icc: detect inconsistent Argyll ICC cLUT profiles, issue #21

The default behavior for Agryll when creating cLUT profiles seem to be
embedding an obviously incorrect XYZ matrix, meaning that for now we cannot use
the matrix for our relative colorimetric transform, and in the future the
profile will be incapable of doing dynamic trade-offs between performance and
accuracy by using matrix over cLUT during interactive operations.

We detect the swapped primaries in combination with presence of cLUTs and then
bail on constructing a valid babl space, as well as provide debug information
about why the profile cannot be used by babl.

diff --git a/babl/babl-icc.c b/babl/babl-icc.c
index 7f38c99d294652812fe8db305f3a7402addc7d57..836388585e7f4c77bbdc8c440c077f1f99b1881f 100644 (file)
--- a/babl/babl-icc.c
+++ b/babl/babl-icc.c
@@ -735,7 +735,7 @@ babl_space_from_icc (const char   *icc_data,
   const char *int_err;
   Babl *ret = NULL;
 
-  sign_t profile_class, color_space;
+  sign_t profile_class, color_space, pcs;
 
 
   if (!error) error = &int_err;
@@ -758,16 +758,24 @@ babl_space_from_icc (const char   *icc_data,
     *error = "not a monitor-class profile";
   else
   {
-  color_space = icc_read (sign, 16);
-  if (strcmp (color_space.str, "RGB "))
-    *error = "not defining an RGB space";
+    color_space = icc_read (sign, 16);
+    if (strcmp (color_space.str, "RGB "))
+      *error = "not defining an RGB space";
   }
   }
 
+  if (!*error)
+  {
+    pcs = icc_read (sign, 20);
+    if (strcmp (pcs.str, "XYZ "))
+      *error = "PCS is not XYZ";
+  }
+
   switch (intent)
   {
     case BABL_ICC_INTENT_RELATIVE_COLORIMETRIC:
       /* that is what we do well */
+
       break;
     case BABL_ICC_INTENT_PERCEPTUAL:
       /* if there is an A2B0 and B2A0 tags, we do not do what that
@@ -848,6 +856,19 @@ babl_space_from_icc (const char   *icc_data,
      wY = icc_read (s15f16, offset + 8 + 4);
      wZ = icc_read (s15f16, offset + 8 + 4 * 2);
 
+      /* detect inconsistent Argyll cLUT + matrix profiles */
+      if (icc_tag (state, "A2B0", NULL, NULL) ||
+          icc_tag (state, "B2A0", NULL, NULL))
+      {
+        if (rz > rx)
+        {
+           *error = "Inconsistent ICC profile detected, profile contains both cLUTs and a matrix with swapped primaries, this likely means it is an intentionally inconsistent Argyll profile is in use; this profile is only capable of high accuracy rendering and does not permit acceleration for interactive previews.";
+           fprintf (stderr, "babl ICC warning: %s\n", *error);
+           babl_free (state);
+           return NULL;
+        }
+      }
+
      ret = (void*)babl_space_match_trc_matrix (trc_red, trc_green, trc_blue,
                                         rx, ry, rz, gx, gy, gz, bx, by, bz);
      if (ret)