Use AOM_TUNE_IQ by default

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

Author: y-guyon

CHANGELOG.md

@@ -47,6 +47,7 @@ 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 luma and chroma with libaom.
 
 ### Removed since 1.3.0
 

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 images (aom v3.12+): 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

@@ -665,15 +665,22 @@ static avifResult aomCodecEncodeImage(avifCodec * codec,
             libavifDefaultTuneMetric = AOM_TUNE_PSNR;
         } else {
             libavifDefaultTuneMetric = AOM_TUNE_SSIM;
+#if defined(AOM_HAVE_TUNE_IQ) && defined(AOM_USAGE_ALL_INTRA) && defined(ALL_INTRA_HAS_SPEEDS_7_TO_9)
+            // AOM_TUNE_IQ is favored for its low perceptual distortion on luma and chroma samples.
+            // AOM_TUNE_IQ sets --deltaq-mode=6 which can only be used in all intra mode.
+            if (image->matrixCoefficients != AVIF_MATRIX_COEFFICIENTS_IDENTITY && (addImageFlags & AVIF_ADD_IMAGE_FLAG_SINGLE)) {
+                libavifDefaultTuneMetric = AOM_TUNE_IQ;
+            }
+#endif
         }
     }
 
     struct aom_codec_enc_cfg * cfg = &codec->internal->cfg;
     avifBool quantizerUpdated = AVIF_FALSE;
     // True if libavif knows that tune=iq is used, either by default by libavif, or explicitly set by the user.
     // False otherwise (including if libaom uses tune=iq by default, which is not the case as of v1.13.1 and earlier versions).
-    // This is only accurate for the first frame but tune=iq is only supported for still images in libavif and
-    // for all-intra coding in libaom (at least up to v1.13.1) anyway.
+    // This is only accurate for the first frame but tune=iq is only supported for all-intra coding in libaom (at least up to v1.13.1),
+    // and thus only supported for still images in libavif.
     const avifBool useTuneIq = useLibavifDefaultTuneMetric ? libavifDefaultTuneMetric == AOM_TUNE_IQ : avifImageUsesTuneIq(codec, alpha);
     const int quantizer = aomQualityToQuantizer(quality, useTuneIq);