RGB specials

cpdfimage.ml

@@ -637,10 +637,10 @@ let test_bpc pdf dict =
   | _ -> 0
 
 let lossless_resample pdf ~pixel_threshold ~length_threshold ~factor ~interpolate ~path_to_convert s dict reference =
-  (* Printf.printf "***lossless_resample IN dictionary: %S\n" (Pdfwrite.string_of_pdf dict); *)
+  (*Printf.printf "***lossless_resample IN dictionary: %S\n" (Pdfwrite.string_of_pdf dict); *)
   let in_components = test_components pdf dict in
   let in_bpc = test_bpc pdf dict in
-  Printf.printf "\n***IN components = %i, bpc = %i\n" in_components in_bpc;
+  (*Printf.printf "\n***IN components = %i, bpc = %i\n" in_components in_bpc;*)
   match lossless_out pdf ~pixel_threshold ~length_threshold ".png" s dict reference with
   | None -> ()
   | Some (_, _, _, 4, _, _) -> Printf.printf "lossless resampling for CMYK not supported yet\n%!"
@@ -668,8 +668,17 @@ let lossless_resample pdf ~pixel_threshold ~length_threshold ~factor ~interpolat
             | Pdf.Stream {contents = Pdf.Dictionary d, data} as s ->
                 let out_components = test_components pdf s in
                 let out_bpc = test_bpc pdf s in
-                Printf.printf "***OUT components = %i, bpc = %i\n" out_components out_bpc;
+                (*Printf.printf "***OUT components = %i, bpc = %i\n" out_components out_bpc;*)
-                if out_components <> in_components || in_bpc <> out_bpc then
+                let rgb_to_grey_special =
+                  let was_rgb =
+                    match Pdf.lookup_direct pdf "/ColorSpace" dict with
+                    | Some (Pdf.Name ("/DeviceRGB" | "/CalRGB")) -> true
+                    | _ -> false
+                  in
+                    in_bpc = out_bpc && in_components = 3 && out_components = 1 && was_rgb
+                in
+                (*Printf.printf "***rgb_to_grey_special = %b\n" rgb_to_grey_special;*)
+                if (out_components <> in_components || in_bpc <> out_bpc) && not rgb_to_grey_special then
                   begin
                     if !debug_image_processing then Printf.printf "wrong bpc / components returned. Skipping.\n%!";
                     !reference
@@ -677,9 +686,7 @@ let lossless_resample pdf ~pixel_threshold ~length_threshold ~factor ~interpolat
                 else
                 begin
                   if !debug_image_processing then Printf.printf "lossless resample %i -> %i (%i%%)\n%!" size newsize (int_of_float (float newsize /. float size *. 100.));
-                  (* We will then test all the files we have available, and make sure all results correct *)
+                  let d' = fold_right (fun (k, v) d -> if k <> "/ColorSpace" || rgb_to_grey_special then add k v d else d) d (match dict with Pdf.Dictionary x -> x | _ -> []) in
-                  (* Then we will see about how to poke convert to do the correct thing - e.g not use PNG, force different output etc. *)
-                  let d' = fold_right (fun (k, v) d -> if k <> "/ColorSpace" then add k v d else d) d (match dict with Pdf.Dictionary x -> x | _ -> []) in
                     (*Printf.printf "***lossless_resample OUT dictionary: %S\n" (Pdfwrite.string_of_pdf (Pdf.Dictionary d')); *)
                     (Pdf.Dictionary d', data)
                 end

cpdfmanual.tex

@@ -3895,6 +3895,8 @@ Method & Effect\\\hline
 
 \noindent It is not currently possible to reprocess lossless JBIG2 into lossy JBIG2, nor is it possible to recompress into CCITT.
 
+NB: CYMK images will be converted to RGB by some of these processes. A future version of cpdf will remove this limitation.
+
 \begin{cpdflib}
 \clearpage
 \section*{C Interface}