[Qgis-developer] Feedback for contrast enhancement ... enhancements ?

Even Rouault even.rouault at spatialys.com
Fri Sep 2 03:38:20 PDT 2016


There has been interest expressed to improve the usability of the contrast 
enhancement function. Basically there are workflows where you want the min/max 
values used by contrast enhancement to be updated each time you pan or zoom 
the canvas. From what I've seen, here's what currently exists:
- a long method (6 clicks): through the Style tab of the layer properties
- a faster method (2 clicks): through the "Strech using current extent" of the 
contextual menu of the layer panel.

When you have several rasters loaded, even the 2-click method is inconvenient, 
so there's a need for a 0-click solution (once the layers have been configured)

Another shortcoming in the current implementation is that the settings of the 
"Load min/max values" foldable group are not persistant, which requires re-
setting them if you're not happy with the default Cumlative count cut method. 
And if you use the "Strech using current extent" menu item, the genuine 
min/max values are used (not the 2-98% cumulative count cut)

So I was thinking to something along the following lines:
- make the min/max settings persistant
- remove the "Load" button and "Clip extent to canvas" checkbox, and replace 
them by 3 radio buttons to determine the scope of statistics : "Whole raster", 
"Current canvas" and "Updated canvas". See the attached proposal_min_max.png. 
The function of the Load button would be replaced by the general Apply / OK 
- when the user manually enters the min/max values, the 2 groups of radio 
buttons (method to compute min/max and scope of statistics) would be unchecked 
so that is is clear that they don't come from computed statistics. I also 
think that when you select "Current canvas", once the Apply/OK buttons have 
been pushed, the checked state of Current canvas shouldn't be saved. This way 
when you display the layer properties you have a good idea of where the 
current min/max values come from.
- make the "Strech using current extent" method honour the way min/max are 
computed in the min/max settings instead of using systematically the min/max. 
So when "Updated canvas" would be selected, it would have no (extra) effect as 

Any opinions ?


Spatialys - Geospatial professional services
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