Use AOM_TUNE_PSNR by default for alpha (#2949)

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2db5f9d..2d09c62 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -45,6 +45,8 @@
   Header OBU, for compatibility with libraries that wrongly ignore the colr box.
 * Use a "quality to quantizer (QP)" mapping formula designed for AOM_TUNE_IQ.
 * 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.
 
 ### Removed since 1.3.0
 

apps/avifenc.c

diff --git a/apps/avifenc.c b/apps/avifenc.c
index c68eea2..1dfee92 100644
--- a/apps/avifenc.c
+++ b/apps/avifenc.c
@@ -320,7 +320,8 @@
         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' (default for aom) or 'iq'.\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("    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

diff --git a/src/codec_aom.c b/src/codec_aom.c
index c235db1..319aecf 100644
--- a/src/codec_aom.c
+++ b/src/codec_aom.c
@@ -660,7 +660,12 @@
     aom_tune_metric libavifDefaultTuneMetric = AOM_TUNE_PSNR; // Meaningless unless useLibavifDefaultTuneMetric.
     if (quality != AVIF_QUALITY_LOSSLESS && !avifAOMOptionsContainExplicitTuning(codec, alpha)) {
         useLibavifDefaultTuneMetric = AVIF_TRUE;
-        libavifDefaultTuneMetric = AOM_TUNE_SSIM;
+        if (alpha) {
+            // Minimize ringing for alpha.
+            libavifDefaultTuneMetric = AOM_TUNE_PSNR;
+        } else {
+            libavifDefaultTuneMetric = AOM_TUNE_SSIM;
+        }
     }
 
     struct aom_codec_enc_cfg * cfg = &codec->internal->cfg;