[FFmpeg-devel] [PATCH] Add a gamma flag to exr loader to avoid banding

[Michael Niedermayer]

On Sun, Apr 27, 2014 at 01:28:15PM -0300, Gonzalo Garramuno wrote:
> On 27/04/14 13:12, Michael Niedermayer wrote:
> >On Sun, Apr 27, 2014 at 06:04:33PM +0200, Michael Niedermayer wrote:
> >>On Sun, Apr 27, 2014 at 10:43:09AM -0300, Gonzalo Garramuno wrote:
> >>>Attached is a patch to add a gamma flag to the exr loader.  This is
> >>>needed to avoid banding artifacts when gammaing the picture.
> >>>Currently, if done with a video filter, the process is done on uints
> >>>instead of full float.
> >>>  Changelog        |    1
> >>>  libavcodec/exr.c |  138 ++++++++++++++++++++++++++++++++++++++++++++++---------
> >>>  2 files changed, 117 insertions(+), 22 deletions(-)
> >>>828c54dedd21b004de00e049b4fa4b2f72ae78b1  exr_gamma.patch
> >>>diff --git a/Changelog b/Changelog
> >>>index daaa1ea..a726823 100644
> >>>--- a/Changelog
> >>>+++ b/Changelog
> >>>@@ -2,6 +2,7 @@ Entries are sorted chronologically from oldest to youngest within each release,
> >>>  releases are sorted from youngest to oldest.
> >>>  version <next>:
> >>>+- EXR loader supports a gamma flag
> >>>  - AC3 fixed-point decoding
> >>>  - shuffleplanes filter
> >>>  - subfile protocol
> >>>diff --git a/libavcodec/exr.c b/libavcodec/exr.c
> >>>index 084025a..8c2425c 100644
> >>>--- a/libavcodec/exr.c
> >>>+++ b/libavcodec/exr.c
> >>>@@ -31,6 +31,7 @@
> >>>   */
> >>>  #include <zlib.h>
> >>>+#include <float.h>
> >>>  #include "libavutil/imgutils.h"
> >>>  #include "libavutil/opt.h"
> >>>@@ -106,8 +107,30 @@ typedef struct EXRContext {
> >>>      EXRThreadData *thread_data;
> >>>      const char *layer;
> >>>+
> >>>+    float gamma;
> >>>+
> >>>  } EXRContext;
> >>>+union FP32 {
> >>>+    uint32_t u;
> >>>+    float f;
> >>>+    struct {
> >>>+        unsigned int Mantissa : 23;
> >>>+        unsigned int Exponent : 8;
> >>>+        unsigned int Sign : 1;
> >>>+    };
> >>>+} FP32;
> >>>+
> >>>+union FP16 {
> >>>+    uint16_t u;
> >>>+    struct {
> >>>+        unsigned int Mantissa : 10;
> >>>+        unsigned int Exponent : 5;
> >>>+        unsigned int Sign : 1;
> >>>+     };
> >>>+} FP16;
> >>that way to access parts of a float is not portable
> >>
> >>see libavutil/intfloat.h
> 
> In what way is not portable?  The bits in the struct?

yes


> 
> >or for 16bit floats you could simply use a LUT
> >
> >
> >[...]
> >
> >
> 
> I am following the blog at:
> 
> http://fgiesen.wordpress.com/2012/03/28/half-to-float-done-quic/
> 
> In there, there are several ways to convert a half to a float.  The
> smallest one is the following (and does not rely on bits):
> 
> staticFP32half_to_float_fast5(FP16h)
> {
> staticconstFP32magic={(254-15)<<23};
> staticconstFP32was_infnan={(127+16)<<23};
> FP32o;
> o.u=(h.u&0x7fff)<<13;// exponent/mantissa bits
> o.f*=magic.f;// exponent adjust
> if(o.f>=was_infnan.f)// make sure Inf/NaN survive
> o.u|=255<<23;
> o.u|=(h.u&0x8000)<<16;// sign bit
> returno;
> }
> 
> Would that be acceptable?

you can do the gamma correction in the LUT for 16bit too

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Frequently ignored answer#1 FFmpeg bugs should be sent to our bugtracker. User
questions about the command line tools should be sent to the ffmpeg-user ML.
And questions about how to use libav* should be sent to the libav-user ML.
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