Support disabling chroma subsampling (#1092)

Author: thenickdude

Readme.md

@@ -171,6 +171,7 @@ var stream = canvas.jpegStream({
     bufsize: 4096 // output buffer size in bytes, default: 4096
   , quality: 75 // JPEG quality (0-100) default: 75
   , progressive: false // true for progressive compression, default: false
+  , disableChromaSubsampling: false // true to disable 2x2 subsampling of the chroma components, default: false
 });
 ```
 

lib/canvas.js

@@ -94,6 +94,8 @@ Canvas.prototype.createJPEGStream = function(options){
       bufsize: clampedBufSize
     , quality: options.quality || 75
     , progressive: options.progressive || false
+    , chromaHSampFactor: options.disableChromaSubsampling ? 1 : 2
+    , chromaVSampFactor: options.disableChromaSubsampling ? 1 : 2
   });
 };
 

lib/jpegstream.js

@@ -54,7 +54,9 @@ JPEGStream.prototype._read = function _read() {
   var bufsize = this.options.bufsize;
   var quality = this.options.quality;
   var progressive = this.options.progressive;
-  self.canvas.streamJPEGSync(bufsize, quality, progressive, function(err, chunk){
+  var chromaHSampFactor = this.options.chromaHSampFactor;
+  var chromaVSampFactor = this.options.chromaVSampFactor;
+  self.canvas.streamJPEGSync(bufsize, quality, progressive, chromaHSampFactor, chromaVSampFactor, function(err, chunk){
     if (err) {
       self.emit('error', err);
     } else if (chunk) {

src/Canvas.cc

@@ -568,15 +568,19 @@ NAN_METHOD(Canvas::StreamJPEGSync) {
     return Nan::ThrowTypeError("quality setting required");
   if (!info[2]->IsBoolean())
     return Nan::ThrowTypeError("progressive setting required");
-  if (!info[3]->IsFunction())
+  if (!info[3]->IsNumber())
+    return Nan::ThrowTypeError("chromaHSampFactor required");
+  if (!info[4]->IsNumber())
+    return Nan::ThrowTypeError("chromaVSampFactor required");
+  if (!info[5]->IsFunction())
     return Nan::ThrowTypeError("callback function required");
 
   Canvas *canvas = Nan::ObjectWrap::Unwrap<Canvas>(info.This());
   closure_t closure;
-  closure.fn = Local<Function>::Cast(info[3]);
+  closure.fn = Local<Function>::Cast(info[5]);
 
   Nan::TryCatch try_catch;
-  write_to_jpeg_stream(canvas->surface(), info[0]->NumberValue(), info[1]->NumberValue(), info[2]->BooleanValue(), &closure);
+  write_to_jpeg_stream(canvas->surface(), info[0]->NumberValue(), info[1]->NumberValue(), info[2]->BooleanValue(), info[3]->NumberValue(), info[4]->NumberValue(), &closure);
 
   if (try_catch.HasCaught()) {
     try_catch.ReThrow();

src/JPEGStream.h

@@ -101,7 +101,7 @@ jpeg_closure_dest(j_compress_ptr cinfo, closure_t * closure, int bufsize){
 }
 
 void
-write_to_jpeg_stream(cairo_surface_t *surface, int bufsize, int quality, bool progressive, closure_t *closure){
+write_to_jpeg_stream(cairo_surface_t *surface, int bufsize, int quality, bool progressive, int chromaHSampFactor, int chromaVSampFactor, closure_t *closure){
   int w = cairo_image_surface_get_width(surface);
   int h = cairo_image_surface_get_height(surface);
   struct jpeg_compress_struct cinfo;
@@ -118,6 +118,9 @@ write_to_jpeg_stream(cairo_surface_t *surface, int bufsize, int quality, bool pr
   if (progressive)
      jpeg_simple_progression(&cinfo);
   jpeg_set_quality(&cinfo, quality, (quality<25)?0:1);
+  cinfo.comp_info[0].h_samp_factor = chromaHSampFactor;
+  cinfo.comp_info[0].v_samp_factor = chromaVSampFactor;
+
   jpeg_closure_dest(&cinfo, closure, bufsize);
 
   jpeg_start_compress(&cinfo, TRUE);