Raster Data Tutorial（光栅数据格式教程-8位）

Raster Data Tutorial

Author: Bill Green (2002)
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INTRODUCTION

A BMP computer image is the easiest to understand because it does not use compression, making pixel data retrieval much easier. The table below shows how the pixel data is stored from the first byte to the last.

一个BMP计算机图像是最容易理解的,因为它不使用压缩,同时也使得读取图像像素点变得容易。下面的表显示了像素数据从第一个字节到最后的存储。

TABLE 1: BMP File StructureByte # to fseek file pointerInformation0Signature（标示）2File size（文件大小）18Width (number of columns)22Height (number of rows)28Bits/pixel（分辨率）46Number of colors used（颜色数）54Start of color table（颜色表）54 + 4*(number of colors)Start of raster data（数据） 

The first 14 bytes are dedicated to the header information of the BMP. The next 40 bytes are dedicated towards the info header, where one can retrieve such characteristics as width, height, file size, and number of colors used. Next, is the color table, which is 4 x (number of colors used) bytes long. So for an 8-bit grayscale image (number of colors is 256), the color table would be 4 x 256 bytes long or 1024 bytes. And the last bit of data in a BMP file is the pixel data, or raster data. The raster data starts at byte 54 (header + info header) + 4 x number of colors (color table). For an 8-bit grayscale image, the raster data would start at byte 54 + 1024 = 1078. The size of the raster data is (width x height) � 1 bytes. Therefore, a 100 row by 100 column 8-bit grayscale image would have (100 x 100) � 1 = 9,999 bytes of raster data starting at byte 1078 and continuing to the end of the BMP.

在上表中的数据中，前十四个字节是BMP图像的头信息。接下来的四十字节是文件头部，包括了图像的宽度、高度、文件大小和使用的颜色数。向后走是颜色表，它的长度为4*(颜色数)的字节长。对于8位的灰度图，颜色表的长度是4*255字节长。最后是BMP数据块或者光栅数据。对于光栅数据而言，它的头部大小等于54（头信息+文件头部）+4*颜色数。对于8位的灰度图，它的数据块的起始位置是54+1024=1078.数据块的大小为(width x height) - 1 bytes。因此对于100*100的图像来说，它的数据块大小为9999字节，从文件的第1078字节处开始到最后。

In terms of image processing, the most important information is the following:
(1) Number of columns � byte #18
(2) Number of rows - byte #22
(3) Raster data � byte #(4 x number of colors) to byte #1078 + (number of columns x number of rows) � 1

In C, the most efficient way of declaring this important information is that of struct.在c语言中，声明如下的结构

typedef struct { int rows; /* number of rows */  int cols; /* number of columns */  unsigned char* data;  /* raster data */}  sImage;

READING BMP RASTER DATA   读取BMP数据

TEST.bmp is a 20 row by 20 column BMP image which we will use to read raster data from. In an 8 bit BMP image, black is 0 and white is 255. The top left corner of TEST.bmp starts at a pixel value of 0 (black) and progressively works its way down the diagonal to pixel value of 255 (white). Thinking of rows and columns in a BMP is not the same as thinking of rows and columns in a matrix. In a matrix row 0 and column 0 would start you at the top left corner of the matrix. However, in a BMP, the rows increase from bottom to top. Therefore, row 0 and column 0 in a BMP would correspond to the bottom left corner.

TEST.bmp是一个20*20的BMP图像，我们接下来将读取这幅图。在8位的BMP图像中，0表示黑和255表示白。如下图测试图像，它的开始像素为0然后逐步沿着斜线像素值增大到255。在BMP图像中的行和列与矩阵的行和列不同。在矩阵中，0行和0列在矩阵的左上角。然而，在BMP图像中，行增加是从下到上。所以，0行和列0在BMP对应于左下角。

(TEST.bmp is scaled up here to a 100 by 100 BMP, so be sure and download the zip file to test out your raster data program.)

TEST.bmp contains 20 rows and 20 columns, so we know we will have 400 bytes of raster data. We also know the raster data will start at byte #(54 + 4 x number of colors). The number of colors of TEST.bmp is 256 because it is a grayscale image with colors ranging from 0 to 255. Therefore, the raster data will start at byte #1078 and the file size will be 1078 + 400 = 1478 bytes. Knowing this, let抯 try our first program to read raster data and print it to a text file.

