TIFF support in PYME¶
Default TIFF support¶
PYME supports the TIFF file format, with some peculiarities as described below. In general, we expect TIFF files to use the .tif extension, with the .tiff extension being reserved for ‘odd’ TIFFs which won’t load or save through the default code paths. As a few ‘normal’ TIFFs use the .tiff extension and several ‘odd’ TIFFs use the .tif extension we recognize that this behaviour is not ideal and this should be a target for refinement in the future. At present manually changing the file extension is the best way of letting PYME know if the file is ‘odd’ or ‘normal’.
By default (TIFFs with the .tif or .lsm extension), PYME uses a slightly modified version of Christoph Gohlke’s excellent tifffile module. This will read 90% of all biologically interesting TIFF formats and is fast and robust. The .tif reader will extract basic OME metadata (pixel sizes and dimension ordering) if present. Unfortunately the tifffile module can’t read all TIFFs.
If reading fails, adding an ‘f’ to the file extension to make it .tiff will mean that the TIFF doesn’t get caught by our default TIFF handler, instead falling through to our handler of last resort - i.e. bioformats. This must be set up independently of PYME (at least on Windows and Linux), and is somewhat painful to install. It has the advantage of being able to load pretty much anything, but is not as stable or fast as we’d like.
PYME supports saving as floating point TIFF with minimal OME metadata, with the extension .tif, and with colour and z channels interleaved according to the OME standard. Additional metadata is packaged within the OME metadata using an XML annotation format. These floating point TIFFs are readable with ImageJ/FIJI, Matlab, and a few other special purpose image processing programs. They are not readable by most general purpose software (word, powerpoint, etc ..).
Why Floating Point?
PYME treats TIFF as a raw or intermediate image format which might be subjected to future quantitative image processing. As such we want to make sure that no information is lost during saving. If saving as the more widely readable 8 bit or 16 bit integer TIFF formats, we would need to quantize the data, resulting in loss of information. An additional problem with having to quantize the data is that the upper and lower bounds of the valid, e.g. 8 bit range would need to be determined and are unlikely to be the same across different images and image types. Saving as floating point removes the need to chose a scaling factor and any potential for loss due to quantization. It is the only safe option for an intermediate data format, despite lack of widespread software support.
Deprecated .tiff format saving¶
In addition to the default .tif format, using the .tiff extension will force PYME to use legacy TIFF export code that was written before we adopted the OME TIFF standard. The main differences to the standard .tif export are:
Colour channels are saved in separate files
Metadata is saved in a stand-alone XML
There is no OME header
This format should not normally be used. ImageJ will not be able to extract the voxel size or other metadata, and the chance of metadata getting separated from the individual files is high. The main use case for this form of export is for software that can’t handle OME interleaved colour channels.
Deprecated TIFF series (.xml) saving¶
There is additionally support for saving image stacks as a folder containing series of individual TIFF files and an XML metadata file. Like the .tiff legacy format, this shouldn’t normally be used, but may be useful when exporting data for exceptionally picky subsequent analysis programs.
Exporting images for use in reports and publications¶
Saving as .tif is not the way you should export data for inclusion in a figure as the default PYME .tif format is unreadable in most publishing programs. Instead you should export the image that is displayed (complete with colourmaps, scaling etc …). This is accomplished by choosing View->Save image as PNG from within an image view.
Alternatively you can copy the displayed image to the clipboard, either from the View menu, or using keyboard shortcuts.
<cmd>-c (<ctrl>-c on windows/linux) will copy the entire area of the currently displayed image to the clipboard
<cmd><shift>-c (<ctrl><shift>-c on windows/linux) will copy only the region which is visible