Use AOM_TUNE_IQ only for non-alpha and non-id MC

Author: y-guyon

apps/avifenc.c

@@ -312,7 +312,7 @@ static void syntaxLong(void)
         printf("    enable-chroma-deltaq=B            : Enable delta quantization in chroma planes. 0=disable (default), 1=enable\n");
         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'. (Default for aom 3.12+ still images: iq, otherwise ssim)\n");
+        printf("    tune=METRIC                       : Tune the encoder for distortion metric, one of 'psnr', 'ssim' or 'iq'. (Default for aom 3.12+ still images YUV: iq, otherwise ssim)\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

@@ -889,8 +889,9 @@ static avifResult aomCodecEncodeImage(avifCodec * codec,
         if (!lossless && !codec->internal->tuningSet) {
             aom_tune_metric tuneMetric = AOM_TUNE_SSIM;
 #if defined(AOM_HAVE_TUNE_IQ)
+            // 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 (aomUsage == AOM_USAGE_ALL_INTRA) {
+            if (!alpha && image->matrixCoefficients != AVIF_MATRIX_COEFFICIENTS_IDENTITY && aomUsage == AOM_USAGE_ALL_INTRA) {
                 tuneMetric = AOM_TUNE_IQ;
             }
 #endif