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 negative image of an 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, we allocate the block of memory as per the imageSize, to read the image information.

int imgDataSize = width * height; // calculate image size

imageData = (unsigned char*)malloc (imgDataSize * sizeof(unsigned char)); // allocate the block of memory as big as the image size
newimageData = (unsigned char*)malloc (imgDataSize * sizeof(unsigned char));

Now, to create the negative image, we need to have the complement of the color which it already has. The input image in our case is a gray-scale image, hence we know that the pixel value would be between 0-255. To find the complement, we just subtract the read image value from 255. The basic equation would be something like,

new_pixel_data = 255 - old_pixel_data;

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

for(i = 0; i < height; i++)
{
	for(j = 0; j < width; j++)
	{                   
		    newimageData[i*width + j] = 255 - imageData[i*width + j]; 
	}   
}

Now that we have the new image data in our newimageData array, it’s time to construct the output image.

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 write the newimageData information to the output image

fwrite( newimageData, sizeof(unsigned char), imgDataSize, fo); // write the values of the negative image.

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

fclose(fo);
fclose(fp);

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

If are able to create the image as above, you have succesfully written your program for image negation!