TEST.bmp是20*20的，所以我们知道这幅图的数据块大小为400字节，同时，我们可以知道，数据块的开始是在文件的第54+4*颜色数。对于这幅侧视图，由于它是灰度图，所以他的颜色数位256，灰度值从0~255变化。因袭，该图像的数据块从第1478字节开始。

To be compiled with Turbo CNote: downloadraster.zip rather than cutting and pasting from below.测试程序

#include (stdio.h)#include (stdlib.h)#include (math.h)/*-------STRUCTURES---------*/typedef struct {int rows; int cols; unsigned char* data;} sImage;/*-------PROTOTYPES---------*/long getImageInfo(FILE*, long, int);int main(int argc, char* argv[]){  FILE*bmpInput, *rasterOutput;  sImageoriginalImage;  unsigned charsomeChar;  unsigned char*pChar;  intnColors;  /* BMP number of colors */  longfileSize; /* BMP file size */  intvectorSize; /* BMP vector size */  intr, c;       /* r = rows, c = cols */  /* initialize pointer */  someChar = '0';  pChar = &someChar;  if(argc < 2)  {    printf("Usage: %s bmpInput.bmp\n", argv[0]);    exit(0);  }  printf("Reading filename %s\n", argv[1]);  /*--------READ INPUT FILE------------*/  bmpInput = fopen(argv[1], "rb");  fseek(bmpInput, 0L, SEEK_END);  /*--------DECLARE OUTPUT TEXT FILE--------*/  rasterOutput = fopen("data.html", "w");  /*--------GET BMP DATA---------------*/  originalImage.cols = (int)getImageInfo(bmpInput, 18, 4);  originalImage.rows = (int)getImageInfo(bmpInput, 22, 4);  fileSize = getImageInfo(bmpInput, 2, 4);  nColors = getImageInfo(bmpInput, 46, 4);  vectorSize = fileSize - (14 + 40 + 4*nColors);  /*-------PRINT DATA TO SCREEN-------------*/  printf("Width: %d\n", originalImage.cols);  printf("Height: %d\n", originalImage.rows);  printf("File size: %ld\n", fileSize);  printf("# Colors: %d\n", nColors);  printf("Vector size: %d\n", vectorSize);  /*----START AT BEGINNING OF RASTER DATA-----*/  fseek(bmpInput, (54 + 4*nColors), SEEK_SET);  /*----------READ RASTER DATA----------*/  for(r=0; r<=originalImage.rows - 1; r++)  {    for(c=0; c<=originalImage.cols - 1; c++)    {      /*-----read data and print in (row,column) form----*/      fread(pChar, sizeof(char), 1, bmpInput);      fprintf(rasterOutput, "(%d, %d) = %d\n", r, c, *pChar);    }  }  fclose(bmpInput);  fclose(rasterOutput);}/*----------GET IMAGE INFO SUBPROGRAM--------------*/long getImageInfo(FILE* inputFile, long offset, int numberOfChars){  unsigned char*ptrC;  longvalue = 0L;  unsigned chardummy;  inti;  dummy = '0';  ptrC = &dummy;  fseek(inputFile, offset, SEEK_SET);  for(i=1; i<=numberOfChars; i++)  {    fread(ptrC, sizeof(char), 1, inputFile);    /* calculate value based on adding bytes */    value = (long)(value + (*ptrC)*(pow(256, (i-1))));  }  return(value);} /* end of getImageInfo */

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Running your raster data program, you will get an ASCII file called data.txt with some entries looking like the following:

(0, 0) = 255(0, 1) = 255(0, 2) = 255(0, 3) = 255     :(1, 0) = 255(1, 1) = 255(1, 2) = 255(1, 3) = 255     :(2, 0) = 255(2, 1) = 255(2, 2) = 255(2, 3) = 255     :(3, 0) = 255(3, 1) = 255(3, 2) = 255(3, 3) = 255     :(4, 0) = 255(4, 1) = 255(4, 2) = 255     :(4, 14) = 207(4, 15) = 207(4, 16) = 255(4, 17) = 255(4, 18) = 255(4, 19) = 255     :(5, 0) = 255(5, 1) = 255(5, 2) = 255(5, 3) = 255     :(5, 14) = 207(5, 15) = 207     :(6, 0) = 255(6, 1) = 255(6, 2) = 255(6, 3) = 255     :(6, 12) = 159(6, 13) = 159     :(7, 0) = 255(7, 1) = 255(7, 2) = 255(7, 3) = 255     :(7, 12) = 159(7, 13) = 159(7, 14) = 255     :(8, 0) = 255(8, 1) = 255(8, 2) = 255     :(8, 10) = 111(8, 11) = 111(8, 12) = 255     :(9, 0) = 255(9, 1) = 255(9, 2) = 255(9, 3) = 255     :(9, 10) = 111(9, 11) = 111(9, 12) = 255     :(10, 0) = 255(10, 1) = 255(10, 2) = 255(10, 3) = 255     :(10, 8) = 79(10, 9) = 79(10, 10) = 255     :

