[qcms] Add a color profile white point transform api

third_party/qcms/src/chain.c

 /* vim: set ts=8 sw=8 noexpandtab: */
 //  qcms
 //  Copyright (C) 2009 Mozilla Corporation
 //  Copyright (C) 1998-2007 Marti Maria
 //
 // Permission is hereby granted, free of charge, to any person obtaining 
 // a copy of this software and associated documentation files (the "Software"), 
 // to deal in the Software without restriction, including without limitation 
 // the rights to use, copy, modify, merge, publish, distribute, sublicense, 
 // and/or sell copies of the Software, and to permit persons to whom the Software 
@@ skipping 879 matching lines @@
                 continue;
         }
         return transform;
 }
 */
 
 static struct qcms_modular_transform* qcms_modular_transform_create(qcms_profile *in, qcms_profile *out)
 {
         struct qcms_modular_transform *first_transform = NULL;
         struct qcms_modular_transform **next_transform = &first_transform;
+        qcms_bool transform_to_pcs_xyz_only = (out == NULL);
 
         if (in->color_space == RGB_SIGNATURE) {
                 struct qcms_modular_transform* rgb_to_pcs;
                 rgb_to_pcs = qcms_modular_transform_create_input(in);
                 if (!rgb_to_pcs) 
                         goto fail;
                 append_transform(rgb_to_pcs, &next_transform);
         } else {
                 assert(0 && "input color space not supported");
                 goto fail;
         }
 
-        if (in->pcs == LAB_SIGNATURE && out->pcs == XYZ_SIGNATURE) {
+        if (in->pcs == LAB_SIGNATURE && (transform_to_pcs_xyz_only || out->pcs == XYZ_SIGNATURE)) {
                 struct qcms_modular_transform* lab_to_pcs;
                 lab_to_pcs = qcms_modular_transform_alloc();
                 if (!lab_to_pcs) 
                         goto fail;
                 append_transform(lab_to_pcs, &next_transform);
                 lab_to_pcs->transform_module_fn = qcms_transform_module_LAB_to_XYZ;
         }
 
+        if (transform_to_pcs_xyz_only)
+                return first_transform;
+
         // This does not improve accuracy in practice, something is wrong here.
         //if (in->chromaticAdaption.invalid == false) {
         //      struct qcms_modular_transform* chromaticAdaption;
         //      chromaticAdaption = qcms_modular_transform_alloc();
         //      if (!chromaticAdaption) 
         //              goto fail;
         //      append_transform(chromaticAdaption, &next_transform);
         //      chromaticAdaption->matrix = matrix_invert(in->chromaticAdaption);
         //      chromaticAdaption->transform_module_fn = qcms_transform_module_matrix;
         //}
@@ skipping 54 matching lines @@
 float* qcms_chain_transform(qcms_profile *in, qcms_profile *out, float *src, float *dest, size_t lutSize)
 {
         struct qcms_modular_transform *transform_list = qcms_modular_transform_create(in, out);
         if (transform_list != NULL) {
                 float *lut = qcms_modular_transform_data(transform_list, src, dest, lutSize/3);
                 qcms_modular_transform_release(transform_list);
                 return lut;
         }
         return NULL;
 }
+
+qcms_bool qcms_profile_white_transform(qcms_profile *profile, float XYZ[3])
+{
+        const float inverse_internal_scale = 1.999969482421875f;
+
+        // Set the output profile to NULL to request a color transform to PCS XYZ only.
+        struct qcms_modular_transform *transform_list = qcms_modular_transform_create(profile, NULL);
+
+        // Now calculate how the profile transforms white input color to PCS XYZ space.
+        if (transform_list != NULL) {
+                XYZ[0] = XYZ[1] = XYZ[2] = 1.0f; // white input
+                qcms_modular_transform_data(transform_list, XYZ, XYZ, 1);
+                // qcms_modular_transform_create internally scales input by 1/1.999969482421875f
+                // but no qcms changelog describes why / how that number was choosen. junov@ "it
+                // might be related to the epsilon of the fixed-point type 2*(1-1/(2^16)), but
+                // there is no explanation, which is disconcerting." Meanwhile, undo the internal
+                // scaling so we return a normalized CIEXYZ value viz., where Y is scaled to 1.0.
+                // A properly created color profile should produce Y=~1.0 in PCS XYZ with white
+                // input (the D50 test). If it does not, then the profile is likely bogus.
+                XYZ[0] *= inverse_internal_scale;
+                XYZ[1] *= inverse_internal_scale;
+                XYZ[2] *= inverse_internal_scale;
+                qcms_modular_transform_release(transform_list);
+                return true;
+        }
+
+        return false;
+}