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 https://i.stack.imgur.com/q5A6v.jpg

blurred image https://i.stack.imgur.com/hFfFk.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!

When you open the file for reading, shouldn't it be in binary mode? — Some programmer dude, Apr 29 '14 at 15:46
It seems, that you are modifying the same bitmap, that you read, so changes applied to fist pixel propagates to seond etc. i believe, that first few rows or columns are ok, but the rest is discoloured. — user3581454, Apr 29 '14 at 15:46
There is one more thing, in BMP, when number of pixels in row isn't multiple of 4, the extra padding bytes are added (4/16/24). Test your code on bitmap with width being multiplicity of 4. — user3581454, Apr 29 '14 at 15:49
@JoachimPileborg I changed it to binary mode, the effect is the same. — Bartosz Karpiński, Apr 29 '14 at 16:10
@user3581454 Added copy of data on which I modify values - it give the same effect. And it wass 1920x1080 so width is multiplicity of 4. — Bartosz Karpiński, Apr 29 '14 at 16:12

score 4 · Accepted Answer · answered Apr 29 '14 at 16:44

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;

Blur effect on bitmap using C

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