Use fast IDCT for JPEG decoding by default (tensorflow#5072)

pxoo · Nov 16, 2016 · 155cbdb · 155cbdb
1 parent 1479838
commit 155cbdb
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Showing 3 changed files with 8 additions and 5 deletions.
diff --git a/tensorflow/core/lib/jpeg/jpeg_mem.cc b/tensorflow/core/lib/jpeg/jpeg_mem.cc
@@ -139,8 +139,11 @@ uint8* UncompressLow(const void* srcdata, FewerArgsForCompiler* argball) {
   cinfo.do_fancy_upsampling = boolean(flags.fancy_upscaling);
   cinfo.scale_num = 1;
   cinfo.scale_denom = ratio;
-  // Activating this has a quality/speed trade-off implication:
-  // cinfo.dct_method = JDCT_IFAST;
+  // Activating this has a quality/speed trade-off implication.
+  // However, most JPEGs are already compressed, and so the faster IDCT
+  // should have no effect on training. The fast setting speeds up training on the
+  // GPU, e.g. by about 30% for smaller networks such as AlexNet.
+  cinfo.dct_method = JDCT_IFAST;
 
   jpeg_start_decompress(&cinfo);
 

diff --git a/tensorflow/core/lib/jpeg/jpeg_mem_unittest.cc b/tensorflow/core/lib/jpeg/jpeg_mem_unittest.cc
@@ -165,7 +165,7 @@ TEST(JpegMemTest, Jpeg2) {
     // Compare the two images
     const int totalerr = ComputeSumAbsoluteDifference(
         imgdata1.get(), refdata1.get(), in_w, in_h, stride1, stride1);
-    CHECK_LE(totalerr, 85000);
+    CHECK_LE(totalerr, 120000);
   }
 
   // check the second image too. Should be bitwise identical to the first.

diff --git a/tensorflow/python/ops/image_ops_test.py b/tensorflow/python/ops/image_ops_test.py
@@ -1928,10 +1928,10 @@ def testSynthetic(self):
       jpeg0, image0, image1, image2 = sess.run([jpeg0, image0, image1, image2])
 
       # The decoded-encoded image should be similar to the input
-      self.assertLess(self.averageError(image0, image1), 0.6)
+      self.assertLess(self.averageError(image0, image1), 0.7)
 
       # We should be very close to a fixpoint
-      self.assertLess(self.averageError(image1, image2), 0.02)
+      self.assertLess(self.averageError(image1, image2), 0.6)
 
       # Smooth ramps compress well (input size is 153600)
       self.assertGreaterEqual(len(jpeg0), 5000)