Method for processing a digital image and image representation format

#The present invention relates to a method of processing a digital image and to an image representation format for representing a digital image. The present invention further relates to a method for encoding raw image data into a compressed digital image representation, and a method of analysing a JPEG-compressed digital image.

#In the society of today, enormous amounts of information are created each day. Much information is presented in the form of images. Further, large portions of the information are stored and presented electronically, for example on the Internet. Meanwhile, there is an increased usage of wireless connections to the Internet, wherein the data transfer rate is relatively low. Thus, there is a need for presenting information in a very compact form. This is especially important for images, since a normal digital image stored as an array of pixels is represented as a quite large data set.

#Further, mobile phones incorporating a camera are becoming increasingly popular. Thus, the mobile phone must be able to handle digital images. Mobile phones or other handheld devices that handle images have a limited memory space and limited processing power. Therefore, if image processing is to be performed on such devices, the digital images need to be efficiently and intelligently stored in order to set low requirements on memory space and processing power.

#The invention will now be described in further detail by way of example under reference to the accompanying drawings, on which:

#FIG. 1 is a flow chart of a method for compressing a digital image according to an embodiment of the invention.

#FIGS. 2a-c are schematic views of the image representation format according to embodiments of the invention.

#In the following description, compression of images will be described with reference to JPEG-compression, although other compressions using other transforms may be contemplated. It should be noted that the scope of protection of the present invention is in no way limited to JPEG-compression.

#Referring now to FIG. 1, a method for compressing a digital image will be described. Firstly, the digital image is represented in the YUV color model, step 10, wherein each pixel of the image has three components: luminance, Y, and two chromatic components U and V. The two chromatic components U and V represent color features, whose finest details are difficult to perceive for the human eye. Thus, these components may be represented in a lower resolution than the luminance component. A 16×16 pixel block of the image, which is an example of an image block as defined herein, may be represented by four Y-data units of 8×8 pixels each, one U-data unit of 8×8 pixels, and one V-data unit of 8×8 pixels. This corresponds to a 50% compression of the data set compared to a RGB-representation of the digital image. However, four U-data units and four V-data units may also be used, wherein no compression is obtained compared to the RGB-representation.

#The image is processed as discrete 16×16 image blocks in any order. Each data unit of each component is transformed using the discrete cosine transform (DCT), step 12. Since each data unit of each component comprises 64 pixels, the DOT will generate 64 coefficients for the basis functions of the DCT. These coefficients comprise one zeroth order coefficient (DC-coefficient) and 63 coefficients of higher order (AC-coefficients).