!strcmp(arg, "--exif")) {
             NEXTARG();
             if (!readEntireFile(arg, &exifOverride)) {

include/avif/avif.h

@@ -176,6 +176,18 @@ typedef enum avifResult
 
 AVIF_API const char * avifResultToString(avifResult result);
 
+// ---------------------------------------------------------------------------
+// avifEncoderHeaderStrategy
+
+typedef enum avifEncoderHeaderStrategy
+{
+    // Encodes as "avif" brand and all its required boxes for maximum compatibility.
+    AVIF_ENCODER_FULL_HEADER,
+    // Encodes as "avir" brand to reduce the encoded file size.
+    // WARNING: Experimental feature. Produces files that are incompatible with older decoders.
+    AVIF_ENCODER_MINIMIZE_HEADER,
+} avifEncoderHeaderStrategy;
+
 // ---------------------------------------------------------------------------
 // avifROData/avifRWData: Generic raw memory storage
 
@@ -1136,6 +1148,9 @@ typedef struct avifScalingMode
 //   call to avifEncoderAddImage().
 typedef struct avifEncoder
 {
+    // Defaults to AVIF_ENCODER_FULL_HEADER
+    avifEncoderHeaderStrategy headerStrategy;
+
     // Defaults to AVIF_CODEC_CHOICE_AUTO: Preference determined by order in availableCodecs table (avif.c)
     avifCodecChoice codecChoice;
 

include/avif/internal.h

@@ -431,14 +431,25 @@ typedef size_t avifBoxMarker;
 
 typedef struct avifBoxHeader
 {
+    // Size of the box in bytes, excluding the number of bytes read to know the size and the type.
     size_t size;
     uint8_t type[4];
 } avifBoxHeader;
 
 typedef struct avifROStream
 {
     avifROData * raw;
+
+    // Index of the next byte in the raw stream.
     size_t offset;
+
+    // Index of the currently selected partial byte. Must be lower than offset.
+    // Only meaningful if numUsedBitsInPartialByte is not zero.
+    size_t offsetOfPartialByte;
+    // Number of bits already used in the currently selected partial byte.
+    size_t numUsedBitsInPartialByte;
+
+    // Error information, if any.
     avifDiagnostics * diag;
     const char * diagContext;
 } avifROStream;
@@ -458,6 +469,8 @@ avifBool avifROStreamReadU32(avifROStream * stream, uint32_t * v);
 avifBool avifROStreamReadU32Endianness(avifROStream * stream, uint32_t * v, avifBool littleEndian);
 avifBool avifROStreamReadUX8(avifROStream * stream, uint64_t * v, uint64_t factor); // Reads a factor*8 sized uint, saves in v
 avifBool avifROStreamReadU64(avifROStream * stream, uint64_t * v);
+avifBool avifROStreamReadBits(avifROStream * stream, uint32_t * v, size_t bitCount);
+avifBool avifROStreamReadVarInt(avifROStream * stream, uint32_t * v);
 avifBool avifROStreamReadString(avifROStream * stream, char * output, size_t outputSize);
 avifBool avifROStreamReadBoxHeader(avifROStream * stream, avifBoxHeader * header); // This fails if the size reported by the header cannot fit in the stream
 avifBool avifROStreamReadBoxHeaderPartial(avifROStream * stream, avifBoxHeader * header); // This doesn't require that the full box can fit in the stream
@@ -467,7 +480,15 @@ avifBool avifROStreamReadAndEnforceVersion(avifROStream * stream, uint8_t enforc
 typedef struct avifRWStream
 {
     avifRWData * raw;
+
+    // Index of the next byte in the raw stream.
     size_t offset;
+
+    // Index of the currently selected partial byte. Must be lower than offset.
+    // Only meaningful if numUsedBitsInPartialByte is not zero.
+    size_t offsetOfPartialByte;
+    // Number of bits already used in the currently selected partial byte.
+    size_t numUsedBitsInPartialByte;
 } avifRWStream;
 
 uint8_t * avifRWStreamCurrent(avifRWStream * stream);
@@ -486,6 +507,8 @@ void avifRWStreamWriteU16(avifRWStream * stream, uint16_t v);
 void avifRWStreamWriteU32(avifRWStream * stream, uint32_t v);
 void avifRWStreamWriteU64(avifRWStream * stream, uint64_t v);
 void avifRWStreamWriteZeros(avifRWStream * stream, size_t byteCount);
+void avifRWStreamWriteBits(avifRWStream * stream, uint32_t v, size_t bitCount);
+void avifRWStreamWriteVarInt(avifRWStream * stream, uint32_t v);
 
 // This is to make it clear that the box size is currently unknown, and will be determined later (with a call to avifRWStreamFinishBox)
 #define AVIF_BOX_SIZE_TBD 0