[gimp/wip/Jehan/issue-3265-png-gama-chunk] Issue #3265: PNG gAMA chunk is silently ignored.

[Jehan <jehanp src gnome org>]

commit 42717cfd8eb9fa1c8516bb160a2dfd53134998f5
Author: Jehan <jehan girinstud io>
Date:   Sun Jan 17 15:28:08 2021 +0100

    Issue #3265: PNG gAMA chunk is silently ignored.
    
    PNG has support for profiles, which is the state-of-the-art way to
    handle color management nowadays. But it also had some very basic color
    management based on a gAMA chunk (for the gamma) and a cHRM chunk
    (primary chromaticities).
    We don't have a core concept with just these and don't want to walk
    backward anyway, but we can easily make basic RGB profiles from these.
    This is what this commit is about. It generates profiles at import which
    were not actually inside the image, but only represented by these 2 PNG
    chunks. What gives it away is that GIMP will propose to convert/keep the
    profile at start (unless configured otherwise, as any profile), and the
    profile name will specify it is a profile generated from PNG chunks'
    values.  One can even discard the profile later (if you didn't convert),
    so it gives a solution if somehow these metadata were not proper.

 plug-ins/common/file-png.c  | 105 ++++++++++++++++++++++++++++++++++++++++++++
 plug-ins/common/meson.build |   2 +-
 2 files changed, 106 insertions(+), 1 deletion(-)
---
diff --git a/plug-ins/common/file-png.c b/plug-ins/common/file-png.c
index 2f3a102a51..b74a628614 100644
--- a/plug-ins/common/file-png.c
+++ b/plug-ins/common/file-png.c
@@ -27,6 +27,7 @@
 #include <errno.h>
 
 #include <glib/gstdio.h>
+#include "lcms2.h"
 
 #include <libgimp/gimp.h>
 #include <libgimp/gimpui.h>
@@ -560,6 +561,21 @@ load_color_profile (png_structp   pp,
   return profile;
 }
 
+/* Copied from src/cmsvirt.c in Little-CMS. */
+static cmsToneCurve *
+Build_sRGBGamma (cmsContext ContextID)
+{
+    cmsFloat64Number Parameters[5];
+
+    Parameters[0] = 2.4;
+    Parameters[1] = 1. / 1.055;
+    Parameters[2] = 0.055 / 1.055;
+    Parameters[3] = 1. / 12.92;
+    Parameters[4] = 0.04045;
+
+    return cmsBuildParametricToneCurve (ContextID, 4, Parameters);
+}
+
 /*
  * 'load_image()' - Load a PNG image into a new image window.
  */
@@ -676,6 +692,95 @@ load_image (GFile        *file,
    */
   profile = load_color_profile (pp, info, &profile_name);
 
