aDStretch

DStretch is a program for the PC and Mac that is used by rock art researchers to make faint rock art visible.  This is a version for Android intended to be used in the field at a rock art site.  Eight enhancements are provided together with settings that can adjust strength and saturation of the enhancements.  Enhancements can be applied to a photo from the camera or from the gallery. The enhancements can be saved.  For ease of use you can designate a default enhancement (I recommend CRGB) that is automatically applied when an image is loaded.
The enhancements are: 
CRGB, YRD, YRE, RGB0: good for red pigments.
YDT, YRD, YWE: general purpose.  
YDT, YBK, YYE: good for yellows.
See DStretch.com for more information on DStretch and the enhancements.

In addition to choosing the default enhancement, you can use the menu to set:
Scale.  This is the strength of the enhancement.
Saturation.  This controls the saturation of the enhancement.  Set to 0 for grayscale, 1.0 for normal.
MPixel Limit.  Android phones can have memory problems with large images.  Set this to a low value to avoid crashes, to a high value to get higher resolution for enhancements.
Output Directory.  This is where the saved enhancements will be put.
Toggle Diagnostics.  Turn on to get diagnostic information.