Allow lossless compression using 4:0:0 on grayscale input. (#1460)

Allow lossless compression using 4:0:0 on grayscale input.

This fixes #1453

CHANGELOG.md

@@ -40,6 +40,7 @@ List of incompatible ABI changes in this release:
 * Add API for multi-threaded YUV to RGB color conversion.
 * Add experimental support for AV2 behind the compilation flag AVIF_CODEC_AVM.
   AVIF_CODEC_CHOICE_AVM is now part of avifCodecChoice.
+* Allow lossless 4:0:0 on grayscale input.
 * Add avifenc --no-overwrite flag to avoid overwriting output file.
 * Add support for all transfer functions when using libsharpyuv.
 

apps/avifenc.c

@@ -1488,14 +1488,13 @@ int main(int argc, char * argv[])
     // Check lossy/lossless parameters and set to default if needed.
     if (lossless) {
         // Pixel format.
-        if (input.requestedFormat != AVIF_PIXEL_FORMAT_NONE && input.requestedFormat != AVIF_PIXEL_FORMAT_YUV444) {
+        if (input.requestedFormat != AVIF_PIXEL_FORMAT_NONE && input.requestedFormat != AVIF_PIXEL_FORMAT_YUV444 &&
+            input.requestedFormat != AVIF_PIXEL_FORMAT_YUV400) {
             fprintf(stderr,
                     "When set, the pixel format can only be 444 in lossless "
-                    "mode.\n");
+                    "mode. 400 also works if the input is grayscale.\n");
             returnCode = 1;
         }
-        // Don't subsample when using AVIF_MATRIX_COEFFICIENTS_IDENTITY.
-        input.requestedFormat = AVIF_PIXEL_FORMAT_YUV444;
         // Quality.
         if ((settings.quality != INVALID_QUALITY && settings.quality != AVIF_QUALITY_LOSSLESS) ||
             (settings.qualityAlpha != INVALID_QUALITY && settings.qualityAlpha != AVIF_QUALITY_LOSSLESS)) {
@@ -1626,10 +1625,12 @@ int main(int argc, char * argv[])
     image->alphaPremultiplied = premultiplyAlpha;
 
     if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (input.requestedFormat != AVIF_PIXEL_FORMAT_NONE) &&
-        (input.requestedFormat != AVIF_PIXEL_FORMAT_YUV444)) {
-        // User explicitly asked for non YUV444 yuvFormat, while matrixCoefficients was likely
-        // set to AVIF_MATRIX_COEFFICIENTS_IDENTITY as a side effect of --lossless,
-        // and Identity is only valid with YUV444. Set matrixCoefficients back to the default.
+        (input.requestedFormat != AVIF_PIXEL_FORMAT_YUV444) && (input.requestedFormat != AVIF_PIXEL_FORMAT_YUV400)) {
+        // User explicitly asked for non YUV444/YUV400 yuvFormat, while
+        // matrixCoefficients was likely set to
+        // AVIF_MATRIX_COEFFICIENTS_IDENTITY as a side effect of --lossless, and
+        // Identity is only valid with YUV444/YUV400. Set matrixCoefficients
+        // back to the default.
         image->matrixCoefficients = AVIF_MATRIX_COEFFICIENTS_BT601;
 
         if (cicpExplicitlySet) {
@@ -1662,7 +1663,8 @@ int main(int argc, char * argv[])
     }
 
     // Check again for y4m input (y4m input ignores input.requestedFormat and retains the format in file).
-    if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444)) {
+    if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444) &&
+        (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV400)) {
         fprintf(stderr, "matrixCoefficients may not be set to identity (0) when subsampling.\n");
         returnCode = 1;
         goto cleanup;
@@ -1797,7 +1799,10 @@ int main(int argc, char * argv[])
 
         if (sourceWasRGB) {
             if (!using444 && !using400) {
-                fprintf(stderr, "WARNING: [--lossless] Input data was RGB and YUV subsampling (-y) isn't YUV444. Output might not be lossless.\n");
+                fprintf(stderr,
+                        "WARNING: [--lossless] Input data was RGB and YUV "
+                        "subsampling (-y) isn't YUV444 or YUV400. Output might "
+                        "not be lossless.\n");
                 lossless = AVIF_FALSE;
             }
 

apps/shared/avifpng.c

@@ -317,11 +317,11 @@ avifBool avifPNGRead(const char * inputFilename,
     const avifBool useYCgCoR = AVIF_FALSE;
 #endif
     if (avif->yuvFormat == AVIF_PIXEL_FORMAT_NONE) {
-        if (avif->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY || useYCgCoR) {
+        if ((rawColorType == PNG_COLOR_TYPE_GRAY) || (rawColorType == PNG_COLOR_TYPE_GRAY_ALPHA)) {
+            avif->yuvFormat = AVIF_PIXEL_FORMAT_YUV400;
+        } else if (avif->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY || useYCgCoR) {
             // Identity and YCgCo-R are only valid with YUV444.
             avif->yuvFormat = AVIF_PIXEL_FORMAT_YUV444;
-        } else if ((rawColorType == PNG_COLOR_TYPE_GRAY) || (rawColorType == PNG_COLOR_TYPE_GRAY_ALPHA)) {
-            avif->yuvFormat = AVIF_PIXEL_FORMAT_YUV400;
         } else {
             avif->yuvFormat = AVIF_APP_DEFAULT_PIXEL_FORMAT;
         }

src/reformat.c

@@ -70,7 +70,8 @@ static avifBool avifPrepareReformatState(const avifImage * image, const avifRGBI
         return AVIF_FALSE;
     }
 
