Add downsampling option to SkJpegEncoder

Bug: 713862
Change-Id: Ibad6ecf836ccfd355499e1bace7bcd4ba772a97a
Reviewed-on: https://skia-review.googlesource.com/15891
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>

2 files changed, 54 insertions, 0 deletions

diff --git a/src/images/SkJpegEncoder.cpp b/src/images/SkJpegEncoder.cpp
index d87fed81bc..4b37cd3be3 100644
--- a/src/images/SkJpegEncoder.cpp
+++ b/src/images/SkJpegEncoder.cpp
@@ -152,6 +152,35 @@ bool SkJpegEncoderMgr::setParams(const SkImageInfo& srcInfo, const SkJpegEncoder
     fCInfo.input_components = numComponents;
     jpeg_set_defaults(&fCInfo);
 
+    if (kGray_8_SkColorType != srcInfo.colorType()) {
+        switch (options.fDownsample) {
+            case SkJpegEncoder::Downsample::k420:
+                SkASSERT(2 == fCInfo.comp_info[0].h_samp_factor);
+                SkASSERT(2 == fCInfo.comp_info[0].v_samp_factor);
+                SkASSERT(1 == fCInfo.comp_info[1].h_samp_factor);
+                SkASSERT(1 == fCInfo.comp_info[1].v_samp_factor);
+                SkASSERT(1 == fCInfo.comp_info[2].h_samp_factor);
+                SkASSERT(1 == fCInfo.comp_info[2].v_samp_factor);
+                break;
+            case SkJpegEncoder::Downsample::k422:
+                fCInfo.comp_info[0].h_samp_factor = 2;
+                fCInfo.comp_info[0].v_samp_factor = 1;
+                fCInfo.comp_info[1].h_samp_factor = 1;
+                fCInfo.comp_info[1].v_samp_factor = 1;
+                fCInfo.comp_info[2].h_samp_factor = 1;
+                fCInfo.comp_info[2].v_samp_factor = 1;
+                break;
+            case SkJpegEncoder::Downsample::k444:
+                fCInfo.comp_info[0].h_samp_factor = 1;
+                fCInfo.comp_info[0].v_samp_factor = 1;
+                fCInfo.comp_info[1].h_samp_factor = 1;
+                fCInfo.comp_info[1].v_samp_factor = 1;
+                fCInfo.comp_info[2].h_samp_factor = 1;
+                fCInfo.comp_info[2].v_samp_factor = 1;
+                break;
+        }
+    }
+
     // Tells libjpeg-turbo to compute optimal Huffman coding tables
     // for the image.  This improves compression at the cost of
     // slower encode performance.
diff --git a/src/images/SkJpegEncoder.h b/src/images/SkJpegEncoder.h
index 07290aeb9f..a8b9dfb540 100644
--- a/src/images/SkJpegEncoder.h
+++ b/src/images/SkJpegEncoder.h
@@ -21,6 +21,23 @@ public:
         kBlendOnBlack,
     };
 
+    enum class Downsample {
+        /**
+         *  Reduction by a factor of two in both the horizontal and vertical directions.
+         */
+        k420,
+
+        /**
+         *  Reduction by a factor of two in the horizontal direction.
+         */
+        k422,
+
+        /**
+         *  No downsampling.
+         */
+        k444,
+    };
+
     struct Options {
         /**
          *  |fQuality| must be in [0, 100] where 0 corresponds to the lowest quality.
@@ -28,6 +45,14 @@ public:
         int fQuality = 100;
 
         /**
+         *  Choose the downsampling factor for the U and V components.  This is only
+         *  meaningful if the |src| is not kGray, since kGray will not be encoded as YUV.
+         *
+         *  Our default value matches the libjpeg-turbo default.
+         */
+        Downsample fDownsample = Downsample::k420;
+
+        /**
          *  Jpegs must be opaque.  This instructs the encoder on how to handle input
          *  images with alpha.
          *