Research on Information Hiding Technology Based on Color Digital Still Images(3)

Based on the above discussion, the different application background and corresponding technical requirements of information hiding technology are summarized in Table 2.1.

Table 2.1 Different Applications of Information Hiding Technology Background and Corresponding Technical Requirements Technical Requirements Robust Watermarking

Technical requirements Robust watermarks (copyright protection) Fragile watermarks (tampered verification) Robustness of extended data requirements Strong or weak intentional attacks Strong or weak Non-geometric distortion (filtering, compression, etc.) strong weaker Stronger geometry is not required Distortion transformation (affine transformation, shearing, etc.) Stronger strong Stronger distortion-free transform Stronger Stronger Smaller amount of embedded data Small Large insignificant signal Signature Good

In addition to different application backgrounds, there are other classification methods for information hiding technology. According to different embedded data hiding methods, information hiding technology can be divided into the following two categories:

(1) Information is embedded in the transform domain, such as the Fourier transform (FFT) domain, the discrete cosine transform (DCT) domain, and the discrete wavelet transform (DWT) domain. Such techniques mainly embed data by modifying certain specified frequency domain coefficients of the main signal. Considering that the modification of the low frequency region coefficients may affect the perception effect of the main signal, and the high frequency coefficients are easily destroyed, the information hiding technology generally selects coefficients on the intermediate frequency region of the signal to embed the signature signal, thereby making it satisfied Imperceptibility, in turn, satisfies robustness to operations such as distortion compression.

(2) Direct embedding of information in the Spatial Domain. The advantage of this kind of method is fast, and it has certain resistance to the operation of the geometric transformation and compression of the main signal, but the robustness to signal filtering, noise and other operations is poor.

According to whether or not the original primary signal without hidden data is required during the detection process, the information hiding technology can be classified into two categories: blind retrival and non-blind retrieving. Because there is a natural dependency between data embedding and data detection or extraction in information hiding technology, the possibility of data recovery must be considered when designing the embedding algorithm. Obviously, if the data is detected without the original primary signal embedded with the signature signal, then as long as the designed embedding algorithm is reversible, and at the same time, it depends on certain signal detection technology, it can theoretically guarantee the success of the detection algorithm. However, if the original primary signal is unknown, the information hiding detection or extraction algorithm will be more complicated to design. While using signal detection technology, it also depends on signal estimation and prediction techniques, as well as clever algorithm design strategies. Because blind extraction information hiding technology has more practical value, it has also received more and more attention.

3 BMP file format for color images
The BMP image file format is a standard image format set by Microsoft for its WINDOWS environment and contains a set of image processing API functions. With the popularity of WINDOWS worldwide, BMP file formats are increasingly supported by various application software. A BMP image file is a bitmap file. The bitmap represents an image divided into grids. Each point of the grid is called a pixel. Each pixel has its own RGB value, that is, an image is composed of a series of pixels. The dot formation. The bitmap file format supports 4-bit RLE (run length encoding) and 8-bit and 24-bit encoding. In this article we only deal with 24-bit format. The structure features of the 24-bit BMP image file are: 1 Each file can only store one color image without compression; 2 The file header is composed of 54 bytes of data segments, which contains the type, size, and size of the bitmap file. Image size and print format; 3 Beginning with the 55th byte, this is the image data portion of the file. The data is arranged in the order of the bottom left corner of the image, from left to right, from bottom to top, every 3 consecutive times. The byte describes the color information of one pixel of the image. The three bytes respectively represent the brightness of the blue, green, and red primary colors in the pixel. If a consecutive three bytes are: 00H, 00H, FFH, it means that The color of this pixel is pure red. Below we discuss in detail the specific structure of the bitmap file.

3.1, The header of the bitmap file contains the type and size information of the bitmap file and the layout information. The structure is as follows [2]:
Typedef struct tagBITMAPFILEHEADER {
UINT bfType;
DWORD bfSize;
UINT bfReserved1;
UINT bfReserved2;
DWORD bfOffBits;
}BITMAPFILEHEADER;

The following is a description of the code elements in this list:
bfType: Specifies the file type, whose value is always "BM".
bfSize: Specifies the size of the entire file in bytes.
bfReserved1: Reserved - Normally 0.
bfReserved2: Reserved - Normally 0.
bfOffBits: Specifies the byte offset from the BitmapFileHeader to the image header.

We now know that the purpose of the bitmap header is to identify the bitmap file. Each program that reads a bitmap file uses a bit header for file verification.

3.2. The header of the bitmap information header is called the information header, which contains the attributes of the image itself. The following describes how to specify the size and color format of a Windows 3.0 (or later) device-independent bitmap (DIB):
Typedef struct tagBITMAPINFOHEADER {
DWORD biSize;
LONG biWidth;
LONG biHeight;
WORD biPlanes;
WORD biBitCount;
DWORD biCompression;
DWORD biSizeImage;
LONG biXPelsPerMeter;
LONG biYPelsPerMeter;
DWORD biClrUsed;
DWORD biClrimportant;
} BITMAPINFOHEADER;

Each element of the above code listing is described as follows:
biSize: Specifies the number of bytes required by the BITMAPINFOHEADER structure.
biWidth: Specifies the width of the bitmap in pixels.
biHeight: Specifies the height of the bitmap in pixels.
biPlanes: Specifies the number of planes of the target device. The value of this member variable must be 1.
biBitCount: Specifies the number of bits per pixel. Its value must be 1, 4, 8, or 24.
biCompression: Specifies the compression type of the compression bitmap. In 24-bit format, this variable is set to 0.
biSizeImage: Specifies the size of the image in bytes. If the format of the bitmap is BI_RGB, setting this member variable to 0 is valid.
biXPelsPerMeter: Specifies the horizontal resolution (in pixels/meters) of the target device for the bitmap. The application can use this value to select a bitmap from the group of resources that best fits the current device characteristics.
biYPelsPerMeter: Specifies the vertical resolution (in pixels/meters) of the target device for the bitmap.
biClrUsed: Specifies the number of color indexes in the color table that the bitmap actually uses. If biBitCount is set to 24, biClrUsed specifies the reference color table that is used to optimize Windows palette performance.
biClrimportant: Specifies the number of color indexes that have a significant effect on the display of bitmaps. If this value is 0, all colors are important.

All the information needed to create an image is now defined.