I performed a quick test and it appears that you can retrieve the raw encoded MJPEG data from directshow using CAP_FFMPEG if you slightly modify the source and link OpenCV against libavdevice. You need to add
avdevice_register_all();
to void CvCapture_FFMPEG::init() and
#include <libavdevice/avdevice.h>.
Unfortunatley this means you need to build OpenCV against FFMPEG libs and not use the precompiled dll - opencv_videoio_ffmpegxxx_64.dll.
Once the modification is in place you simply need to add the following environmental variable
OPENCV_FFMPEG_CAPTURE_OPTIONS=input_format;dshow
and open as
VideoCapture cap(“video=<NAME_FROM_DEVICE_MANAGER>”);
cap.set(CAP_PROP_FORMAT, -1);
to retrieve the raw data, or pass it straight through to cuda for decoding by opening as
cv::Ptr<cv::cudacodec::VideoReader> reader = cv::cudacodec::createVideoReader(String(<NAME_FROM_DEVICE_MANAGER>));
where <NAME_FROM_DEVICE_MANAGER> is the name of your webcam in device manager, in my case this would be
VideoCapture cap(“video=HD Webcam”);
I can’t guarantee that these modifications won’t break OpenCV videoio somewhere so proceed with caution. I also cannot guarantee that all the info you need to decoded the retrieved frame will be present (see below), although I can’t think of a reason why not.
I am not familiar with the format of MJPEG but the following is the start of the hex from a raw frame, does it look correct?
ff d8 ff c0 00 11 08 01 e0 02 80 03 01 21 00 02 11 01 03 11 01 ff db 00 84 00 08 05 05 08 05 05 08 08 08 08 0b 08 08 0b 0e 17 0e 0e 0b 0b 0e 1d 14 14 11 17 22 1d 22 22 1f 1d 1f 1f 26
28 34 2e 26 28 31 28 1f 1f 2e 40 2e 31 37 3a 3c 3c 3c 22 2b 42 45 40 3a 45 34 3a 3c 3a 01 08 0b 0b 0e 0b 0e 1a 0e 0e 1a 3a 26 1f 26 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a
3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a ff c4 01 a2 00 00 01 05 01 01 01 01 01 01 00 00 00 00 00 00 00 00 01 02 03 04 05 06
07 08 09 0a 0b 01 00 03 01 01 01 01 01 01 01 01 01 00 00 00 00 00 00 01 02 03 04 05 06 07 08 09 0a 0b 10 00 02 01 03 03 02 04 03 05 05 04 04 00 00 01 7d 01 02 03 00 04 11 05 12 21 31
41 06 13 51 61 07 22 71 14 32 81 91 a1 08 23 42 b1 c1 15 52 d1 f0 24 33 62 72 82 09 0a 16 17 18 19 1a 25 26 27 28 29 2a 34 35 36 37 38 39 3a 43 44 45 46 47 48 49 4a 53 54 55 56 57 58