-    if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444)) {
+    if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444) &&
+        (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV400)) {
         return AVIF_FALSE;
     }
 
@@ -384,7 +385,9 @@ avifResult avifImageRGBToYUV(avifImage * image, const avifRGBImage * rgb)
                 }
 
                 // Populate any subsampled channels with averages from the 2x2 block
-                if (image->yuvFormat == AVIF_PIXEL_FORMAT_YUV420) {
+                if (image->yuvFormat == AVIF_PIXEL_FORMAT_YUV400) {
+                    // Do nothing on chroma planes.
+                } else if (image->yuvFormat == AVIF_PIXEL_FORMAT_YUV420) {
                     // YUV420, average 4 samples (2x2)
 
                     float sumU = 0.0f;

tests/test_cmd_avm_lossless.sh

@@ -62,7 +62,7 @@ pushd ${TMP_DIR}
   "${AVIFDEC}" "${ENCODED_FILE}" "${DECODED_FILE}"
 
   # Combining some arguments with lossless should fail.
-  for option in "-y 400" "--min 0 --max 1" "-r limited" "--cicp 2/2/8"; do
+  for option in "-y 420" "--min 0 --max 1" "-r limited" "--cicp 2/2/8"; do
     "${AVIFENC}" -c avm $option -s 10 -l "${DECODED_FILE}" -o "${ENCODED_FILE}" && exit 1
   done
 

tests/test_cmd_icc_profile.sh

@@ -95,7 +95,7 @@ pushd ${TMP_DIR}
 
   "${AVIFENC}" -s 8 -l "${INPUT_GRAY_PNG}" -o "${ENCODED_FILE}"
   "${AVIFDEC}" "${ENCODED_FILE}" "${DECODED_FILE}"
-  "${IMAGEMAGICK}" "${DECODED_FILE}" -profile "${SRGB_ICC}" "${CORRECTED_FILE}"
+  "${IMAGEMAGICK}" "${DECODED_FILE}" -define png:color-type=2 -profile "${SRGB_ICC}" "${CORRECTED_FILE}"
   "${ARE_IMAGES_EQUAL}" "${REFERENCE_GRAY_PNG}" "${CORRECTED_FILE}" 0 45
 popd
 

tests/test_cmd_lossless.sh

@@ -43,6 +43,7 @@ ARE_IMAGES_EQUAL="${BINARY_DIR}/tests/are_images_equal"
 
 # Input file paths.
 INPUT_PNG="${TESTDATA_DIR}/paris_icc_exif_xmp.png"
+INPUT_GRAY_PNG="${TESTDATA_DIR}/kodim03_grayscale_gamma1.6.png"
 # Output file names.
 ENCODED_FILE="avif_test_cmd_lossless_encoded.avif"
 DECODED_FILE="avif_test_cmd_lossless_decoded.png"
@@ -62,7 +63,7 @@ pushd ${TMP_DIR}
   "${AVIFDEC}" "${ENCODED_FILE}" "${DECODED_FILE}"
 
   # Combining some arguments with lossless should fail.
-  for option in "-y 400" "--min 0 --max 1" "-r limited" "--cicp 2/2/8"; do
+  for option in "-y 420" "--min 0 --max 1" "-r limited" "--cicp 2/2/8"; do
     "${AVIFENC}" $option -s 10 -l "${DECODED_FILE}" -o "${ENCODED_FILE}" && exit 1
   done
 
@@ -76,6 +77,12 @@ pushd ${TMP_DIR}
   "${AVIFENC}" -s 10 -l "${INPUT_PNG}" -o "${ENCODED_FILE}"
   "${AVIFDEC}" "${ENCODED_FILE}" "${DECODED_FILE_LOSSLESS}"
   "${ARE_IMAGES_EQUAL}" "${INPUT_PNG}" "${DECODED_FILE_LOSSLESS}" 0
+
+  # 400 test.
+  echo "Testing 400 lossless"
+  "${AVIFENC}" -y 400 -s 10 -l "${INPUT_GRAY_PNG}" -o "${ENCODED_FILE}"
+  "${AVIFDEC}" "${ENCODED_FILE}" "${DECODED_FILE_LOSSLESS}"
+  "${ARE_IMAGES_EQUAL}" "${INPUT_GRAY_PNG}" "${DECODED_FILE_LOSSLESS}" 0
 popd
 
 exit 0