Use AOM_TUNE_IQ by default for YUV (AOMediaCodec#2830)

Enable TUNE_IQ by default starting with libaom v3.13.0.
Update avifenc --help.
Add CHANGELOG entry.

Co-authored-by: Vincent Rabaud <vrabaud@google.com>

Author: y-guyon

CHANGELOG.md

@@ -56,6 +56,8 @@ The changes are relative to the previous release, unless the baseline is specifi
 * Set tuning before applying the user-provided specific aom codec options.
 * Use AOM_TUNE_PSNR by default when encoding alpha with libaom because
   AOM_TUNE_SSIM causes ringing for alpha.
+* Use AOM_TUNE_IQ by default when encoding still non-RGB color samples with
+  libaom v3.13.0 or later.
 * Converting an image containing a gain map using avifenc with the --grid flag
   now also splits the gain map into a grid.
 

apps/avifenc.c

@@ -321,7 +321,7 @@ static void syntaxLong(void)
         printf("    end-usage=MODE                    : Rate control mode, one of 'vbr', 'cbr', 'cq', or 'q'\n");
         printf("    sharpness=S                       : Bias towards block sharpness in rate-distortion optimization of transform coefficients in 0..7. (Default: 0)\n");
         printf("    tune=METRIC                       : Tune the encoder for distortion metric, one of 'psnr', 'ssim' or 'iq'.\n");
-        printf("                                        (Default for color: ssim, default for alpha: psnr)\n");
+        printf("                                        (Default for color: still non-RGB images (libaom v3.13.0+): iq, otherwise: ssim; default for alpha: psnr)\n");
         printf("    film-grain-test=TEST              : Film grain test vectors in 0..16. 0=none (default), 1=test1, 2=test2, ... 16=test16\n");
         printf("    film-grain-table=FILENAME         : Path to file containing film grain parameters\n");
         printf("\n");

src/codec_aom.c

@@ -757,6 +757,20 @@ static avifResult aomCodecEncodeImage(avifCodec * codec,
                 libavifDefaultTuneMetric = AOM_TUNE_PSNR;
             } else {
                 libavifDefaultTuneMetric = AOM_TUNE_SSIM;
+#if defined(AOM_HAVE_TUNE_IQ)
+                // AOM_TUNE_IQ has been tuned for the YCbCr family of color spaces, and is favored for
+                // its low perceptual distortion. AOM_TUNE_IQ partially generalizes to, and benefits
+                // from other "YUV-like" spaces (e.g. YCgCo and ICtCp) including monochrome (luma only).
+                // AOM_TUNE_IQ sets --deltaq-mode=6 which can only be used in all intra mode.
+                // AOM_TUNE_IQ was introduced in libaom v3.12.0 but it has significantly different bit
+                // allocation characteristics compared to v3.13.0. AOM_TUNE_IQ is used by default
+                // starting with v3.13.0 for fewer behavior changes in libavif.
+                static const int aomVersion_3_13_0 = (3 << 16) | (13 << 8);
+                if (image->matrixCoefficients != AVIF_MATRIX_COEFFICIENTS_IDENTITY && aomUsage == AOM_USAGE_ALL_INTRA &&
+                    aomVersion >= aomVersion_3_13_0) {
+                    libavifDefaultTuneMetric = AOM_TUNE_IQ;
+                }
+#endif
             }
         }
     }