enable-chroma-deltaq enabled by default in libavif 1.4.0?

[radarhere]

In libavif 1.3.0 and libavif 1.4.0, enable-chroma-deltaq is supposed to be disabled by default

libavif/apps/avifenc.c

Line 320 in d145e1a

printf(" enable-chroma-deltaq=B : Enable delta quantization in chroma planes. 0=disable (default), 1=enable\n");

libavif/apps/avifenc.c

Line 310 in 1aadfad

printf(" enable-chroma-deltaq=B : Enable delta quantization in chroma planes. 0=disable (default), 1=enable\n");

However, when I try libavif 1.4.0, I find that it is enabled by default in some circumstances.

I looked through the release notes and the diff, but I'm not seeing anything that could have led to this. Do you have any explanation for why this would be?

Here is the simplified C code that I've put together to trigger the issue.

avifEncoder *encoder = avifEncoderCreate();
if (!encoder) {
	PyErr_SetString(PyExc_MemoryError, "Can't allocate encoder");
	return NULL;
}

int advanced;
if (!PyArg_ParseTuple(args, "I", &advanced)) {
	return NULL;
}
if (advanced != -1) {
	avifResult result = avifEncoderSetCodecSpecificOption(encoder, "enable-chroma-deltaq", advanced == 0 ? "0" : "1");
	printf("Set enable-chroma-deltaq to %d\n", advanced);
	if (result != AVIF_RESULT_OK) {
		PyErr_Format(exc_type_for_avif_result(result), "Setting advanced codec options failed: %s", avifResultToString(result));
		return NULL;
	}
} else {
	printf("Do not set enable-chroma-deltaq\n");
}

avifImage *image = avifImageCreateEmpty();
if (image == NULL) {
	PyErr_SetString(PyExc_ValueError, "Image creation failed");
	return NULL;
}

image->yuvRange = AVIF_RANGE_LIMITED;
image->yuvFormat = AVIF_PIXEL_FORMAT_YUV420;
image->width = 128;
image->height = 128;
image->depth = 8;

image->matrixCoefficients = AVIF_MATRIX_COEFFICIENTS_BT601;

avifRGBImage rgb;
avifRGBImageSetDefaults(&rgb, image);

rgb.format = AVIF_RGB_FORMAT_RGB;

avifResult result = avifRGBImageAllocatePixels(&rgb);
if (result != AVIF_RESULT_OK) {
	PyErr_Format(exc_type_for_avif_result(result), "Pixel allocation failed: %s", avifResultToString(result));
	return NULL;
}

memset(rgb.pixels, 255, rgb.rowBytes * image->height);

result = avifImageRGBToYUV(image, &rgb);
if (result != AVIF_RESULT_OK) {
	PyErr_Format(exc_type_for_avif_result(result), "Conversion to YUV failed: %s", avifResultToString(result));
	return NULL;
}

result = avifEncoderAddImage(encoder, image, 0, AVIF_ADD_IMAGE_FLAG_SINGLE);
if (result != AVIF_RESULT_OK) {
	PyErr_Format(exc_type_for_avif_result(result), "Failed to encode image: %s", avifResultToString(result));
	return NULL;
}

avifRWData raw = AVIF_DATA_EMPTY;
result = avifEncoderFinish(encoder, &raw);

if (result != AVIF_RESULT_OK) {
	PyErr_Format(exc_type_for_avif_result(result), "Failed to finish encoding: %s", avifResultToString(result));
	avifRWDataFree(&raw);
	return NULL;
}

If I run this code with libavif 1.3.0, using Python to compare the output when not specifying enable-chroma-deltaq and when specifying enable-chroma-deltaq as 0, I find that it matches.
If I run this code with libavif 1.4.0, I find that not specifying enable-chroma-deltaq doesn't match 0, but instead matches 1.

[wantehchang]

Andrew: Thank you for testing libavif 1.4.0 and filing a bug report. This change is caused by the following change in the release notes:

* Use AOM_TUNE_IQ by default when encoding still non-RGB color samples with
  libaom v3.13.0 or later.

In the aom/aomcx.h header in libaom v3.13.0, AOM_TUNE_IQ is described as follows:

 * AOM_TUNE_IQ and AOM_TUNE_SSIMULACRA2 are restricted to all intra mode
 * (AOM_USAGE_ALL_INTRA). Setting the tuning option to either AOM_TUNE_IQ or
 * AOM_TUNE_SSIMULACRA2 causes the following options to be set (expressed as
 * command-line options):
 *   * --enable-qm=1
 *   * --qm-min=2
 *   * --qm-max=10
 *   * --sharpness=7
 *   * --dist-metric=qm-psnr
 *   * --enable-cdef=3
 *   * --enable-chroma-deltaq=1
 *   * --deltaq-mode=6
 *   * --screen-detection-mode=2
 * AOM_TUNE_IQ additionally sets the following options:
 *   * --enable-adaptive-sharpness=1

AOM_TUNE_IQ sets several libaom encoder options, including enable-chroma-deltaq. The default value of enable-chroma-deltaq is still 0 in libaom. But if libavif sets AOM_TUNE_IQ by default, it appears as if enable-chroma-deltaq had a default value of 1.

Although you can set the codec-specific option "tune=ssim" (for libaom) to restore the behavior in libavif v1.3.0, we recommend using the new default "tune=iq" option in libavif v1.4.0.

---

Next we consider how this is documented in the avifenc help message:

aom-specific advanced options:
    ...

    When used with libaom 3.0.0 or later, any key-value pairs supported by the aom_codec_set_option() function
    can be used. When used with libaom 2.0.x or older, the following key-value pairs can be used:

    aq-mode=M                         : Adaptive quantization mode. 0=off (default), 1=variance, 2=complexity, 3=cyclic refresh
    cq-level=Q                        : Constant/Constrained Quality level in 0..63, end-usage must be set to cq or q
    enable-chroma-deltaq=B            : Enable delta quantization in chroma planes. 0=disable (default), 1=enable
    ...

The part that you looked at is only applicable when avifenc is used with libaom 2.0.x or older. Since libaom v3.0.0 was released five years ago (on 2021-02-24), you are unlikely to use libaom 2.0.x or older. Please ignore that part of the avifenc help message.

[radarhere]

Thanks very much for the explanation.