Make alpha and gain map quality default to the same as color in API. (#2631)

Author: maryla-uc

src/write.c

@@ -1831,11 +1831,22 @@ static avifResult avifEncoderAddImageInternal(avifEncoder * encoder,
     }
 
     // -----------------------------------------------------------------------
-    // Map quality and qualityAlpha to quantizer and quantizerAlpha
+    // Map quality settings to quantizer values.
     encoder->data->quantizer = avifQualityToQuantizer(encoder->quality, encoder->minQuantizer, encoder->maxQuantizer);
-    encoder->data->quantizerAlpha = avifQualityToQuantizer(encoder->qualityAlpha, encoder->minQuantizerAlpha, encoder->maxQuantizerAlpha);
-    encoder->data->quantizerGainMap =
-        avifQualityToQuantizer(encoder->qualityGainMap, AVIF_QUANTIZER_BEST_QUALITY, AVIF_QUANTIZER_WORST_QUALITY);
+    // If alpha quality, and min and max alpha quantizer have their default values, default to the same quality as color.
+    if (encoder->qualityAlpha == AVIF_QUALITY_DEFAULT && encoder->minQuantizerAlpha == AVIF_QUANTIZER_BEST_QUALITY &&
+        encoder->maxQuantizerAlpha == AVIF_QUANTIZER_WORST_QUALITY) {
+        encoder->data->quantizerAlpha = encoder->data->quantizer;
+    } else {
+        encoder->data->quantizerAlpha =
+            avifQualityToQuantizer(encoder->qualityAlpha, encoder->minQuantizerAlpha, encoder->maxQuantizerAlpha);
+    }
+    if (encoder->qualityGainMap == AVIF_QUALITY_DEFAULT) {
+        encoder->data->quantizerGainMap = encoder->data->quantizer; // Default to the same quality as color.
+    } else {
+        encoder->data->quantizerGainMap =
+            avifQualityToQuantizer(encoder->qualityGainMap, AVIF_QUANTIZER_BEST_QUALITY, AVIF_QUANTIZER_WORST_QUALITY);
+    }
 
     // -----------------------------------------------------------------------
     // Handle automatic tiling