of iDStretch in the field  iDStretch on iPhone


Now you can perform DStretch enhancements in the field on your iPhone or iPad.

iDStretch is available on the Apple App Store.  App Store Link
It is simple and easy to use.  Eight enhancements are included to cover all the pigments commonly seen at rock art sites.  Several of the enhancements are specifically for red pigments, by far the most common pigment found at rock art sites.  To use the app just load an image, either from your album or using the camera.  Then just hit the *Enh* button to cycle through the enhancements.  The available enhancements are:
CRGB (for reds), YDT (general purpose), YRD (for reds), YBK (for blacks), YRE (for reds), YYE (for yellows), YWE (for whites), and RGB0 (for reds).  These enhancements will be familiar to those who have used DStretch.  If you use the camera then iDStretch automatically saves the original image for you.  You do not need wi-fi or a phone connection for this app.  There is a settings page that allows you to change the scale (strength) of the enhancement, change the saturation (i.e. make it grayscale), change which enhancements are used, and reset the image. 

Exif Data

Exif data is information added to a photo.  Exif holds camera, exposure, and other data which may include location data.  This can be a concern if photos of rock art sites with exif location data are posted to the web.  As far as I can tell if you take a picture with the iDStretch app exif data is not recorded and if you save an enhancement it is not passed to the enhancement.  If you take a picture with the phone camera app then it is recorded.  In this case you should erase the exif data before posting a photo from a sensitive site.  There are apps available that can do this.

Image Quality

For iDStretch the most important image quality issues are light, noise suppression and jpeg compression.  Low light requires high ISO which triggers noise suppression which then smears the colors.  The poor flash (or complete lack for iPads) exacerbates this problem.  Jpeg compression also degrades the colors (little blocks are the characteristic artifacts).  Usually the iPhone user doesn't think about ISO and jpeg compression and there is no setting for them in the Apple camera app.  I know of at least one app, camera+, which gives control over ISO and jpeg quality.  It can greatly improve DStretch results.

iPhones have a better camera than iPads.  iPads have better displays. (The iPad Pro is gorgeous.)  One solution to this problem is to take the photos on an iPhone and use air drop to send to an iPad.  But this may not be needed; the image display and zoom on an iPhone are certainly adequate for field visualization.  I like the iPads so much that I wish there was an easy way to get images from my regular camera into it in the field.  For the RX10 the camera wi-fi can send images to the iPad.  I just tested this in the field in the Mojave Desert and it worked reasonably well.  I have also tested a Transcend Wi-Fi SD Card.  It is a little slow, but it worked to send images from a Canon G1X camera to an iPad.  An Eye-Fi Pro X2 card also worked with that camera and seemed to be faster.

Field Results

My first chance to test iDStretch came on a trip to Baja California.  I had an iPhone 5. The first site we visited was a rock shelter. Inside the light was dim, a problem for any small camera.  The iPhone flash was useful, but is not as strong as a good camera flash.

Rockshelter is inside of the big rock.

An actual iPhone5 photo from the interior (click the image for the full size image).
Interior of shelter using iPhone 5 camera.

The saved CRGB enhancement.
The saved YDT enhancement.

A revelation in the Mojave Preserve

Near an historic inscription my wife Sheila noticed some faint red paint.
Look at the full size image to see the inscription.  The panel with faint paint is to the left.
Mojave Preserve

Out came the iPhone 5.  Very hard to see, but there is some faint paint at center and right.
Both in the unstained rock on the right and the water stain at center.
iPhone 5 image
After the automatic CRGB enhancement there was no doubt: A complex and very interesting panel.
Was the painting intentionally placed at a spot where the water runs down the rock?  If so is
this a depiction of a (David Whitley) sort of "Water Baby"?
CRGB enhancement of iPhone 5 image
For comparison here is an enhancement of a RX10 image of the same panel.
The enhancement was made from a tiff image developed from raw data with
noise reduction turned off.  It is easier to see that water running down the stain area has erased
part of the painted image.
RX10 image

More Examples