lavfi: add ICC profile support via lcms2

This introduces an optional dependency on lcms2 into FFmpeg. lcms2 is a
widely used library for ICC profile handling, which apart from being
used in almost all major image processing programs and video players,
has also been deployed in browsers. As such, it's both widely available
and well-tested.

Add a few helpers to cover our major use cases. This commit merely
introduces the helpers (and configure check), even though nothing uses
them yet.

It's worth pointing out that the reason the cmsToneCurves for each
AVCOL_TRC are cached inside the context, is because constructing a
cmsToneCurve requires evaluating the curve at 4096 (by default) grid
points and constructing a LUT. So, we ideally only want to do this once
per curve. This matters for e.g. ff_icc_profile_detect_transfer, which
essentially compares a profile against all of these generated LUTs.
Re-generating the LUTs for every iteration would be unnecessarily
wasteful.

The same consideration does not apply to e.g. cmsCreate*Profile, which
is a very lightweight operation just involving struct allocation and
setting a few pointers.

The cutoff value of 0.01 was determined by experimentation. The lowest
"false positive" delta I saw in practice was 0.13, and the largest
"false negative" delta was 0.0008. So a value of 0.01 sits comfortaby
almost exactly in the middle.

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

1 files changed, 3 insertions, 0 deletions

diff --git a/configure b/configure
index b364f503a5..c4d2bbedf9 100755
--- a/configure
+++ b/configure
@@ -215,6 +215,7 @@ External library support:
   --disable-iconv          disable iconv [autodetect]
   --enable-jni             enable JNI support [no]
   --enable-ladspa          enable LADSPA audio filtering [no]
+  --enable-lcms2           enable ICC profile support via LittleCMS 2 [no]
   --enable-libaom          enable AV1 video encoding/decoding via libaom [no]
   --enable-libaribb24      enable ARIB text and caption decoding via libaribb24 [no]
   --enable-libass          enable libass subtitles rendering,
@@ -1814,6 +1815,7 @@ EXTERNAL_LIBRARY_LIST="
     gnutls
     jni
     ladspa
+    lcms2
     libaom
     libass
     libbluray
@@ -6510,6 +6512,7 @@ enabled gmp               && require gmp gmp.h mpz_export -lgmp
 enabled gnutls            && require_pkg_config gnutls gnutls gnutls/gnutls.h gnutls_global_init
 enabled jni               && { [ $target_os = "android" ] && check_headers jni.h && enabled pthreads || die "ERROR: jni not found"; }
 enabled ladspa            && require_headers "ladspa.h dlfcn.h"
+enabled lcms2             && require_pkg_config lcms2 "lcms2 >= 2.13" lcms2.h cmsCreateContext
 enabled libaom            && require_pkg_config libaom "aom >= 1.0.0" aom/aom_codec.h aom_codec_version
 enabled libaribb24        && { check_pkg_config libaribb24 "aribb24 > 1.0.3" "aribb24/aribb24.h" arib_instance_new ||
                                { enabled gpl && require_pkg_config libaribb24 aribb24 "aribb24/aribb24.h" arib_instance_new; } ||