Advantages of integrated JPEG image compression

The implementation of image compression directly in the camera offers many advantages for system design and flexible, cost-effective use.

Function principle of JPEG image compression

In 1992 the standard ISO/IEC 10918 was published by the Joint Photographic Experts Group (JPEG) and comprises several parts, in particular the actual compression of individual photos as well as the format for saving JFIF (JPEG File Interchange Format). It is one of the most widely used image formats.

The compression is done in several substeps. Color images are converted from the RGB color space to the YCbCr color model. The model consists of the luminance Y (brightness) and the two color components Cb (blue-yellow chrominance) and Cr (red-green chrominance). The color information is reduced by low-pass filtering and undersampling. In doing so, advantage is taken of the fact that the human eye resolves colors significantly lower than differences in brightness.

From left to right: original image (RGB color space) and its splitting into the components Y, Cb and Cr.
After conversion, each component (Y, Cb and Cr) is divided into 8×8 blocks and its discrete cosine transformation (DCT). The actual data reduction is achieved by a subsequent quantisation. The final step of the image compression is a frequency-correlated re-sorting of the coefficients and an entropy coding. For high performance, these processes can be executed in parallel.
Procedure of the JPEG image compression
JPEG compression is generally lossy, as the image quality is reduced. For monochrome images, a compression ratio in the range of 5:1 is visually lossless. At a rate of 10:1, the quality is sufficient for many applications. However, higher compression rates lead to strong artefacts in the image such as blocking and steps at edges. When using RGB color images, compression ratios of 12:1 for visual lossless or 35:1 for good quality can be used.

Realization in the LXT cameras

LXT cameras are based on modern CMOS sensors with high resolution and speed. Uncompressed images are transferred directly to the PC via the fast 10 GigE interface. JPEG compression can be optionally activated as a feature in Camera Explorer or SDK. For this purpose, a pixel format suitable for the application must first be selected (Mono8 or YCbCr422_8). Afterwards, the integrated JPEG image compression can be activated by the parameter “Image Compression Mode” = JPEG simply by pressing a button.
The JPEG compression rate can be flexibly configured in the range 30–100% via the “Image Compression Quality” parameter and thus enables an individual adjustment of the image quality to the respective application.
From left to right: original image, 80% compression (8:1), 30% compression (16:1)
Monochrome or color 8-bit images are supported. A high-quality color calculation (partly in a 5×5 environment) as well as conversion to YCbCr422_8 is carried out for color cameras. JPEG image compression is sequential with full sensor performance. The additional latency due to compression can be neglected. The implementation is based on the GigE Vision 2.0 standard per JPEG payload type. The generated JPEG stream contains all headers and can be saved directly into a JPEG file. A timestamp and the ROI position in the JPEG stream are also transmitted in the metadata. A preview image or parallel transmission of original and JPEG image is not provided.

Application examples

JPEG image compression can be used wherever image sequences are needed over a longer period of time. Especially if the bandwidth of the transmission or the storage medium is limited or the PC cannot perform the additional processing, e.g. for several cameras. Since the image quality is somewhat reduced compared to the original, use for metrological applications is not recommended.

Software support

Various software options are available for easy integration and use of the VLXT.JP cameras.

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