avifenc.c Use better formula in quantizerToQuality
The original formula was apparently derived by inverting the
quality-to-quantizer conversion formula in src/write.c. That derivation
is not ideal because it included the `+ 50` rounding term. It is better
to omit the `+ 50` rounding term, invert the formula, and then add a
`+ 31` rounding term.
The new formula is already used in avifenc.c.
diff --git a/apps/avifenc.c b/apps/avifenc.c
index b378ec5..9e129fb 100644
--- a/apps/avifenc.c
+++ b/apps/avifenc.c
@@ -1424,7 +1424,8 @@
static int quantizerToQuality(int minQuantizer, int maxQuantizer)
{
const int quantizer = (minQuantizer + maxQuantizer) / 2;
- return (int)(100 - (100 * quantizer - 50) / 63.0);
+ const int quality = ((63 - quantizer) * 100 + 31) / 63;
+ return quality;
}
int main(int argc, char * argv[])
