Formation of YUV (YCbCr) Image from JPEG Data

Abstract

JPEG file format is supported by a wide variety of applications on a variety of platforms and is the most common file format used. JPEG images are constructed from RGB data. Compression allows more images to be saved in the same amount of disk space while preserving the ICC Color profile and caption information. Note, however, that JPEG compression results in loss of image information that can not afterwards be restored. EXIF is an extension of the JPEG format that allows thumbnail data and information about the image to be included in a JPEG file. Images are saved in uncompressed YCbCr format, where the picture data is represented in one luminance (Y) and 2 color channels (Cb and Cr) instead of the normal 3 color channels (RGB). The two color channels occupy the same space as the luminance channel and the file size is therefore two thirds of the size of a RGB file. YCbCr is a more efficient mode of the image representation than RGB and has the same quality but occupies less space. We use YCbCr if we want the best combination of high quality and the lowest file size in an uncompressed finished file. YUV models human perception of color more closely than the standard RGB model used in computer graphics hardware. The primary advantages of luminance/chrominance systems such as YUV are that they remain compatible with black and white analog television. The Y channel saves nearly all the data recorded by black and white cameras, so it produces a signal suitable for reception on old monochrome displays. In this case, the U and V are simply discarded. If displaying color, all three channels are used, and the original RGB information can be decoded. Another advantage of YUV is that some of the information can be discarded in order to reduce bandwidth. The human eye has fairly little color sensitivity: the accuracy of the brightness information of the luminance channel has far more impact on the image discerned than that of the other two. By understanding this human shortcoming, standards such as NTSC are used to reduce the amount of data consumed by the chrominance channels and considerably leaving the eye to extrapolate more of the color. NTSC saves only 11% of the original blue and 30% of the red. The green information is usually preserved in the Y channel. Therefore, the resulting U and V signals can be substantially compressed.