RLE algorithm applied to bmp file - c

Question

I am currently facing issues when trying to apply the run-length algorithm to a .bmp picture. When I am doing the encoding, all is working well, but at decoding there is a problem and I cannot figure it out, namely I cannot read the number of repetitive pixels and thus my output is a blank image. When I use the debugger, the number of repetitive pixels is not readed correctly from the binary file (the result of applying RLE on the bitmap file). Here is my code
#include<stdlib.h>
#include <stdint.h>
//structure defiens bitmap header
struct BITMAPFILEHEADER{
uint8_t type[2];//type of file (bit map)
uint32_t size;//size of file
uint16_t reserved1;//
uint16_t reserved2;//
uint32_t offsetbits;//off set bits
} __attribute__ ((packed));
struct BITMAPINFOHEADER{
uint32_t size;//bitmap size
// uint16_t w2;
uint32_t width;//width of bitmap
//uint16_t h2;
uint32_t height;//hight of bitmap
uint16_t planes;
uint16_t bitcount;
uint32_t compression;// compression ratio (zero for no compression)
uint32_t sizeimage;//size of image
long xpelspermeter;
long ypelspermeter;
uint32_t colorsused;
uint32_t colorsimportant;
} __attribute__ ((packed));
//const char* INPUT_FILE = "/home/bogdan/bee.bmp";
const char* INPUT_FILE = "/home/bogdan/Linux.bmp";
const char* ENCODED_FILE = "/home/bogdan/encoded.bin";
const char* DECODED_FILE = "/home/bogdan/decoded.bmp";
typedef struct SINGLE_PIXEL{
uint8_t green;//Green level 0-255
uint8_t red; //Red level 0-255
} PIXEL;
int comparePixels(PIXEL, PIXEL);
void encode();
void decode(char*);
int main()
{
encode();
decode(ENCODED_FILE);
return 0;
}
void encode() {
uint32_t i=0;//to count pixels read
uint32_t pixno=0;//number of pixels to read
struct BITMAPFILEHEADER source_head;//to store file header
struct BITMAPINFOHEADER source_info;//to store bitmap info header
PIXEL pixel;// the current pixel
FILE *in;// bitmap imput pointer file
FILE *out;//output file pointer
if(!(in=fopen(INPUT_FILE,"rb")))//open in binary read mode
{
printf("\ncan not open file");//error at opening file
exit(-1);
}
out=fopen(ENCODED_FILE,"wb");//opne in binary write mode
//read the headers to source file
fread(&source_head,sizeof(struct BITMAPFILEHEADER),1,in);
fread(&source_info,sizeof(struct BITMAPINFOHEADER),1,in);
//write the headers to the output file
fwrite(&source_head,sizeof(struct BITMAPFILEHEADER),1,out);
fwrite(&source_info,sizeof(struct BITMAPINFOHEADER),1,out);
//cumpute the number of pixels to read
pixno=source_info.width*source_info.height;
// init list of pixels
PIXEL pixArr[pixno];
printf("total pixels: %d", pixno);
//printf("w:%f h:%u pn:%lu", (source_head.size/1024.0/1024), source_info.height, pixno);
uint32_t sum = 0;
//read, modify and write pixels
for(i=0;i<pixno;++i)
{
//read pixel form source file
fread(&pixel,sizeof(PIXEL),1,in);
pixArr[i] = pixel;
}
for (i = 0; i < pixno; i++) {
// printf ("i = %d\tred = %d green = %d blue = %d\n",i, pixArr[i].red, pixArr[i].green, pixArr[i].blue);
int runlength = 1;
while ((i + 1) < pixno) {
if (comparePixels(pixArr[i], pixArr[i+1]) == 0){
// printf ("i = %d\t red = %d green = %d blue = %d\n",i, pixArr[i].red, pixArr[i].green, pixArr[i].blue);
runlength++;
i++;
} else {
break;
}
}
//fprintf(out, "%d", runlength);
fwrite(&runlength, sizeof(runlength), 1, out);
fwrite(&pixel,sizeof(PIXEL),1,out);
sum += runlength;
runlength = 0;
}
//write the modification to the output file
//close all fiels
fclose(in);
fclose(out);
printf("sum = %d",sum);
}
void decode(char * filePath) {
uint32_t i=0;//to count pixels read
uint32_t j=0;
uint32_t totalPixels=0;//number of pixels to read
uint32_t pixelRepetition = 1;
struct BITMAPFILEHEADER source_head;//to store file header
struct BITMAPINFOHEADER source_info;//to store bitmap info header
PIXEL pixel;// the current pixel
FILE *in;// bitmap encoded pointer file
FILE *out;//decoded bitmap file pointer
if (!(in = fopen(filePath, "rb"))) {
printf("\ncan not open file");
exit(-1);
}
out = fopen(DECODED_FILE, "wb");
//read the headers to source file
fread(&source_head,sizeof(struct BITMAPFILEHEADER),1,in);
fread(&source_info,sizeof(struct BITMAPINFOHEADER),1,in);
//write the headers to the output file
fwrite(&source_head,sizeof(struct BITMAPFILEHEADER),1,out);
fwrite(&source_info,sizeof(struct BITMAPINFOHEADER),1,out);
totalPixels=source_info.width*source_info.height;
while(i < totalPixels) {
fread(&pixelRepetition, sizeof(pixelRepetition), 1, out);
fread(&pixel,sizeof(PIXEL),1,in);
for (j = 0; j < pixelRepetition; j++) {
fwrite(&pixel,sizeof(PIXEL),1,out);
}
i += pixelRepetition;
}
fclose(in);
fclose(out);
}
int comparePixels(PIXEL px1, PIXEL px2) {
if (px1.red == px2.red && px1.green == px2.green && px1.blue == px2.blue) {
return 0;
} else {
return -1;
}
}
My implementation is working as follow: I read the header of a bmp file and put it directly to another file. After that I compare pixels to see if they have the same RGB values. If this is true (according to comparePixels function) I put the number of consecutive identical pixels and one pixel (the one which is repeated) into the file. This is the encode phase. At decoding, I am reading the header of the image and then I am trying to read the number of repetitions (1 default, means no repetitive pixels) and the pixel which is repeated. I will really appreciate any type of help. Thank you.

Answer 1

You are repeatedly writing out only the very last pixel you read in:
fwrite(&runlength, sizeof(runlength), 1, out);
fwrite(&pixel,sizeof(PIXEL),1,out);
You need to write out your current pixel instead:
fwrite(pixArr[i], sizeof(PIXEL),1,out);