1. Field of the Invention
The present invention relates to an image display system, and more specifically, relates to scramble processing of image data to protect content in a multi-image display.
2. Description of the Related Art
In recent years, in the field of AV equipment such as a video recording apparatus, broadcast monitors, and TV monitors, there has been a trend towards digitalization and higher resolution. As a result, illegal copying of copyright content has been perceived as a problem, and some sort of protection function has come to be employed. For example, when an image is output from a digital broadcasting tuner or a HD video recording apparatus to a TV monitor, a digital interface (I/F) having a content protection function referred to as HDMI (High-Definition Multimedia Interface) is used. Further, there is a model in which an image input processing unit and a display apparatus are placed in separate housings and the image is transferred from the image input processing unit to the display apparatus. Some sort of content protection for an image transfer is also required in such a model.
A TV monitor displays the image data input by this HDMI. Other types of TV monitor receive video input from a plurality of sources and can perform a multi-window display by scaling down the video inputs to arbitrary sizes, and displaying them on one screen at the same time. Japanese Patent Application Laid-Open No. 2000-217079 discusses content protection in a case where an image having protection and an image requiring no protection are multi-image synthesized at the image input processing unit and an image is transferred to a display apparatus. The image having protection and the image requiring no protection are multi-image synthesized so as to output the synthesized image to a display apparatus after protection is applied.
However, in the above case, if the image requiring no protection is input from an image source where the pixel data value of image data is known in advance, the type of data the known image data is converted into can be analyzed. Thus, the scrambling method of the image originally having protection can be found out. An image from the image source, in which the pixel data value of the image data is known in advance, is, for example, a pattern image from a test pattern generator. Generally speaking, sufficient encryption processing is performed in the authentication processing between equipment. However, in the case of uncompressed image data that requires the transmission of vast amounts of data in real time, complicated scramble processing is not possible. Hence, by analyzing the type of data the known image data is converted into, it is possible to illegally descramble the scrambled image data.
As a specific example, shown in FIG. 3, in the case of a system where a user can arbitrarily set up a window size, an image 3(a) having protection is slightly reduced (for example, approximately by 1%), and at the back of that image, an image 3(b) of a ramp waveform is disposed. As to the image 3(b), the value of the image data is incremented by for each pixel. Thus, synthesized image 3(c) is outputted. In a headmost line, the image data applied with a scramble is incremented as an address value in order and stored in a memory. Then, the relationship between the address and the data of the memory becomes the relationship between the scramble data and the original data, and thereby constitutes a data table for descrambling.
If the image data in the subsequent line is substituted one by one with reference to this data table, all scrambles are released, and the scramble of the image 3(a) requiring content protection is also released. Even in a case where the scramble method is changed at intervals of several frames, if a data table is prepared for descrambling for each frame, it is possible to descramble the scrambled image data.