Description

Lab13. Image Processing
Today’s digital images are composed of individual picture elements, call pixels. With a large number of pixels, the image can be clear. But, with a large number of pixels the image file would need a larger disk space for storage. Thus, most of the popular image file standards involve some compression scheme to reduce the storage overhead. In this lab, we’ll concentrate on one of the simplest color image format, portable pixmap format (PPM).
An image can be represented by a two-dimension array of pixels, the x-direction is the width and y-direction is the height of the image. In PPM, each pixel is represented by 3 bytes for red, green and blue colors. Thus, the total number of color levels is 224, ∼16M. The format of a PPM file is
P6
W H
255
R0G0B0R1G1B1 ··· RNGNBN
Where W and H are two integers and 255 is the intensity level for each color components (R, G, and B). Since each color component R, G, B takes one byte, the intensity level is 255. After the header, the remaining file consists of 3N, N=W×H, bytes, each byte representing the intensity of one of the color components. This file can be read as following: the first line read using a string of two characters, the second line read using “%d %d” for two integers, the third line read using a string of 3 characters, then the remaining file read using 3N characters. Note that the new-line character after the 3rd line should be handled carefully to ensure the image are read in correctly. Also, the pixels are arranged in column-major fashion, instead of row-major matrix storage use by C compiler.
In this lab, you will need to read in an image file, an EE Department logo and the NTHU logo. You will need to covert the original color image to a blue-and-white image, add the EE Department logo to the lower right corner of the image and the NTHU logo to the center of the image. The EE department logo retains its colors, but the NTHU logo modifies the image to a purple tone.
To convert a color image to a black-and-white image, the following method has been popular.
Let Rc, Gc and Bc be the intensities of a color pixel and Rg, Gg, Bg be the intensities of the gray-scale image, then
Rg = Gg = Bg = Rc × 0.2126+ Gc × 0.7152+ Bc × 0.0722.
Note that when R = G = B at a pixel, this pixel has a gray color. And the levels of gray scale is 256. When R = G = B =255, the color is white and when R = G = B =0, the pixel has the color black.
However, in this lab we will convert the image to a blue-and-while image, which can be done simply making the blue component equals to the maximum intensity at all time. Thus, when the gray level of the pixel is 255, we see a white pixel as it should be. And when the gray level is 0, we will see a blue pixel instead. In this way, a blue-and-white image is obtained.
When placing EE Department logo, the pixels of the original image are simple replaced by the logo pixels when the logo pixel is not white. But in placing the NTHU logo, we simply set the red component of the pixel to be 255 when the NTHU logo pixel is not white. This will make the logo purple since both blue and red color components are always on.
Five image files are available for you to test your program: pic1.ppm, pic3.ppm, pic7.ppm, pic8.ppm, and pic9.ppm. The EE Department and the NTHU logos are also given: EElogo.ppm and NTHU.ppm.
Your program needs to read an image file, EE Department log and NTHU logo files and produce an output file. Thus, the execution of your program should be invoked by the following command line
$ ./a.out ∼ee231002/lab13/pic1.ppm ∼ee231002/lab13/EElogo.ppm ∼ee231002/lab13/NTHU.ppm pic1out1.ppm
Where pic1.ppm and pic1_out1.ppm can be replaced by other image input and output files.
FILE *fin,*fout; int k;
fin=fopen(“data.in”,”r”); fout=fopen(“data.out”,”w”); fscanf(fin,”%d”,&k); fprintf(fout,”%d “,k); fclose(fout); fclose(fin);
Of course, your can use fopen(argv[1],”r”); to open a file specified by the command line argument.
Notes.
1. Create a directory lab13 and use it as the working directory.
2. Name your program source file as lab13.c.
3. The first few lines of your program should be comments as the following.
/* EE231002 Lab13. Image Processing
*/
4. After you finish verifying your program, you can submit your source code by
$ ∼ee231002/bin/submit lab13 lab13.c
If you see a ”submitted successfully” message, then you are done. In case you want to check which file and at what time you submitted your labs, you can type in the following command:
$ ∼ee231002/bin/subrec
It will show the last few submission records.
5. You should try to write the program as efficient as possible. The format of your program should be compact and easy to understand. These are part of the grading criteria.
6. An example of data structure to store the PPM image is shown below. You can but not required to use this data structure.
typedef struct sPIXEL { // a single pixel
unsigned char r,g,b;
} PIXEL; // three color components
typedef struct sIMG { // an image of PPM style
char header[3]; // header, either P3 or P6
int W,H; // width and height of the image
int level; // intensity level of each color component
PIXEL **PX;
} IMG; // two-dimensional array for all the pixels