add optional flag to keep original colortype

3 files changed, 27 insertions, 5 deletions

diff --git a/src/zopflipng/zopflipng_bin.cc b/src/zopflipng/zopflipng_bin.cc
index fc2ddfa..ea764b9 100644
--- a/src/zopflipng/zopflipng_bin.cc
+++ b/src/zopflipng/zopflipng_bin.cc
@@ -125,6 +125,13 @@ void ShowHelp() {
          " web images because web browsers do not use these chunks. By default"
          " ZopfliPNG only keeps (and losslessly modifies) the following chunks"
          " because they are essential: IHDR, PLTE, tRNS, IDAT and IEND.\n"
+         "--keepcolortype: Keep original color type (RGB, RGBA, gray,"
+         " gray+alpha or palette) and bit depth of the PNG.\n"
+         " This results in a loss of compression opportunities, e.g. it will no"
+         " longer convert a 4-channel RGBA image to 2-channel gray+alpha if the"
+         " image only had translucent gray pixels.\n"
+         " May be useful if a device does not support decoding PNGs of a"
+         " particular color type.\n"
          "\n"
          "Usage examples:\n"
          "Optimize a file and overwrite if smaller: zopflipng infile.png"
@@ -239,6 +246,8 @@ int main(int argc, char *argv[]) {
                  " --keepchunks=gAMA,cHRM,sRGB,iCCP\n");
           return 0;
         }
+      } else if (name == "--keepcolortype") {
+        png_options.keep_colortype = true;
       } else if (name == "--prefix") {
         use_prefix = true;
         if (!value.empty()) prefix = value;
diff --git a/src/zopflipng/zopflipng_lib.cc b/src/zopflipng/zopflipng_lib.cc
index bff53a0..15188db 100644
--- a/src/zopflipng/zopflipng_lib.cc
+++ b/src/zopflipng/zopflipng_lib.cc
@@ -35,6 +35,7 @@ ZopfliPNGOptions::ZopfliPNGOptions()
   , lossy_transparent(false)
   , lossy_8bit(false)
   , auto_filter_strategy(true)
+  , keep_colortype(false)
   , use_zopfli(true)
   , num_iterations(15)
   , num_iterations_large(5)
@@ -379,7 +380,7 @@ int ZopfliPNGOptimize(const std::vector<unsigned char>& origpng,
   lodepng::State inputstate;
   error = lodepng::decode(image, w, h, inputstate, origpng);
 
-  bool keep_colortype = false;
+  bool keep_colortype = png_options.keep_colortype;
 
   if (!png_options.keepchunks.empty()) {
     // If the user wants to keep the non-essential chunks bKGD or sBIT, the
@@ -391,10 +392,12 @@ int ZopfliPNGOptimize(const std::vector<unsigned char>& origpng,
     // possible.
     std::set<std::string> keepchunks;
     ChunksToKeep(origpng, png_options.keepchunks, &keepchunks);
-    keep_colortype = keepchunks.count("bKGD") || keepchunks.count("sBIT");
-    if (keep_colortype && verbose) {
-      printf("Forced to keep original color type due to keeping bKGD or sBIT"
-             " chunk.\n");
+    if (keepchunks.count("bKGD") || keepchunks.count("sBIT")) {
+      if (!keep_colortype && verbose) {
+        printf("Forced to keep original color type due to keeping bKGD or sBIT"
+               " chunk.\n");
+      }
+      keep_colortype = true;
     }
   }
 
diff --git a/src/zopflipng/zopflipng_lib.h b/src/zopflipng/zopflipng_lib.h
index 7fdeeca..b2bcd12 100644
--- a/src/zopflipng/zopflipng_lib.h
+++ b/src/zopflipng/zopflipng_lib.h
@@ -105,6 +105,16 @@ struct ZopfliPNGOptions {
   // Automatically choose filter strategy using less good compression
   bool auto_filter_strategy;
 
+  // Keep original color type (RGB, RGBA, gray, gray+alpha or palette) and bit
+  // depth of the PNG.
+  // This results in a loss of compression opportunities, e.g. it will no
+  // longer convert a 4-channel RGBA image to 2-channel gray+alpha if the image
+  // only had translucent gray pixels.
+  // May be useful if a device does not support decoding PNGs of a particular
+  // color type.
+  // Default value: false.
+  bool keep_colortype;
+
   // PNG chunks to keep
   // chunks to literally copy over from the original PNG to the resulting one
   std::vector<std::string> keepchunks;