Question

I’m trying use this code to get image resolution from file

bool GetImageSizeEx(const char *fn, int *x,int *y)
{ 
    FILE *f=fopen(fn,"rb"); 

    if (f==0) return false;

    fseek(f,0,SEEK_END); 
    long len=ftell(f); 
    fseek(f,0,SEEK_SET); 

    if (len<24) {
        fclose(f); 
        return false;
    }

  // Strategy:
  // reading GIF dimensions requires the first 10 bytes of the file
  // reading PNG dimensions requires the first 24 bytes of the file
  // reading JPEG dimensions requires scanning through jpeg chunks
  // In all formats, the file is at least 24 bytes big, so we'll read that always
    unsigned char buf[24]; fread(buf,1,24,f);


  // For JPEGs, we need to read the first 12 bytes of each chunk.
  // We'll read those 12 bytes at buf+2...buf+14, i.e. overwriting the existing buf.
    if (buf[0]==0xFF && buf[1]==0xD8 && buf[2]==0xFF && buf[3]==0xE0 && buf[6]=='J' && buf[7]=='F' && buf[8]=='I' && buf[9]=='F')
    {
        long pos=2;
        while (buf[2]==0xFF)
        {
            if (buf[3]==0xC0 || buf[3]==0xC1 || buf[3]==0xC2 || buf[3]==0xC3 || buf[3]==0xC9 || buf[3]==0xCA || buf[3]==0xCB) 
                break;

            pos += 2+(buf[4]<<8)+buf[5];
            if (pos+12>len) break;
            fseek(f,pos,SEEK_SET); 
            fread(buf+2,1,12,f);   
        }
    }


    fclose(f);

  // JPEG: (first two bytes of buf are first two bytes of the jpeg file; rest of buf is the DCT frame
    if (buf[0]==0xFF && buf[1]==0xD8 && buf[2]==0xFF)
    {     
        *y = (buf[7]<<8) + buf[8]; 
        *x = (buf[9]<<8) + buf[10]; 

        return true;
    }

  // GIF: first three bytes say "GIF", next three give version number. Then dimensions
    if (buf[0]=='G' && buf[1]=='I' && buf[2]=='F')
    { 
        *x = buf[6] + (buf[7]<<8);
        *y = buf[8] + (buf[9]<<8);
        return true;
    }

  // PNG: the first frame is by definition an IHDR frame, which gives dimensions
    if ( buf[0]==0x89 && buf[1]=='P' && buf[2]=='N' && buf[3]=='G' && buf[4]==0x0D && buf[5]==0x0A && buf[6]==0x1A && buf[7]==0x0A && buf[12]=='I' && buf[13]=='H' && buf[14]=='D' && buf[15]=='R')
    {
        *x = (buf[16]<<24) + (buf[17]<<16) + (buf[18]<<8) + (buf[19]<<0);
        *y = (buf[20]<<24) + (buf[21]<<16) + (buf[22]<<8) + (buf[23]<<0);

        return true;
    }

    return false;
}

return always x == 26112 and y == 30825 for all images any type and any resolution

I compiled with VS 2010 SP1 on Win7 x64

Update

I’m trying to open jpg files which was created by Photoshop. Photoshop encode JPEG Files with Exif Metadata

Debugger show (right click to fullsize)

so code look now:

if ((buf[0]==0xFF && buf[1]==0xD8 && buf[2]==0xFF && buf[3]==0xE0 && buf[6]=='J' && buf[7]=='F' && buf[8]=='I' && buf[9]=='F') ||
        (buf[0]==0xFF && buf[1]==0xD8 && buf[2]==0xFF && buf[3]==0xE1 && buf[6]=='E' && buf[7]=='x' && buf[8]=='i' && buf[9]=='f'))
    {
        long pos=2;
        while (buf[2]==0xFF)
        {
            if (buf[3]==0xC0 || buf[3]==0xC1 || buf[3]==0xC2 || buf[3]==0xC3 || buf[3]==0xC9 || buf[3]==0xCA || buf[3]==0xCB) 
                break;

            pos += 2+(buf[4]<<8)+buf[5];
            if (pos+12>len) break;
            fseek(f,pos,SEEK_SET); 
            fread(buf+2,1,12,f);   
        }
    }

Answer 1

I ran your code for all three formats and it works perfectly fine for various resolutions! Only thing I would suggest is call your GetImageSizeEx appropriately (Escaping the backslash)

Something like

int x,y;

GetImageSizeEx("C:\\Users\\IBM_ADMIN\\Desktop\\pavan2.png", &x, &y);
printf("%d %d", x, y );

A few possible cases for the failure your noticing might be that the fopen itself is failing because of some wrong filename etc and x and y are never modified inside GetImageSizeEx. Or another case wherein you read a wrong file and you are not able to read the appropriate headers and hence again, x and y are not modified at all! And may be you never initialized x and y in your calling function and hence some garbage is being shown?

PS: I compiled it with gcc 4.6.1 mingw on Windows 7