Color processing allows to
- correct color balance*,
- to manipulate color to better visualize structures
- and to filter orthomosaics.
Any combination is possible.
Generally, all effects are applied on the selected image. To apply two or more effects in sequence, the processed image must be loaded before a second effect can be applied. Processing the same effect/filter with different setting again overwrites the previous version.
Enhancement and manipulation
Depending on weather conditions, flight characteristics, camera setup etc. some orthomosaics can show a low brightness or a low contrast.
Enhance combines color space stretching of luminace or color schemes, automated gamma correction, global and local contrast enhancements. It can be used to correct images or to make the colors look more vibrant.
To compensate illumination Gamma Correction can be used, which adjusts the overall brightness of an image. Gamma values above 1 results in darker with higher contrast while gamma values below 1 results in lighter colors and less contrast.
White balance aims to correct images with a wrong exposure. A threshold can be set to adjusted the stretching of the output color space. Common thresholds are 2-5%.
Contrast limited adaptive histogram equalization (CLAHE) is a local contrast enhancement technique. Local histogram equalization guarantees that the contrast is also enhanced in very light or very dark region of the orthomosaic.
To improve the contrast of an orthomosaic Histogram Equalization can be used. This equalizes the intensity of the orthomosaic. It should be applied on image Intensity, but for certain analysis it might be interesting to apply it on the RGB channels as well.
Often some (parts) of the RGB image channels are correlated to each other. Decorrelation Stretch enhances the color separation by removing channel-to-channel correlation and by stretching the contrast of the new uncorrelated channels. Mavis provides a Decorrelation Stretch in various color spaces.
An example for correcting, enhancing and manipulating colors in Mavis:
Spatial filters can be applied to remove noise in the orthomosaics.
Mavis provides Blur, Gaussian, Median, Bilateral and Mean Shift filters. They all have different characteristics. Blur is the most common filter, Gaussian produces less artifacts, Median is resistant to outliers, Bilateral preserves edges best and Mean Shift accounts for spatial and color distance.
*Color calibraiton algorithms are provided in the tools section