Following the previous blog posts, I hope you now get a pretty clear idea about how to go ahead with image processing in C. In this blog post, we will discuss about creating a gray image from a color image and you will also understand that how to read color image.

For this blog post, we will again use our usual, lena’s face.

The complete code for this post can be found here.

Starting with the usual procedure, image is nothing but a file for C. So go ahead and open the file.

FILE* fp = fopen("/path/to/your/image.bmp", "rb");   //read the file//

Following the Bitmap basics, strip out the image header.

fread(imageHeader, sizeof(unsigned char), 54, fp); // read the 54-byte from fp to imageHeader

Extract the required information from the image header,

• Width
• Height
• bitDepth

// extract image height and width from imageHeader      
int width = *(int*)&imageHeader[18];
int height = *(int*)&imageHeader[22];
int bitDepth = *(int*)&imageHeader[28];

Next, read the colorTable if it exists.

if(bitDepth <= 8)	// COLOR TABLE Present
	fread(colorTable, sizeof(unsigned char), 1024, fp); // read the 1024-byte from fp to colorTable

Now Open the output file and write the image header and colortable to it.

FILE *fo = fopen("/path/to/output/image.bmp", "wb");

fwrite(imageHeader, sizeof(unsigned char), 54, fo); // write the header back.
if(bitDepth <= 8)       			    // COLOR TABLE Present
        fwrite(colorTable, sizeof(unsigned char), 1024, fo); // write the color table back

Now, we will read the image data in buffer from input file using getc which is explained below. For that we will declare two-dimensional buffer to read data character by character using getc.

int imgDataSize = width * height;		//calculate image size
unsigned char buffer[imgDataSize][3];   		
    buffer[i][2] = getc(fp);                   //blue
    buffer[i][1] = getc(fp);                   //green
    buffer[i][0] = getc(fp);                   //red

Now, to convert the RGB image into gray, we need to apply conversion formula of RGB to gray using all red, green and blue pixels. As the input image in our case is a rgb 24-bit (8-bit for each color).We will get character by character values into buffer from all color planes and put these value back to output image character by character.

The basic equation to get character values would be something like,

y = (buffer[i][0]*0.3) + (buffer[i][1]*0.59) + (buffer[i][2]*0.11);	

Now, it’s time to write the C code to convert RGB image to gray image for the same. We know that the image is arranged in terms of rows and columns, hence running for loops should solve our purpose.

for(i=0;i<size;i++)
{
    y = 0;
    buffer[i][2] = getc(fp);					//blue
    buffer[i][1] = getc(fp);					//green
    buffer[i][0] = getc(fp);					//red
		
    y = (buffer[i][0]*0.3) + (buffer[i][1]*0.59) + (buffer[i][2]*0.11); //conversion formula of rgb to gray

    putc(y,fo);
    putc(y,fo);
    putc(y,fo);
}

As shown above, we read the image data character by character so we will write it into file in that way only using putc.

Now that everything is complete, time to close the files.

fclose(fo);
fclose(fp);

Compile the program, and run it. You should have a gray image of lena formed in your specified directory.

If you are able to create the image as above, you have succesfully written your program to convert the RGB image into gray !

Author:- Priya Shah