[gimp] Issue #6886: fix loading PNG images with linear profile or gAMA chunk.

[Jehan <jehanp src gnome org>]

commit 42a5326545f10009dfc45ca5db0cee8bc09bc483
Author: Jehan <jehan girinstud io>
Date:   Thu Mar 24 22:17:18 2022 +0100

    Issue #6886: fix loading PNG images with linear profile or gAMA chunk.
    
    Our main issue is that GIMP does not accept indexed images with linear
    backend. MR !460 was an attempt to fix this, but some weird pixel data
    conversions were happening so the result was very wrong.
    
    We could try to investigate this and add support for linear RGB palette
    colors, but it's unclear how necessary it is (i.e. is there any indexed
    image format for which we'd want this? PNG is not one such format as
    will be explained below), especially that palette are u8 only right now
    and linear RGB u8 is not the best idea anyway.
    
    In cases reported in #6886 though, our code would say the input is
    linear mostly because it had a 1.0 gAMA chunk (or we could have a case
    with a linear profile too). But then we don't need to set such data as
    being in linear RGB. Just set it as non-linear RGB (R'G'B') with the
    associated linear profile and that's it.
    Setting a linear backend to an imported PNG would only be useful if PNG
    had some flag explicitly asking for this, bypassing any profile TRC. But
    as far as I can see in its spec, there is no such thing.

 plug-ins/common/file-png.c | 33 ++++++++++++---------------------
 1 file changed, 12 insertions(+), 21 deletions(-)
---
diff --git a/plug-ins/common/file-png.c b/plug-ins/common/file-png.c
index d92c926981..ac4da57ae7 100644
--- a/plug-ins/common/file-png.c
+++ b/plug-ins/common/file-png.c
@@ -598,7 +598,6 @@ load_image (GFile        *file,
   GimpImageType     layer_type;           /* Type of drawable/layer */
   GimpColorProfile *profile      = NULL;  /* Color profile */
   gchar            *profile_name = NULL;  /* Profile's name */
-  gboolean          linear       = FALSE; /* Linear RGB */
   FILE             *fp;                   /* File pointer */
   volatile GimpImage *image      = NULL;  /* Image -- protected for setjmp() */
   GimpLayer        *layer;                /* Layer */
@@ -681,8 +680,7 @@ load_image (GFile        *file,
     png_set_swap (pp);
 
   /*
-   * Get the iCCP (color profile) chunk, if any, and figure if it's
-   * a linear RGB profile
+   * Get the iCCP (color profile) chunk, if any.
    */
   profile = load_color_profile (pp, info, &profile_name);
 
@@ -790,30 +788,23 @@ load_image (GFile        *file,
     }
 
   if (profile)
-    {
-      *profile_loaded = TRUE;
-
-      linear = gimp_color_profile_is_linear (profile);
-    }
+    *profile_loaded = TRUE;
 
   /*
-   * Get image precision and color model
+   * Get image precision and color model.
+   * Note that we always import PNG as non-linear. The data might be
+   * actually linear because of a linear profile, or because of a gAMA
+   * chunk with 1.0 value (which we convert to a profile above). But
+   * then we'll just set the right profile and that's it. Other than
+   * this, PNG doesn't have (that I can see in the spec) any kind of
+   * flag saying that data is linear, bypassing the profile's TRC so
+   * there is basically no reason to explicitly set a linear precision.
    */
 
   if (png_get_bit_depth (pp, info) == 16)
-    {
-      if (linear)
-        image_precision = GIMP_PRECISION_U16_LINEAR;
-      else
-        image_precision = GIMP_PRECISION_U16_NON_LINEAR;
-    }
+    image_precision = GIMP_PRECISION_U16_NON_LINEAR;
   else
-    {
-      if (linear)
-        image_precision = GIMP_PRECISION_U8_LINEAR;
-      else
-        image_precision = GIMP_PRECISION_U8_NON_LINEAR;
-    }
+    image_precision = GIMP_PRECISION_U8_NON_LINEAR;
 
   if (png_get_bit_depth (pp, info) < 8)
     {