Introduction
Cexif is a small class to read the EXIF data stored in JPEG images, normally generated by digital cameras.
The code is based on Jhead, written by Matthias Wandel. Jhead offers a lot of switches to parse an image with EXIF tags, but it's plain C. I simply rearranged the functions and the global variables into a simple class.
Another useful article on this topic is here [^], with a bit of details on the EXIF structure. The official document for the "Digital Still Camera Image File Format Standard (Exchangeable image file format for Digital Still Cameras: Exif) Version 2.1" can be downloaded from http://www.bbs-informatique.fr/pdf/photo/Exif_2-1.pdf [^].
You can find some EXIF images at http://www.exif.org/samples.html
Using the code
You can use this class in 2 ways. The simplest is:
Cexif exif;
exif.DecodeExif(hFile);
where hFile is a valid file handle. If the file contains EXIF data, exif.m_exifinfo->IsExif is true, and you can read the fields in exif.m_exifinfo to create a report. The second way is:
EXIFINFO m_exifinfo;
memset(&m_exifinfo,0,sizeof(EXIFINFO));
Cexif exif(&m_exifinfo);
exif.DecodeExif(hFile);
In this case the result is stored in the variable m_exifinfo, and you can delete the exif object without losing the EXIF data.
More EXIF tags
typedef struct tag_ExifInfo {
char Version [5];
char CameraMake [32];
char CameraModel [40];
char DateTime [20];
int Height, Width;
int Orientation;
int IsColor;
int Process;
int FlashUsed;
float FocalLength;
float ExposureTime;
float ApertureFNumber;
float Distance;
float CCDWidth;
float ExposureBias;
int Whitebalance;
int MeteringMode;
int ExposureProgram;
int ISOequivalent;
int CompressionLevel;
float FocalplaneXRes;
float FocalplaneYRes;
float FocalplaneUnits;
float Xresolution;
float Yresolution;
float ResolutionUnit;
float Brightness;
char Comments[MAX_COMMENT];
unsigned char * ThumbnailPointer;
unsigned ThumbnailSize;
bool IsExif;
} EXIFINFO;
If needed, the EXIFINFO structure can be extended to read more tags.
- Declare a new field in the
EXIFINFO structure (example: float ExposureTime;)
- Define a new tag in exif.cpp (example:
#define TAG_EXPOSURETIME 0x829A).
- Add a new case in the
ProcessExifDir function.
Example:
case TAG_EXPOSURETIME:
m_exifinfo->ExposureTime = (float)ConvertAnyFormat(ValuePtr, Format);
break;
ConvertAnyFormat converts automatically the value stored in the tag.
To Do
Cexif::EncodeExif : to append the EXIF data into a standard JPEG image.
