2004.10.21 18:10 "Re: [Tiff] Changing TIFF value", by Andrey Kiselev

Hello. I'm having some trouble successfully changing TIFF values. I have scanned some of the archives. I'm adding the functionality in a Windows DLL.

The primary problem seems to be that I cannot read my changed value successfully. Note that I am attempting to change the ARTIST field. I'm all but certain that the field does not exist in the original TIFF file.

Here's my two functions:

extern "C" __declspec (dllexport) BOOL WINAPI ReadTag(char *filename, long tag, char *result)
{
        TIFF *image;
        long returncode, counter;

        // Open the TIFF file
        if((image = TIFFOpen(filename, "r")) == NULL)
        {
                return FALSE;
        }

        returncode = TIFFGetField(image, tag, result);

                                              ^^^^^^

The bug is here. Should be

        returncode = TIFFGetField(image, tag, &result);

        // Close the file
        TIFFClose(image);

        return TRUE;
}

extern "C" __declspec (dllexport) BOOL WINAPI ChangeTag(char *filename, long tag, char *value)
{
        TIFF *image;
        long returncode;

        // Open the TIFF file.
        if((image = TIFFOpen(filename, "a")) == NULL)
        {
                return FALSE;
        }

        returncode = TIFFSetField(image, tag, value);

        // Write data.
        returncode = TIFFRewriteDirectory(image);

        // Close the file
        TIFFClose(image);

        return TRUE;
}

Here's the body of the code:

ReadTag("C:\\tmp\\sample.tif", 315, resultbuffer);
ChangeTag("C:\\tmp\\sample.tif", 315, "RICHARD");
ReadTag("C:\\tmp\\sample.tif", 315, resultbuffer);

When I open the TIFF file after running the code in a binary editor, I see "RICHARD", which implies that ChangeTag had some success. As a matter of fact, I get an additional "RICHARD" every time I run the code, which isn't so great.

TIFFRewriteDirectory() adds the new directory at the end of the file every time being called and we loose the space, occuped by the old directories. So you should minimize the amount of TIFFRewriteDirectory() calls.

Andrey

Andrey V. Kiselev
Home phone: +7 812 5274898 ICQ# 26871517