Hi all,
@Mario:
In general, it is not recommended to preprocess the images, especially if the preprocessing changes the geomerty of the images.
In some extreme cases (see https://support.pix4d.com/hc/en-us/articles/203341269-How-to-Process-Images-with-Big-Color-Variance) we recommend some preprocessing as a last effort to solve processing problems.
If you are working with multispectral images, then you should do some preprocessing based on the instructions of the following article:
https://support.pix4d.com/hc/en-us/articles/204894705-Camera-Requirements-For-Precision-Agriculture
- The calibration target image does need to be taken on the ground. If you have a big calibration target that is visible from the air and covers many pixels, then it is fine to take the image during the flight. If this is not possible, then we recommend to take the calibration target image before or after flying (under the same weather/lightening conditions)
- If you can take good images (not overexposed or underexposed) of your targets, it does not matter what is their color. We would recommend you to work with the grey tiles as it is easier to take images that are not overexposed or underexposed.
- For more information about how to work with a spectroradiometer, see the post: https://community.pix4d.com/t/4749-Radiometric-Calibration-Target
Best regards,