Is there a way to write a GeoTiff file with Color and Alpha Channel Data using geotiffwrite?

Is there a way to write a GeoTiff file with Color and Alpha Channel Data using geotiffwrite? I tried defining a TiffTag for "ExtraSamples" as shown below, but I got an warning message. The image (img) is a n x m x 4 RGB image where the 4 index on dimension 3 is the alpha channel.

geotiffwrite('img.tif', img, R, 'TiffTags', struct('ExtraSamples', Tiff.ExtraSamples.Unspecified))

Warning Message:

Warning: Sum of Photometric color channels and ExtraSamples does not match the value specified in SamplesPerPixel.

Writing with this configuration will error in a future release. To correct the configuration, define the non-color channels as ExtraSamples.

I get an error if I try defining the "SamplesPerPixel" TiffTag as 4. I don't know how to define the non-color channels as ExtraSamples using geotiffwrite.

I used the following example for Tiff objects to try to create the alpha channel data:

Write Tiff Image with Color and Alpha Channel Data

Answer 1

[Shubham]

Hi Sonoma,

Yes, it is possible to write a GeoTiff file with color and alpha channel data using geotiffwrite function in MATLAB. However, you need to correctly define the "ExtraSamples" TiffTag in order to do so.

From the error and warning messages you received, it seems like the "ExtraSamples" TiffTag was not defined correctly. Instead of defining it as "Unspecified", you can define the non-color channels as "ExtraSamples". For example, you can modify the "TiffTags" struct as follows:

TiffTags.Photometric = Tiff.Photometric.RGB;
TiffTags.BitsPerSample = 8;
TiffTags.SamplesPerPixel = 4;
TiffTags.ExtraSamples = Tiff.ExtraSamples.UnassociatedAlpha;

This sets the "Photometric" tag to RGB, "BitsPerSample" to 8 (assuming your image has 8-bit color depth), "SamplesPerPixel" to 4 (indicating that there are 4 color channels), and "ExtraSamples" to "UnassociatedAlpha" (indicating that the extra channel is an alpha channel that is not associated with any specific color).

Then, you can pass this "TiffTags" struct to the geotiffwrite function like this:

geotiffwrite('img.tif', img, R, 'TiffTags', TiffTags);

This should allow you to write a GeoTiff file with color and alpha channel data using geotiffwrite function in MATLAB.

[Sonoma Rich]

Shubham,

when I run the script using your suggestion, I get the following error:

Error using geotiffwrite (line 245)
Expected TiffTags.BitsPerSample to match one of these values:

'Artist', 'Compression', 'Copyright', 'DateTime', 'DocumentName', 'DotRange', 'ExtraSamples', 'FillOrder', 'Group3Options',
'Group4Options', 'HalfToneHints', 'HostComputer', 'ICCProfile', 'ImageDepth', 'ImageDescription', 'InkNames', 'InkSet', 'JPEGColorMode',
'JPEGQuality', 'Make', 'MaxSampleValue', 'MinSampleValue', 'Model', 'NumberOfInks', 'Orientation', 'PageName', 'PageNumber',
'PhotometricInterpretation', 'Photoshop', 'PlanarConfiguration', 'PrimaryChromaticities', 'RPCCoefficientTag', 'ReferenceBlackWhite',
'ResolutionUnit', 'RichTIFFIPTC', 'RowsPerStrip', 'SGILogDataFmt', 'SMaxSampleValue', 'SMinSampleValue', 'SToNits', 'Software',
'TargetPrinter', 'Thresholding', 'TileLength', 'TileWidth', 'TransferFunction', 'WhitePoint', 'XMP', 'XPosition', 'XResolution',
'YCbCrCoefficients', 'YCbCrPositioning', 'YCbCrSubSampling', 'YPosition', 'YResolution', 'ZipQuality'

The input, 'BitsPerSample', did not match any of the valid values.

I am running Matlab 2020b. It this feature used on later versions of Matlab.

[Taylor Shropshire]

I also had the same issue and have not found a solution. Shubham could you provide more information?

Its possible to modify a tiff object (T) as you describe. This can then be used to create a tiff image write(T,rgb_with_alpha). Although I don't think there is a way to convert the tiff object to a geotiff.

setTag(T,'BitsPerSample',8);

setTag(T,'SamplesPerPixel',4);

setTag(T,'ExtraSamples',Tiff.ExtraSamples.UnassociatedAlpha);

Creating a TiffTags Structure does gives the error described by Sonoma Rich

TiffTags.Photometric = Tiff.Photometric.RGB;

TiffTags.BitsPerSample = 8;

TiffTags.SamplesPerPixel = 4;

TiffTags.ExtraSamples = Tiff.ExtraSamples.UnassociatedAlpha;

geotiffwrite('file.tif', img, R, 'TiffTags', TiffTags);

Thanks for any support.

[Taylor Shropshire]

I stumbled upon a very easy solution... geotiffwrite will create transparency for all grid cells that have RGB=[NaN NaN NaN]. I used QGIS to load the file. Hope this helps someone else.