Lua image functions

Through Lua scripting, SIPI allows a wide aea of utilities to analyze, manipulate and convert images to/from different formats. This functioanlity allows to use SIPI e.g. for offering image upload and converting these images into IIIF conformant long-term storage formats (e.g. JPEG2000). It allows to programmatically modify an image before delivering it to the client, ot to extract data from the images.

The basic concept is a specialized Lua image object that offers all methods to manipulate images.

This method creates a new image object by reading an image file that has to be located somewhere on the SIPI server.

The simple forms are:

img ="filepath")
img =

The first variant opens a file given by "filepath", the second variant opens an uploaded file directly using the integer index to the uploaded files.

If the index of an uploaded file is passed as an argument, this method adds additional metadata to the SipiImage object that is constructed: the file's original name, its MIME type, and its SHA256 checksum. When the SipiImage object is then written to another file, this metadata will be stored in an extra header record.

If a filename is passed, the method does not add this metadata.

The more complex form is as follows:

img ="filename", {

This creates a new Lua image object and loads the given image into. The second form allows to indicate a region, the size or a reduce factor and the original filename. The hash parameter indicates that the given checksum should be calculated out of the pixel values and written into the header. All parameters are optional, but at least one has to given. The meaning of the parameters are:

  • region: A region in IIIF format the image should be cropped to.
  • size: The size of the resulting image as valid IIIF size string.
  • reduce: An much faster alternative to size, if the image size will be reduced by a integer factor (2=half size, 3=one thrid size etc.)
  • original: The original file name that should be recorded in the metadata
  • hash: The Hash algorithm that will be used for the hash of the pixel values. Valid entries are md5, sha1, sha256, sha384 and sha512.

For example to read an image and include the SIPI preservation metadata, the function is called as follows:"path_to_file", { original="my_image.tif", hash="md5" }

This call will include the preservation metadata (please note that in this case the original filename is mandatory, since Lua has know direct knowledge about the original filename. The filepath given as first parameter must not and normally does not correspond to the original filename). The hash-parameter indicates to use the md5-algorithm for the has of the pixel values.


success, dims = img.dims()
if success then
    server.print('nx=', dims.nx, ' ny=', dims.ny)

Returns the pixel dimensions of the image as dims object.


success, errormsg = img.crop(<IIIF-region-string>)

Crops the image to the given rectangular region. The parameter must be a valid IIIF-region string.


success, errormsg = img.scale(<iiif-size-string>)

Resizes the image to the given size as iiif-conformant size string.


success, errormsg = img.rotate(<iiif-rotation-string>)

Rotates and/or mirrors the image according the given iiif-conformant rotation string.


success, errormsg = img.watermark(wm-file-path)

Applies the given watermark file to the image. The watermark file must be a single channel 8-Bit gray value TIFF file.

SipiImage.write(filepath, [compression_params])

success, errormsg = img.write(filepath)
success, errormsg = img.write('HTTP.jpg')

The first version write the image to a file in the SIPI server, the second writes the file to the HTTP connection (which is done whenever the basename of the output file is HTTP):


  • filepath: Path to output file. The file format is determined by the filename extension. Supported are

    • jpg : writes a JPEG file
    • tif : writes a TIFF file
    • png : writes a png file
    • jpx : writes a JPGE2000 file
    • pdf : writes a PDF file
  • compression_params: (optional) An optional Lua table with compression parameters (which are dependent on the chosen output file format!) can be given. All compression parameters are optional. Buf if a compression parameter table is give, it must have at least one entry.

  • JPEG format:
    • quality: Number between 1 and 100 (1 highest compression, worst quality, 100 lowest compression, best quality)
  • JPEG2000 format:
    • Creversible: Use the reversible compression algorithms of JPEG2000. Must be string yes or no. Defaults to yes.
    • Clayers: Number of layers to use.
    • Clevels: Number of levels to use.
    • Corder: Ordering of file components. Must be one of the following strings: LRCP, RLCP, RPCL, PCRL or CPRL.
    • Cprecincts: A kakadu conformant precinct string.
    • rates: rates string as used in kakadu.


success, errormsg = img.send(format)

Sends the file to the HTTP connection. As format-strings are allowed:

  • jpg : writes a JPEG file
  • tif : writes a TIFF file
  • png : writes a png file
  • jpx : writes a JPGE2000 file


img.mimetype_consistency(mimetype, original_filename)

This method checks if the supplied mimetype (e.g. from received from the browser during upoad), the file according to the magic number (file signature) and the file extension are consistent. The parameters are:

  • mimetype: The expected mimetype.
  • filename: The original filename with extension.

Please note that mimetype handling can be quite complex, since the correspondence between file extensions and mimetypes is not unambiguous. In addition the file signature can not identify all mimetypes. For example, A "comma seperated value" value file (extension .csv) can have a mimetype of application/csv, text/csv, text/x-csv, application/ and more. However, the file signature will usually return text/plain. SIPI tries to cope with these ambiguities.