+  if (! profile && ! png_get_valid (pp, info, PNG_INFO_sRGB) &&
+      (png_get_valid (pp, info, PNG_INFO_gAMA) ||
+       png_get_valid (pp, info, PNG_INFO_cHRM)))
+    {
+      /* This is kind of a special case for PNG. If an image has no
+       * profile, and the sRGB chunk is not set, and either gAMA or cHRM
+       * (or ideally both) are set, then we generate a profile from
+       * these data on import. See #3265.
+       */
+      cmsToneCurve    *gamma_curve[3];
+      cmsCIExyY        whitepoint;
+      cmsCIExyYTRIPLE  primaries;
+      cmsHPROFILE      cms_profile = NULL;
+      gdouble          gamma = 1.0 / DEFAULT_GAMMA;
+
+      if (png_get_valid (pp, info, PNG_INFO_gAMA) &&
+          png_get_gAMA (pp, info, &gamma) == PNG_INFO_gAMA)
+        {
+          gamma_curve[0] = gamma_curve[1] = gamma_curve[2] = cmsBuildGamma (NULL, 1.0 / gamma);
+        }
+      else
+        {
+          gamma_curve[0] = gamma_curve[1] = gamma_curve[2] = Build_sRGBGamma (NULL);
+        }
+
+      if (png_get_valid (pp, info, PNG_INFO_cHRM) &&
+          png_get_cHRM (pp, info, &whitepoint.x, &whitepoint.y,
+                        &primaries.Red.x, &primaries.Red.y,
+                        &primaries.Green.x, &primaries.Green.y,
+                        &primaries.Blue.x, &primaries.Blue.y) == PNG_INFO_cHRM)
+        {
+          whitepoint.Y = primaries.Red.Y = primaries.Green.Y = primaries.Blue.Y = 1.0;
+        }
+      else
+        {
+          /* Rec709 primaries and D65 whitepoint as copied from
+           * cmsCreate_sRGBProfileTHR() in Little-CMS.
+           */
+          cmsCIExyY       d65_whitepoint   = { 0.3127, 0.3290, 1.0 };
+          cmsCIExyYTRIPLE rec709_primaries =
+            {
+                {0.6400, 0.3300, 1.0},
+                {0.3000, 0.6000, 1.0},
+                {0.1500, 0.0600, 1.0}
+            };
+
+          memcpy (&whitepoint, &d65_whitepoint, sizeof whitepoint);
+          memcpy (&primaries, &rec709_primaries, sizeof primaries);
+        }
+      cms_profile = cmsCreateRGBProfile (&whitepoint, &primaries, gamma_curve);
+      cmsFreeToneCurve (gamma_curve[0]);
+      g_warn_if_fail (cms_profile != NULL);
+
+      if (cms_profile != NULL)
+        {
+          /* Customize the profile description to show it is generated
+           * from PNG metadata.
+           */
+          gchar      *profile_desc;
+          cmsMLU     *description_mlu;
+          cmsContext  ContextID = cmsGetProfileContextID (cms_profile);
+
+          /* Note that I am not trying to localize these strings on purpose
+           * because cmsMLUsetASCII() expects ASCII. Maybe we should move to
+           * using cmsMLUsetWide() if we want the generated profile
+           * descriptions to be localized. XXX
+           */
+          if ((png_get_valid (pp, info, PNG_INFO_gAMA) && png_get_valid (pp, info, PNG_INFO_cHRM)))
+            profile_desc = g_strdup_printf ("Generated RGB profile from PNG's gAMA (gamma %.4f) and cHRM 
chunks",
+                                            1.0 / gamma);
+          else if (png_get_valid (pp, info, PNG_INFO_gAMA))
+            profile_desc = g_strdup_printf ("Generated RGB profile from PNG's gAMA chunk (gamma %.4f)",
+                                            1.0 / gamma);
+          else
+            profile_desc = g_strdup_printf ("Generated RGB profile from PNG's cHRM chunk");
+
+          description_mlu  = cmsMLUalloc (ContextID, 1);
+
+          cmsMLUsetASCII (description_mlu,  "en", "US", profile_desc);
+          cmsWriteTag (cms_profile, cmsSigProfileDescriptionTag, description_mlu);
+
+          profile = gimp_color_profile_new_from_lcms_profile (cms_profile, NULL);
+
+          g_free (profile_desc);
+          cmsMLUfree (description_mlu);
+          cmsCloseProfile (cms_profile);
+        }
+    }
+
   if (profile)
     {
       *profile_loaded = TRUE;
diff --git a/plug-ins/common/meson.build b/plug-ins/common/meson.build
index f459aa9f92..74693a589c 100644
--- a/plug-ins/common/meson.build
+++ b/plug-ins/common/meson.build
@@ -47,7 +47,7 @@ common_plugins_list = [
   },
   { 'name': 'file-pix', },
   { 'name': 'file-png',
-    'deps': [ gtk3, gegl, libpng, ],
+    'deps': [ gtk3, gegl, libpng, lcms ],
   },
   { 'name': 'file-pnm', },
   { 'name': 'file-psp',