avcodec/ffv1: flip half of float16 and Compactify floats

float16 (and more so float32) have many odd values
half the values are negative, many are larger than "1.0"
and many values are very close to 0.

Storing the 16bits as is, looses compression because of the mixture
of dense and sparse regions and also many completely unused ones.

This simply remaps the 65536 values so no unused values remain
This improves compression by about 1.5% for the ACES_OT_VWG_SampleFrames testset
(this testset contains all kind of funny values including many images
 with negative rgb values)

The space needed for the map is insignificant compared to the
compression gained

This patch also flips half the float range as it can be done
using the same table.

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <[email protected]>

3 files changed, 71 insertions, 0 deletions

diff --git a/libavcodec/ffv1.h b/libavcodec/ffv1.h
index 87f0b2c02f..4ed06310f7 100644
--- a/libavcodec/ffv1.h
+++ b/libavcodec/ffv1.h
@@ -105,6 +105,7 @@ typedef struct FFV1SliceContext {
             uint64_t (*rc_stat2[MAX_QUANT_TABLES])[32][2];
         };
     };
+    uint16_t   fltmap[4][65536];
 } FFV1SliceContext;
 
 typedef struct FFV1Context {
diff --git a/libavcodec/ffv1dec_template.c b/libavcodec/ffv1dec_template.c
index e983d1ba64..e11da1a66c 100644
--- a/libavcodec/ffv1dec_template.c
+++ b/libavcodec/ffv1dec_template.c
@@ -155,6 +155,20 @@ static int RENAME(decode_rgb_frame)(FFV1Context *f, FFV1SliceContext *sc,
 
     memset(RENAME(sc->sample_buffer), 0, 8 * (w + 6) * sizeof(*RENAME(sc->sample_buffer)));
 
+    if (f->flt) {
+        for (int p= 0; p<3 + transparency; p++) {
+            int j = 0;
+            int lu = 0;
+            uint8_t state[2] = {128, 128};
+            for (int i= 0; i<65536; i++) {
+                int u = get_rac(&sc->c, state + lu);
+                sc->fltmap[p][j] = i ^ ((i&0x8000) ? 0 : 0x7FFF);
+                j+= u;
+                lu = u;
+            }
+        }
+    }
+
     for (y = 0; y < h; y++) {
         for (p = 0; p < 3 + transparency; p++) {
             int ret;
@@ -185,6 +199,13 @@ static int RENAME(decode_rgb_frame)(FFV1Context *f, FFV1SliceContext *sc,
                 b += g;
                 r += g;
             }
+            if (f->flt) {
+                r = sc->fltmap[0][r & 0xFFFF];
+                g = sc->fltmap[1][g & 0xFFFF];
+                b = sc->fltmap[2][b & 0xFFFF];
+                if (transparency)
+                    a = sc->fltmap[3][a & 0xFFFF];
+            }
 
             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 bc14926ab9..0d1b0a2843 100644
--- a/libavcodec/ffv1enc_template.c
+++ b/libavcodec/ffv1enc_template.c
@@ -148,6 +148,47 @@ static int RENAME(encode_rgb_frame)(FFV1Context *f, FFV1SliceContext *sc,
     memset(RENAME(sc->sample_buffer), 0, ring_size * MAX_PLANES *
            (w + 6) * sizeof(*RENAME(sc->sample_buffer)));
 
+    if (f->flt) {
+        memset(sc->fltmap, 0, sizeof(sc->fltmap));
+
+        for (y = 0; y < h; y++) {
+            for (x = 0; x < w; x++) {
+                int b, g, r, av_uninit(a);
+
+                if (sizeof(TYPE) == 4 || transparency) {
+                    g = *((const uint16_t *)(src[0] + x*2 + stride[0]*y));
+                    b = *((const uint16_t *)(src[1] + x*2 + stride[1]*y));
+                    r = *((const uint16_t *)(src[2] + x*2 + stride[2]*y));
+                    if (transparency)
+                        a = *((const uint16_t *)(src[3] + x*2 + stride[3]*y));
+                } else {
+                    b = *((const uint16_t *)(src[0] + x*2 + stride[0]*y));
+                    g = *((const uint16_t *)(src[1] + x*2 + stride[1]*y));
+                    r = *((const uint16_t *)(src[2] + x*2 + stride[2]*y));
+                }
+
+                sc->fltmap[0][r] = 1;
+                sc->fltmap[1][g] = 1;
+                sc->fltmap[2][b] = 1;
+                if (transparency)
+                    sc->fltmap[3][a] = 1;
+            }
+        }
+        for (int p= 0; p<3 + transparency; p++) {
+            int j = 0;
+            int lu = 0;
+            uint8_t state[2] = {128, 128};
+            for (int i= 0; i<65536; i++) {
+                int ri = i ^ ((i&0x8000) ? 0 : 0x7FFF);
+                int u = sc->fltmap[p][ri];
+                sc->fltmap[p][ri] = j;
+                j+= u;
+                put_rac(&sc->c, state + lu, u);
+                lu = u;
+            }
+        }
+    }
+
     for (y = 0; y < h; y++) {
         for (i = 0; i < ring_size; i++)
             for (p = 0; p < MAX_PLANES; p++)
@@ -180,6 +221,14 @@ static int RENAME(encode_rgb_frame)(FFV1Context *f, FFV1SliceContext *sc,
                 r = *((const uint16_t *)(src[2] + x*2 + stride[2]*y));
             }
 
+            if (f->flt) {
+                r = sc->fltmap[0][r];
+                g = sc->fltmap[1][g];
+                b = sc->fltmap[2][b];
+                if (transparency)
+                    a = sc->fltmap[3][a];
+            }
+
             if (sc->slice_coding_mode != 1) {
                 b -= g;
                 r -= g;