lavfi: add ff_detect_color_primaries helper

Related to #9673, this helper exists to facilitate "guessing" the right
primary tags from a given set of raw primary coefficients.

The cutoff value of 0.001 was chosen by experimentation. The smallest
"false positive" delta observed in practice was 0.023329, while the
largest "false negative" delta was 0.00016. So, a value of 0.001 sits
comfortably in the middle.

Signed-off-by: Niklas Haas <git@haasn.dev>

1 files changed, 25 insertions, 0 deletions

diff --git a/libavfilter/colorspace.c b/libavfilter/colorspace.c
index 25e99f4759..8d7b882375 100644
--- a/libavfilter/colorspace.c
+++ b/libavfilter/colorspace.c
@@ -170,6 +170,31 @@ const struct ColorPrimaries *ff_get_color_primaries(enum AVColorPrimaries prm)
     return p;
 }
 
+enum AVColorPrimaries ff_detect_color_primaries(const struct ColorPrimaries *prm)
+{
+    double delta;
+
+    for (enum AVColorPrimaries p = 0; p < AVCOL_PRI_NB; p++) {
+        const struct ColorPrimaries *ref = &color_primaries[p];
+        if (!ref->prim.xr)
+            continue;
+
+        delta = fabs(prm->prim.xr - ref->prim.xr) +
+                fabs(prm->prim.yr - ref->prim.yr) +
+                fabs(prm->prim.yg - ref->prim.yg) +
+                fabs(prm->prim.yg - ref->prim.yg) +
+                fabs(prm->prim.yb - ref->prim.yb) +
+                fabs(prm->prim.yb - ref->prim.yb) +
+                fabs(prm->wp.xw - ref->wp.xw) +
+                fabs(prm->wp.yw - ref->wp.yw);
+
+        if (delta < 0.001)
+            return p;
+    }
+
+    return AVCOL_PRI_UNSPECIFIED;
+}
+
 void ff_fill_rgb2yuv_table(const struct LumaCoefficients *coeffs,
                            double rgb2yuv[3][3])
 {