Under Linux, the general ordinary USB camera V4L2 video capture has two ways: V4L2_PIX_FMT_MJPEG and V4L2_PIX_FMT_YUYV.
The V4L2_PIX_FMT_MJPEG acquisition method is a picture of the MJPEG compressed, the image format is JPEG / JPG, the suffix is .jpg or .jpeg. The collected .jpeg data is written directly to a file, the resulting stream does not play directly, and the video format that is packaged into AVI can be placed normally.
The V4L2_PIX_FMT_YUYV acquisition method is the original image format file in the yuyv format, the suffix is .yuv, the obtained YUV data file can be played directly with the professional YUV format viewer such as Pyuv, YUV data can be HEVC (H265), H264, VP8, VP9, etc., encoding compression, compressed by H.264 can be obtained, and the H264 code stream can be played directly with a VLC player.
Comparative analysis of M-JPEG and H.264 compression effect:
Uniform conditions: 50 frame images (1080P resolution, frame rate 5fps, video duration 10s)
Performance comparison: encoding time, compression ratio, image quality. Be
Original image effect:
Yuyv
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Two compression methods are compressed:
M-JPEG compression:
Yuyv - RGB - MJPEG (libjpeg-turbo)
Quality (Q) Coding (Yuyv) Post-Encoding (JPG) Compression Compression Delivery Time 90202500k17005k1: 1179s80202500k10903k1: 1878s70202500k9255k1: 2180s
Compression effect:
Q = 70
Q = 80
Q = 90
H.264 compression
The AVC specification is divided into three, from low to heights: Baseline, Main, HIGH
Baseline (Minimum PROFILE) level supports I / P frames, only supports non-interleaved and CAVLC, usually used for low-order or extra fault-tolerant applications, such as video calls, mobile phones, etc .; Main (primary profile level) / P / B frame, supported non-interlaced, but also provides support for CAVLC and CABAC, for mainstream consumer electronic product specifications such as low decoding (relatively) MP4, portable video playback PSP and iPod, etc .; High (High-end Profile, also called FREXT) increased 8x8 internal prediction, custom quantization, lossless video coding and more YUV format (such as 4: 4: 4) Application of HDTV in Broadcast and Video Disc Storage (Bluetooth Movies). Be
Yuyv - H.264 (x264)
profile level the average rate (ABR) before encoded (YUYV) coding (h264) compression ratio consuming baseline 2Mbps 202500K 1932K1: 10465shigh1440K1: 14094sbaseline 3Mbps2646K1: 7670shigh2031K1: 9999sbaseline 4Mbps3354K1: 6074shigh2612K1: 77105sbaseline 5Mbps3999K1: 5076shigh3223K1: 62108sbaseline 6Mbps4349K1: 4680shigh3621K1: 55108s
Image Effect:
Baseline-2Mbps
HIGH-2MBPS
Be
Baseline-3Mbps
HIGH-3MBPS
Be
Baseline-4Mbps
Be
HIGH-4MBPS
Be
Baseline-5Mbps
HIGH-5MBPS
Be
Baseline-6Mbps
Be
HIGH-6MBPS
in conclusion:
1. In terms of compression ratio, H.264 is far better than M-JPEG, H.264 compression ratio can generally reach 1:50 or even 1: 100 or more, and M-JPEG compression ratio is generally less than 1:20; Due to the high compression ratio, the amount of image data compressed by H.264 is much smaller than M-JPEG, more beneficial to real-time transmission, and less network bandwidth.
2. Image Quality: Since the M-JPEG compression ratio is small, the image quality and clarity close to the original image can be reached. In order to meet the high compression ratio, H.264 is slightly lost in the image quality.
Conclusion: The choice of two compression methods is most important to use the scene, reasonably select. If the real-time requirements are higher, H.264 is more suitable because the amount of data is small, it is easy to transmit; if the real-time requirements are not very high, the amount of large data volume can be accepted, and the image quality requires higher requirements, Consider M-JPEG mode.
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