Image editing

Besides sharpening, some image editing options for adjusting brightness/gamma/contrast/color balance etc. can be useful in practice. Of course, the situation permits only a limited feature set. Extensive image manipulation must be done separately with one of the many available graphics programs.

The plugin comes with a small graphical editor which operates analog to the 'curves' tool in Gimp. It can be started by clicking the 'ImgEd' button. The green state of the button signals activeness. As long as the image editor is open, images will be transformed to a modified version acc. to the current curve adjustments. Clicking the 'ImgEd' button again closes the editor, and images will come out without modifications again. The editor can also be closed in the traditional way (X in the top window bar).
The curve can be adjusted with the mouse pointer (left mouse button pressed), releasing the mouse button triggers application of the current transformation to the image.
The transformation acts on the (-usually-) postprocessed/sharpened version of the data, this means that the displayed version is also the one which gets written to the disk (either with auto-saving or a manual click on 'Save')

Editor modes



Three modes are available, linear, gamma and bezier mode. Initially, the curve is set to the unit transform in linear mode.
  • linear: by clicking on one of the two endpoint circles, linear brightness and contrast modification (incl. clipping) can be adjusted.
  • gamma: clicking on the curve adds a handle point on the curve and switches the editor to a gamma curve mode.
  • bezier: clicking on the curve again adds a second handle point and activates the bezier curve mode. In bezier mode, the handle points define the slopes at the left and right endpoints. The handle point which is set first gets always connected with the left clip point (endpoint circle), the second one with the right clip point.
Gamma and bezier mode can be used on top of an optional linear adjustment of the endpoints. One example for the application of a bezier curve is limiting contrast enhancement to the brighter region of the image, and at the same time saving the terminator region from getting too dark (for planets showing a phase).

Mono and color mode

For mono images, there is of course only one channel. In color mode (when debayering is active), 4 channels are available, one for each color component (R,G,B) and one for the overall value. Switching between them can be done with help of a drop down list (leftmost element in the first button row). The value curve gets applied on top of each color channel curve.

Resetting

2 options for resetting the transformation to the initial unit state are available. 'Reset channel' resets only the current active channel, whereas 'Reset' resets all 4 channels (in color mode).

Inversion

With the last button row, the actually chosen transform curve can be inverted. In mono mode, this is self evident. Inversion in color mode is somewhat tricky: Imagine all color channels and the value channel set to the initial unit state. Simply inverting all curves would result in -- no change at all! The inverted value channel simply reverses the inversion of each color channel. Besides that, the accumulated action of inverting different color component curves plus the value curve is difficult to comprehend. So in color mode, a click on 'Invert' acts as reset plus inverting the value channel. Then you have the expected effect of inverted color appearance and a unique, comprehensible starting point. There is always the option to invert each channel separately with the 'Invert channel' switch for more complex adjustments.

State of development

In previous releases, rather strict constraints for clip and handle point moving were enforced. From version 1.0.0wip9 onward the policy is much more relaxed. This allows for all kinds of whicky-whacky image manipulation curves which -hopefully- don't run into mathematically invalid cases. Let's try it out.