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Overwriting Existing Thumbnails

The The ImageMagick media filters can differentiate thumbnails created by the DSpace default thumbnail generator and thumbnails that were manually uploaded by a user.  The media filter reads the bitstream description field to make this determination.  A regular expression can be provided to define the set of thumbnails that should be overwritten by the ImageMagick thumbnail generator. Thumbnail descriptions matching this pattern will be overwritten even if the -f option is not passed to the filter media process.

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Thumbnail descriptions that do not match either of the patterns listed above are presumed to be manually uploaded thumbnails.  These thumbnails will not be replaced even if the -f option is passed to the filter media process.

Flatten

DSpaces uses the JPEG as format for thumbnails. While JPEG doesn't support transparency, PDF, PNG and other formats doesdo. As those formats are used as outgoing material in DSpace, DSpace has to care about transparency during the generation of the thumbnails. In combination combinations of special specific versions of ImageMagick and Ghostscript it may occur, that completely transparent areas will become black. As a solution ImageMagick recommends to flatten images extracted from PDFs before they are stored as JPEG.

Since DSpace 5.2 the ImageMagick media filter flatten flattens thumbnails extracted from PDFs. If you run in to problems you lead back to into problems caused by flattening of the extracted images, you can switch the flattening off by setting the following property in dspace.cfg to false:

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PDFs optimized for printing most probably use CMYK color schemes. While most Printers work with CMYK colors, monitors use RGB color schemes. Following From this it follows that it is a good idea to convert CMYK color schemes to RGB when creating RGBthumbnail images. Furthermore the JSPUI currently does not support thumbnails using CMYK color schemes (see DS-2834). If you are using ghostscript Ghostscript at least in version 9, DSpace can create thumbnails that use RGB even if the source PDF was using CMYK. For the transformation from CMYK to RGB ghostscript Ghostscript needs ICC profiles. Default ones are provided by most Ghostscript installations from version 9 upwards. The following configuration options tells tell DSpace where those ICC porfiles are located. Giving ICC profiles to ImageMagick is optional. If the configuration properties in DSpace are unset, no profiles will be supplied to ImageMagick, which won't convert the thumbnailsprofiles are located.

# org.dspace.app.mediafilter.ImageMagickThumbnailFilter.cmyk_profile = /usr/share/ghostscript/9.18/iccprofiles/default_cmyk.icc
# org.dspace.app.mediafilter.ImageMagickThumbnailFilter.srgb_profile = /usr/share/ghostscript/9.18/iccprofiles/default_rgb.icc

Giving ICC profiles to ImageMagick is optional. If these configuration properties in DSpace are unset, no profiles will be supplied to ImageMagick, which won't convert the thumbnails.

Additional Customization

The ImageMagick conversion software provides a large number of conversion options.  Subclasses of these media filters could be written to take advantage of the additional conversion properties available in the software.

Note: The PDF thumbnail generator is hard-coded to generate a thumbnail from the first page of the PDF.

Giving ICC profiles to ImageMagick is optional. If the configuration properties in DSpace are unset, no profiles will be supplied to ImageMagick, which won't convert the thumbnails.