Blur effect on bitmap using C

Question

I'm writing program applying blur filter on mp4. I'm exctracting bmp files from mp4 with ffmpeg. But the blur result is wrong. Some parts of picture have been correctly blurred but other parts have wrong colors.

original image http://i.imgur.com/XS4yqNd.jpg

blurred image http://i.imgur.com/IbcxxA4.jpg

This is the code for reading bmp, applying blur and writing to bmp file.

int blur(char* input, char *output) {

  //variable dec:
  FILE *fp,*out;
  bitmap_header* hp;
  int n,x,xx,y,yy,ile, avgR,avgB,avgG,B,G,R;
  char *data;
  int blurSize = 5;


  //Open input file:
  fp = fopen(input, "r");
  if(fp==NULL){
    //cleanup
  }

  //Read the input file headers:
  hp=(bitmap_header*)malloc(sizeof(bitmap_header));
  if(hp==NULL)
    return 3;

  n=fread(hp, sizeof(bitmap_header), 1, fp);
  if(n<1){
    //cleanup
  }
  //Read the data of the image:
  data = (char*)malloc(sizeof(char)*hp->bitmapsize);
  if(data==NULL){
    //cleanup
  }

  fseek(fp,sizeof(char)*hp->fileheader.dataoffset,SEEK_SET);
  n=fread(data,sizeof(char),hp->bitmapsize, fp);
  if(n<1){
    //cleanup
  }

for(xx = 0; xx < hp->width; xx++)
{
    for(yy = 0; yy < hp->height; yy++)
    {
        avgB = avgG = avgR = 0;
        ile = 0;

        for(x = xx; x < hp->width && x < xx + blurSize; x++)
        {


            for(y = yy; y < hp->height && y < yy + blurSize; y++)
            {
                avgB += data[x*3 + y*hp->width*3 + 0];
                avgG += data[x*3 + y*hp->width*3 + 1];
                avgR += data[x*3 + y*hp->width*3 + 2];
                ile++;
            }
        }

        avgB = avgB / ile;
        avgG = avgG / ile;
        avgR = avgR / ile;

        data[xx*3 + yy*hp->width*3 + 0] = avgB;
        data[xx*3 + yy*hp->width*3 + 1] = avgG;
        data[xx*3 + yy*hp->width*3 + 2] = avgR;
    }
}

    //Open output file:
  out = fopen(output, "wb");
  if(out==NULL){
    //cleanup
  }

  n=fwrite(hp,sizeof(char),sizeof(bitmap_header),out);
  if(n<1){
    //cleanup
  }
  fseek(out,sizeof(char)*hp->fileheader.dataoffset,SEEK_SET);
  n=fwrite(data,sizeof(char),hp->bitmapsize,out);
  if(n<1){
    //cleanup
  }

  fclose(fp);
  fclose(out);
  free(hp);
  free(data);
  return 0;
}

I don't know where is the problem. Please help!

Answer 1

It seems char is signed on your platform. Because of this all color values bigger than 127 are interpreted as negative numbers (256 less than original value). This causes all weird color effects.

To correct it you can declare data as unsigned char*:

unsigned char *data;