Old images scanned from film positives or negatives, and images made under poor lighting conditions can be improved with a few simple tools. For more difficult cases, please download and install CastCor which offers many more options. Some of the options offered here may be found in many image treatment programmes, but those described below have proved to be very useful for treating archaeological air photographs. External image treatment programmes may also be used as long as they do not change the size, orientation or EXIF data of an image if control, calibration points or geo-referencing have been added by AirPhotoSE. If required, radial lens distortion should be removed first with an external programme like RadCor
The enhancement menu offers several options for improving the appearance of the currently displayed image file.
The filter menu offers options for extracting edges and boundaries, removing scratches and dirt and sharpening images.
Try the options on an image and click on the Undo button and repeat until you find the treatment which gives the best results.
If the image contains scratches or dirt marks, use the Scratch/Dirt Removal filtering option to remove these first before applying any enhancement or filtering options.
The Contrast/Brightness option modifies contrast and brightness so that the balance of the image is improved. It is the weakest of the options available but should be tried first since it introduces neither artefacts nor colour distortion.
The Auto Colour option proposed by Rizzi et al. corrects for colour problems in dark and light parts of an image. It also reduces fog if present. Colours may be unnaturally strong in places.
The Luminance option due to Meylan greatly improves the visibility of fine structure in dark parts of an image without affecting the remainder.
The Retinex method, proposed by Edwin Land, the inventor of the Polaroid process, and as implemented by Barnard is much stronger and also works on the light areas. Colours are distorted.
Local Adaptive Contrast due to Wallis improves the visibility of fine structure in the whole image and sharpens it at the same time. It does not affect the colours.
The White Balance option corrects for colour cast automatically by computing an optimal white point and modifying the colours of all pixels.
ColorBoost using Van der Weijer's algorithm exaggerates colour differences, giving an image a false-colour film appearance. This may be useful as a preliminary step for images where luminance differences along boundaries are weak. Buildings and roads are usually sharpened considerably. Archaeological markings are less visible, so after placing control points, back off with the Undo button and save the original with the new control points.
Greyscale converts a colour image to a image.
Black / White converts a colour or greyscale image to a black and white image. Weak fine grey lines are enhanced for greater visibility.
Positive-Negative reverses greyscale images to negatives or positives. It also converts scanned colour negatives to colour positives.
Misc. Changes offers some typical photo correction items found in other software with operation using a simple "before and after window" pair.
The options may be applied in any sequence. The Undo button may be clicked on to go back one step if the result is not useful. The edge extraction filter may give better results after a combination of several of the enhancement options.
See the bibliography for detailed references for the enhancement and filtering options.
Edge extraction is intended for converting vertical and satellite imagery including pictures from Google Earth or an orthophoto from a GeoPortal site to a black and white line drawing which may be used as a background for overlaying rectified images without interference from the colours of the original. It usually results in too much detail to be useful for extracting archaeological features.