[gimp] Issue #1070: exporting to jpeg from 32-bit float linear image...

[Jehan <jehanp src gnome org>]

commit 8594275bb7a807feae997146d6dbf3ca72a1a0b1
Author: Jehan <jehan girinstud io>
Date:   Fri Apr 12 12:57:18 2019 +0200

    Issue #1070: exporting to jpeg from 32-bit float linear image...
    
    ... produces jpeg in linear color space.
    The problem was that we were anyway always exporting to non-linear while
    attaching a linear profile. A first approach would have been to export
    in linear instead when the work image is linear, as proposed by mitch in
    a first version of the patch. Yet as Elle Stone notes, it is not a great
    idea to export 8-bit images as linear.
    Instead let's continue to always export as non-linear, as we were
    already doing. Yet when we also save a profile, and this one was
    originally linear, let's convert it to sRGB TRC before exporting.

 plug-ins/file-jpeg/jpeg-save.c | 47 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 44 insertions(+), 3 deletions(-)
---
diff --git a/plug-ins/file-jpeg/jpeg-save.c b/plug-ins/file-jpeg/jpeg-save.c
index 9ea605fc1a..5094afbd7a 100644
--- a/plug-ins/file-jpeg/jpeg-save.c
+++ b/plug-ins/file-jpeg/jpeg-save.c
@@ -550,14 +550,55 @@ save_image (const gchar  *filename,
 
       if (profile)
         {
-          const guint8 *icc_data;
-          gsize         icc_length;
+          GimpColorProfile *saved_profile;
+          const guint8     *icc_data;
+          gsize             icc_length;
+          gboolean          linear;
 
-          icc_data = gimp_color_profile_get_icc_profile (profile, &icc_length);
+          switch (gimp_image_get_precision (orig_image_ID))
+            {
+            case GIMP_PRECISION_U8_LINEAR:
+            case GIMP_PRECISION_U16_LINEAR:
+            case GIMP_PRECISION_U32_LINEAR:
+            case GIMP_PRECISION_HALF_LINEAR:
+            case GIMP_PRECISION_FLOAT_LINEAR:
+            case GIMP_PRECISION_DOUBLE_LINEAR:
+              linear = TRUE;
+              break;
+
+            case GIMP_PRECISION_U8_NON_LINEAR:
+            case GIMP_PRECISION_U16_NON_LINEAR:
+            case GIMP_PRECISION_U32_NON_LINEAR:
+            case GIMP_PRECISION_HALF_NON_LINEAR:
+            case GIMP_PRECISION_FLOAT_NON_LINEAR:
+            case GIMP_PRECISION_DOUBLE_NON_LINEAR:
+
+            case GIMP_PRECISION_U8_PERCEPTUAL:
+            case GIMP_PRECISION_U16_PERCEPTUAL:
+            case GIMP_PRECISION_U32_PERCEPTUAL:
+            case GIMP_PRECISION_HALF_PERCEPTUAL:
+            case GIMP_PRECISION_FLOAT_PERCEPTUAL:
+            case GIMP_PRECISION_DOUBLE_PERCEPTUAL:
+              linear = FALSE;
+              break;
+            }
+
+          /* Though it would be technically possible to export with a
+           * linear profile, it is not such a great idea to export 8-bit
+           * formats as linear. This is why our current JPEG exporter
+           * only exports 8-bit non linear JPEG images.
+           */
+          if (linear)
+            saved_profile = gimp_color_profile_new_srgb_trc_from_color_profile (profile);
+          else
+            saved_profile = profile;
 
+          icc_data = gimp_color_profile_get_icc_profile (saved_profile, &icc_length);
           jpeg_icc_write_profile (&cinfo, icc_data, icc_length);
 
           g_object_unref (profile);
+          if (linear)
+            g_object_unref (saved_profile);
         }
     }