FFMPEG - how -preset effects the compression
https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/qsvenc.h#L82-L88
{ "veryfast", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = MFX_TARGETUSAGE_BEST_SPEED }, INT_MIN, INT_MAX, VE, "preset" }, \
{ "faster", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = MFX_TARGETUSAGE_6 }, INT_MIN, INT_MAX, VE, "preset" }, \
{ "fast", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = MFX_TARGETUSAGE_5 }, INT_MIN, INT_MAX, VE, "preset" }, \
{ "medium", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = MFX_TARGETUSAGE_BALANCED }, INT_MIN, INT_MAX, VE, "preset" }, \
{ "slow", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = MFX_TARGETUSAGE_3 }, INT_MIN, INT_MAX, VE, "preset" }, \
{ "slower", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = MFX_TARGETUSAGE_2 }, INT_MIN, INT_MAX, VE, "preset" }, \
{ "veryslow", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = MFX_TARGETUSAGE_BEST_QUALITY }, INT_MIN, INT_MAX, VE, "preset" }, \
https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/qsvenc.c#L504-L511
} else if (avctx->compression_level >= 0) {
if (avctx->compression_level > MFX_TARGETUSAGE_BEST_SPEED) {
av_log(avctx, AV_LOG_WARNING, "Invalid compression level: "
"valid range is 0-%d, using %d instead\n",
MFX_TARGETUSAGE_BEST_SPEED, MFX_TARGETUSAGE_BEST_SPEED);
avctx->compression_level = MFX_TARGETUSAGE_BEST_SPEED;
}
}