Mac Video Capture AVFoundation: Calculate compressed frame size for MJPEG.

media/video/capture/mac/video_capture_device_avfoundation_mac.mm

Index: media/video/capture/mac/video_capture_device_avfoundation_mac.mm
diff --git a/media/video/capture/mac/video_capture_device_avfoundation_mac.mm b/media/video/capture/mac/video_capture_device_avfoundation_mac.mm
index f3c7ca76bd283694c1ef9cda38430123222b59b6..fb1f0c4208b7b6439e8b2b7984c1877e3bfac5b0 100644
--- a/media/video/capture/mac/video_capture_device_avfoundation_mac.mm
+++ b/media/video/capture/mac/video_capture_device_avfoundation_mac.mm
@@ -6,6 +6,8 @@
 
 #import <CoreVideo/CoreVideo.h>
 
+#include <cstring>  // For memchr.
+
 #include "base/logging.h"
 #include "base/mac/foundation_util.h"
 #include "media/video/capture/mac/video_capture_device_mac.h"
@@ -30,6 +32,20 @@ media::VideoPixelFormat FourCCToChromiumPixelFormat(FourCharCode code) {
   }
 }
 
+// Returns frame size by finding the End Of Image marker, or 0 if not found.
+size_t JpegFrameSize(const char* sample, size_t sampleSize) {
+  // Jump to next marker (0xff), check for End Of Image (0xd9), repeat.
+  const char* end = sample + sampleSize - 1;
+  for (const char* it = sample;
+       (it = static_cast<const char*>(memchr(it, 0xff, end - it)));

[torbjorng, 2015/02/12 14:32:57]

I'm not too fond of side-effects in loop termination criteria, consider putting
memchr code in loop body for readability.
Also, while pointer-interpreted-as-boolean is OK in the language, is it really
accepted by all compilers?

+       ++it) {
+    if (it[1] == static_cast<char>(0xd9))
+      return 2 + (it - sample);
+  }
+  DLOG(WARNING) << "JPEG End Of Image (EOI) marker not found.";
+  return 0;
+}
+
 @implementation VideoCaptureDeviceAVFoundation
 
 #pragma mark Class methods
@@ -306,6 +322,15 @@ media::VideoPixelFormat FourCCToChromiumPixelFormat(FourCharCode code) {
       // Expect the MJPEG data to be available as a contiguous reference, i.e.
       // not covered by multiple memory blocks.
       CHECK_EQ(lengthAtOffset, frameSize);
+
+      // If |frameSize| is suspiciously high (>= 8 bpp), calculate the actual
+      // size by finding the end of image marker. The purpose is to speed up the
+      // jpeg decoding in the browser.

[tommi (sloooow) - chröme, 2015/02/11 15:57:44]

what about adding a note about the validation step in libvpx?  Since that's
where we're spending cycles right now, it will help a future reader understand
the context.

+      if (static_cast<int>(frameSize) >= dimensions.width * dimensions.height)
+        frameSize = JpegFrameSize(baseAddress, frameSize);

[tommi (sloooow) - chröme, 2015/02/11 15:57:44]

If JpegFrameSize fails and returns 0, should we keep the previous value?

+
+      if (frameSize == 0)
+        return;
     }
   } else {
     videoFrame = CoreMediaGlue::CMSampleBufferGetImageBuffer(sampleBuffer);