EXPLANATION

Notice how entry (4, 14) is 207 meaning that the pixel in the row 4, column 14 is very close to white. This is not the case if you are thinking in terms of matrices. Just remember that in BMPs, the raster data is stored from left to right and bottom to top.

从上面的结果中可以看到，（4,14）的像素灰度值为297接近白色，如果在矩阵中，这并非如此。记住,在bmp,光栅数据存储从左到右和从下到上。

The program begins by opening the BMP file entered at the command prompt. For example if your program is called raster.c, at the command prompt you would enter:

raster test.bmp

上面的测试程序中，需要你在命令行中输入BMP的路径。如果你的BMP在工程目录下，就可以直接输入：

raster test.bmp

 

We want to read a binary BMP file which required us to add 搑b� in our fopen statement for read binary. We also have to set the file pointer to the beginning of the BMP file � byte #0. We accomplish these 2 tasks with the following lines of code:若我们想读取二进制文件，需要在文件打开类型中加入“rb”.同时还可以设置读取指针（文件流指针）的位置。以下是代码

bmpInput = fopen(argv[1], "rb");fseek(bmpInput, 0L, SEEK_END);

From the pixel data table we see that byte #18 contains the width of the BMP file. So to get the corresponding number of columns we have to set the file pointer to byte number 18. We also have to set the pointer to read the height, number of colors, file size, vector size, etc. Instead of tediously performing this task repeatedly, it would be much easier to write a subprogram that does it for you each time it is called in main. The subprogram is a function which means it returns a value to main. For example, to get the width of a BMP, all you would have to type is:

originalImage.rows = (int) getImageInfo(bmpInput, 18, 4);

The 18 corresponds to where you want to start reading data from and the 4 corresponds to how many bytes you want to read.

MANIPULATING A BMP FILE  对BMP文件的操作

As our first image processing task, we want to take the TEST.bmp and reflect it. In other words, instead of having decrease from black to white down the diagonal, we want it to decrease from white to black. If the pixel value was black, we want it to be white.

对于我们的第一个任务，想把TESTT.BMP图像反转（颜色取反）。

reflect(k, k) = 255 � TEST(k, k)

Aside from this simple algorithm, reflect.bmp and TEST.bmp are identical, so we would like to copy the header information and the color table from TEST.bmp to reflect.bmp. This can be done with the following 2 procedures. A procedure is a subprogram that does not return a value.

对于这个简单的算法，这两个图像的头部是相同的，所以我们想把同文件头只是复制一下，以下是实现代码：

/*-------------COPIES HEADER AND INFO HEADER----------------*/void copyImageInfo(FILE* inputFile, FILE* outputFile){  unsigned char*ptrC;  unsigned chardummy;  inti;  dummy = �0�;  ptrC = &dummy;  fseek(inputFile, 0L, SEEK_SET);  fseek(outputFile, 0L, SEEK_SET);  for(i=0; i<=50; i++)  {    fread(ptrC, sizeof(char), 1, inputFile);    fwrite(ptrC, sizeof(char), 1, outputFile);  }}
/*----------------COPIES COLOR TABLE-----------------------------*/void copyColorTable(FILE* inputFile, FILE* outputFile, int nColors){  unsigned char*ptrC;  unsigned chardummy;  inti;  dummy = �0�;  ptrC = &dummy;  fseek(inputFile, 54L, SEEK_SET);  fseek(outputFile, 54L, SEEK_SET);  for(i=0; i<=(4*nColors); i++)  /* there are (4*nColors) bytesin color table */  {    fread(ptrC, sizeof(char), 1, inputFile);     fwrite(ptrC, sizeof(char), 1, outputFile);  }}

The following code reflects a BMP file. The differences from raster.c are in red:

To be compiled with Turbo C
Note: download reflect.zip rather than cutting and pasting from below.

