mritopng 2.2 - Auto Contrast

This version contains the ability to apple auto-contrast when converting the DICOM files. This provides the following benefits

• Ensure a more uniform output across files when converting multiple files
• Match the processing of commercial processors a little more closely

The behavior is backward compatible. To enable auto-contrast, you need to add either the -c or --auto-contrast flag when running the command.

This ability is also added to the functions as an optional paramenter which defaults to False

mritopng 2.1

This fix addresses an issue where the input DICOM files might have readings below 0. In the example files provided by a user here, A value of -1024 is seen for the background.

I've modified the logic to simply clip such values to 0.