Image Processing Reference
In-Depth Information
Wavelet Denoising
The Denoise slider controls the reduction of noise in the image. The default
setting, 0 (zero), doesn't reduce any noise. Be aware that the higher the
setting, the more the image looses sharpness. Apply it only when necessary.
Dark Frame Subtraction
Many cameras apply dark frame subtraction for long exposures to reduce
noise and remove so-called hot pixels. The disadvantage of this procedure
is that it makes the exposure twice as long because the dark frame exposure
time needs to be as long as the exposure time of the real image.
You can make your own dark frame by putting on the lens cover and
taking a picture. However, you have to do this where and when taking the
original shot on location, not afterwards. The camera chip's temperature is
a crucial value responsible for the amount of noise. And of course, the file
format (RAW) and the settings for ISO and exposure time ought to be the
same as for the original photography. After copying the original and the dark
frame to your hard disk, you can load the dark frame into UFRaw. Click the
load dark frame button (it looks like a folder) to open your dark picture. The
option UFRaw offers is practical if your camera does not have the dark frame
subtraction feature or if you decide to disable it to save time.
After having tried several of the white balance default settings, I used
Auto WB as a basis and corrected it with the spot white balance eyedropper
(see figure 2.5 ). I set the value for Temperature to 5888 Kelvin and Green to
1.004. I left the interpolation method on AHD for the best image quality. With
a mouse click I applied the button for color smoothing. So as not to reduce the
sharpness too much, I refrained from denoising the image.
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