Question

I have a program that takes an image and writes that out to a TIFF file. The image can be grey scale (8 bit), grey scale with alpha channel (16 bit), RGB (24 bit), or ARGB (32 bit). I don’t have any problems writing out the images without an alpha channel, but for the images with alpha, when I try to set the extra samples tag, I get sent to the TIFF error handling routine set by TIFFSetErrorHandler. The message that’s passed in is <filename>: Bad value 1 for "ExtraSamples" in the _TIFFVSetField module. Some sample code below:

#include "tiff.h"
#include "tiffio.h"
#include "xtiffio.h"
//Other includes

class MyTIFFWriter
{
public:
    MyTIFFWriter(void);

    ~MyTIFFWriter(void);

    bool writeFile(MyImage* outputImage);
    bool openFile(std::string filename);
    void closeFile();

private:
    TIFF* m_tif;
};

//...

bool MyTIFFWriter::writeFile(MyImage* outputImage)
{
    // check that we have data and that the tiff is ready for writing
    if (outputImage->getHeight() == 0 || outputImage->getWidth() == 0 || !m_tif)
        return false;

    TIFFSetField(m_tif, TIFFTAG_IMAGEWIDTH, outputImage->getWidth());
    TIFFSetField(m_tif, TIFFTAG_IMAGELENGTH, outputImage->getHeight());
    TIFFSetField(m_tif, TIFFTAG_COMPRESSION, COMPRESSION_NONE);
    TIFFSetField(m_tif, TIFFTAG_PLANARCONFIG, PLANARCONFIG_CONTIG);

    if (outputImage->getColourMode() == MyImage::ARGB)
    {
        TIFFSetField(m_tif, TIFFTAG_PHOTOMETRIC, PHOTOMETRIC_RGB);
        TIFFSetField(m_tif, TIFFTAG_BITSPERSAMPLE, outputImage->getBitDepth() / 4);
        TIFFSetField(m_tif, TIFFTAG_SAMPLESPERPIXEL, 4);
        TIFFSetField(m_tif, TIFFTAG_EXTRASAMPLES, EXTRASAMPLE_ASSOCALPHA);  //problem exists here
    } else if (/*other mode*/)
        //apply other mode settings

    //...

    return (TIFFWriteEncodedStrip(m_tif, 0, outputImage->getImgDataAsCharPtr(), 
        outputImage->getWidth() * outputImage->getHeight() *
        (outputImage->getBitDepth() / 8)) != -1);
}

As far as I can see, the tag never gets written to the file. Luckily, GIMP still recognizes that the additional channel is alpha, but some other programs that need to read these TIFFs aren’t so generous. Am I missing tags that must be set before TIFFTAG_EXTRASAMPLES? Am I missing other tags that need to be there? Any help would be appreciated.

Answer 1

I found the solution. The Extra_Samples field is not an uint16 but rather first a count (which is uint16) and then an array of that size (of type uint16). The call should thus look like this:

uint16 out[1];
out[0] = EXTRASAMPLE_ASSOCALPHA;

TIFFSetField( outImage, TIFFTAG_EXTRASAMPLES, 1, &out );

The reason for this is that more than one extra sample is allowed.

Hope this helps!

Cheers,
Kaspar