[FFmpeg-devel] [PATCH 1/2] avcodec/ffv1: flip half of float16

[Michael Niedermayer]

float16 is a sign bit followed by "positive is larger" exponent and mantisse
that way the up direction changes between negative and positive numbers
flipping the exponent and mantisse for positive numbers gives a compression
gain of 0.5% for both RCT and noRCT modes using the
ACES_OT_VWG_SampleFrames testset

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
---
 libavcodec/ffv1dec_template.c | 10 ++++++++++
 libavcodec/ffv1enc_template.c | 11 +++++++++++
 2 files changed, 21 insertions(+)

diff --git a/libavcodec/ffv1dec_template.c b/libavcodec/ffv1dec_template.c
index e983d1ba648..0be17b54bbb 100644
--- a/libavcodec/ffv1dec_template.c
+++ b/libavcodec/ffv1dec_template.c
@@ -185,6 +185,16 @@ static int RENAME(decode_rgb_frame)(FFV1Context *f, FFV1SliceContext *sc,
                 b += g;
                 r += g;
             }
+            if (f->flt) {
+                if (!(r&0x8000))
+                    r ^= 0x7FFF;
+                if (!(g&0x8000))
+                    g ^= 0x7FFF;
+                if (!(b&0x8000))
+                    b ^= 0x7FFF;
+                if (transparency && !(a&0x8000))
+                    a ^= 0x7FFF;
+            }
 
             if (lbd)
                 *((uint32_t*)(src[0] + x*4 + stride[0]*y)) = b + ((unsigned)g<<8) + ((unsigned)r<<16) + ((unsigned)a<<24);
diff --git a/libavcodec/ffv1enc_template.c b/libavcodec/ffv1enc_template.c
index bc14926ab95..4d20cb93627 100644
--- a/libavcodec/ffv1enc_template.c
+++ b/libavcodec/ffv1enc_template.c
@@ -180,6 +180,17 @@ static int RENAME(encode_rgb_frame)(FFV1Context *f, FFV1SliceContext *sc,
                 r = *((const uint16_t *)(src[2] + x*2 + stride[2]*y));
             }
 
+            if (f->flt) {
+                if (!(r&0x8000))
+                    r ^= 0x7FFF;
+                if (!(g&0x8000))
+                    g ^= 0x7FFF;
+                if (!(b&0x8000))
+                    b ^= 0x7FFF;
+                if (transparency && !(a&0x8000))
+                    a ^= 0x7FFF;
+            }
+
             if (sc->slice_coding_mode != 1) {
                 b -= g;
                 r -= g;
-- 
2.48.1