get Horizontal Resolution from .tif in C++ - c++

Question

When a person right-clicks a .tif file, then clicks Properties, then clicks the Details tab, they can see Horizontal Resolution. My question is: How can I read this number in from a .tif file using C++?
deAngel, I tried your code on a specific .tif image I have and ithe code froze. Could you send me an e-mail to steve61706#yahoo.com and then I can send you the .tif file that caused the code to freeze? I really need this to work. Thank you so much.

Answer 1

Use libTIFF
Read the fields (sample code here)
Profit!

Answer 2

If you just want to get one or two fields from the header then adding libtiff may be overkill. In this case you can parse the file yourself and locate the tags that you want, the format is quite simple to parse. See the file format details and tag reference.

Answer 3

Below is an example which reads the horizontal and vertical resolution from a TIFF file.
As reference I used the TIFF Revision 6.0 sections 2 and 8 (TIFF structure, IFD structure and data types).
Please note: the example only treats the LSB byte order case and only the first image in the TIFF file. A good implementation must also take into account the MSB order and multiple images per file (which could have different resolutions). I appologize for bugs that may have slipped in (unintentionally). Source was compiled with Borland C++ 3.1. I tried to keep it as simple as I could...
#include <iostream.h>
#include <fstream.h>
//converts a 4-byte LSB order array to ULong
unsigned long lsbToUL(unsigned char *buff)
{
unsigned long val=0;
int i=0;
for(i=3;i>0;i--)
val=(val+buff[i])<<8;
val=val+buff[0];
return val;
}
//converts a 2-byte LSB order array to UInt
unsigned int lsbToUI(unsigned char *buff)
{
unsigned int val=0;
val=val+buff[1];
val=val<<8;
val=val+buff[0];
return val;
}
//reads the resolution values of the first image of a TIFF file
void tiffRes(char *path, double *xRes, double *yRes, int *unit)
{
unsigned int XRES=0x011A; //TIFF XResolution tag
unsigned int YRES=0x011B; //TIFF YResolution tag
unsigned int UNIT=0x0128; //TIFF Resolution Unit tag
unsigned char buff[5];
unsigned long seekAddr=0;
unsigned int tag=0;
unsigned long xAddr=0,yAddr=0;
unsigned int resUnit=0;
ifstream f;
f.open(path,ios::binary);
if(f.fail())
{
cout<<"File error"<<endl;
return;
}
f.get(buff,3); //reads the byte order
if(buff[0]==0x49 && buff[1]==0x49)
{
//this example treats the LSB case
f.seekg(4); //seeks the offset of first IFD from header
f.get(buff,5); //reads the address of the first IFD
seekAddr=lsbToUL(buff);
seekAddr=seekAddr+2;
f.seekg(seekAddr); //seeks the addr of the first TIFF tag
//skipping the no. of directory entries as it is not used
f.get(buff,3); //reads the first TIFF tag
tag=lsbToUI(buff);
//reading the file for XRES,YRES and UNIT TIFF tags
//should be a better way of doing this
while(xAddr==0 || yAddr==0 || resUnit==0)
{
if(tag==UNIT || tag==XRES || tag==YRES)
{
if(tag==UNIT)
{
f.seekg(seekAddr+8);
f.get(buff,3); //read the Resolution Unit
resUnit=lsbToUI(buff);
*unit=resUnit;
}
else
{
f.seekg(seekAddr+8);
f.get(buff,5); //reads the address of the XRes or YRes
if(tag==XRES)
xAddr=lsbToUL(buff); //XRes address
else
yAddr=lsbToUL(buff); //YRes address
}
}
seekAddr=seekAddr+12; //computes the address of next IFD
f.seekg(seekAddr); //seeks the next IFD
f.get(buff,3); //reads the next tag
tag=lsbToUI(buff);
}
//actual reading of the resolution values
//X resolution
f.seekg(xAddr);
f.get(buff,5);
*xRes=lsbToUL(buff);
f.seekg(xAddr+4);
f.get(buff,5);
*xRes=*xRes/(double)lsbToUL(buff);
//Y resolution
f.seekg(yAddr);
f.get(buff,5);
*yRes=lsbToUL(buff);
f.seekg(yAddr+4);
f.get(buff,5);
*yRes=*yRes/(double)lsbToUL(buff);
f.close();
}
else
{
//Not covered by the example
f.close();
if(buff[0]==0x4D && buff[1]==0x4D)
cout<<"MSB mode used. Not covered by the example..."<<endl;
else
cout<<"Unkown byte order..."<<endl;
}
}
void main()
{
double *xRes=new double;
double *yRes=new double;
int *unit=new int;
typedef char string[8];
string resUnit[4]={"","NO_UNIT","INCH","CM"};
char *path="test.tif";
*xRes=0;
*yRes=0;
*unit=0;
tiffRes(path,xRes,yRes,unit);
cout<<"UNIT: "<<resUnit[(*unit)]<<endl;
cout<<"X res: "<<(*xRes)<<endl;
cout<<"Y res: "<<(*yRes)<<endl;
}