#include (stdio.h)#include (stdlib.h)#include (math.h)/*-------STRUCTURES---------*/typedef struct {int rows; int cols; unsigned char* data;} sImage;/*-------PROTOTYPES---------*/long getImageInfo(FILE*, long, int);void copyImageInfo(FILE* inputFile, FILE* outputFile);void copyColorTable(FILE* inputFile, FILE* outputFile, int nColors);int main(int argc, char* argv[]){  FILE*bmpInput, *bmpOutput;  sImageoriginalImage;  unsigned charsomeChar;  unsigned char*pChar;  intnColors;  /* BMP number of colors */  longfileSize; /* BMP file size */  intvectorSize; /* BMP vector size */  intr, c;       /* r = rows, c = cols */  /* initialize pointer */  someChar = '0';  pChar = &someChar;  if(argc < 2)  {    printf("Usage: %s bmpInput.bmp\n", argv[0]);    exit(0);  }  printf("Reading filename %s\n", argv[1]);  /*--------READ INPUT FILE------------*/  bmpInput = fopen(argv[1], "rb");  fseek(bmpInput, 0L, SEEK_END);  /*--------DECLARE OUTPUT FILE--------*/  bmpOutput = fopen("reflect.bmp", "wb");  /*--------GET BMP DATA---------------*/  originalImage.cols = (int)getImageInfo(bmpInput, 18, 4);  originalImage.rows = (int)getImageInfo(bmpInput, 22, 4);  fileSize = getImageInfo(bmpInput, 2, 4);  nColors = getImageInfo(bmpInput, 46, 4);  vectorSize = fileSize - (14 + 40 + 4*nColors);  /*-------PRINT DATA TO SCREEN-------------*/  printf("Width: %d\n", originalImage.cols);  printf("Height: %d\n", originalImage.rows);  printf("File size: %ld\n", fileSize);  printf("# Colors: %d\n", nColors);  printf("Vector size: %d\n", vectorSize);  copyImageInfo(bmpInput, bmpOutput);  copyColorTable(bmpInput, bmpOutput, nColors);  /*----START AT BEGINNING OF RASTER DATA-----*/  fseek(bmpInput, (54 + 4*nColors), SEEK_SET);  /*----------READ RASTER DATA----------*/  for(r=0; r<=originalImage.rows - 1; r++)  {    for(c=0; c<=originalImage.cols - 1; c++)    {      /*-----read data, reflect and write to output file----*/      fread(pChar, sizeof(char), 1, bmpInput);      if(*pChar == 255) *pChar = 255;      else *pChar = 255 - *pChar;      fwrite(pChar, sizeof(char), 1, bmpOutput);    }  }  fclose(bmpInput);  fclose(bmpOutput);}/*----------GET IMAGE INFO SUBPROGRAM--------------*/long getImageInfo(FILE* inputFile, long offset, int numberOfChars){  unsigned char*ptrC;  longvalue = 0L;  unsigned chardummy;  inti;  dummy = '0';  ptrC = &dummy;  fseek(inputFile, offset, SEEK_SET);  for(i=1; i<=numberOfChars; i++)  {    fread(ptrC, sizeof(char), 1, inputFile);    /* calculate value based on adding bytes */    value = (long)(value + (*ptrC)*(pow(256, (i-1))));  }  return(value);} /* end of getImageInfo *//*-------------COPIES HEADER AND INFO HEADER----------------*/void copyImageInfo(FILE* inputFile, FILE* outputFile){  unsigned char*ptrC;  unsigned chardummy;  inti;  dummy = '0';  ptrC = &dummy;  fseek(inputFile, 0L, SEEK_SET);  fseek(outputFile, 0L, SEEK_SET);  for(i=0; i<=50; i++)  {    fread(ptrC, sizeof(char), 1, inputFile);    fwrite(ptrC, sizeof(char), 1, outputFile);  }}/*----------------COPIES COLOR TABLE-----------------------------*/void copyColorTable(FILE* inputFile, FILE* outputFile, int nColors){  unsigned char*ptrC;  unsigned chardummy;  inti;  dummy = '0';  ptrC = &dummy;  fseek(inputFile, 54L, SEEK_SET);  fseek(outputFile, 54L, SEEK_SET);  for(i=0; i<=(4*nColors); i++)  /* there are (4*nColors) bytesin color table */  {    fread(ptrC, sizeof(char), 1, inputFile);     fwrite(ptrC, sizeof(char), 1, outputFile);  }}