Let's start by adjusting the colour balance. There is a very practical
Paint.net plug-in, named Whitepoint, available here. To install it, unzip the archive
and place the Whitepoint.dll file
into C:\Program Files\Paint.NET\Effects.
(Note: "Program Files" may be named "Programs" in Vista and Windows 7+).
If you took the precaution, as I advised you, to have in the photo a neutral zone (the background, a white reflector or a gray ground), just take this supposed neutral colour with the colour pipette, then launch Adjustments / Whitepoint. You can then verify that the reference point has its R, G and B components almost equal.
Then, you can try to improve the image (levels, brightness, contrast, framing). The Levels and Curves functions in the Adjustments menu are very useful. The first is more difficult to apprehend than the second, but gives often faster results. Avoid Brightness and Contrast that may remove pixels (either lighter or darker), thus reducing the richness of the image.
The curve to the right allows to lighten the shadows, to see a little more details for example on the tires of a car.
Original image: the background is greyish, but very
Lada Niva Brekina réf. 27201
In the Layers window, let’s duplicate the image. On this copy, apply the Effects / Color / Threshold function which transforms any pixel into either pure white or pure black. The extreme contrast in short… By playing with the slider, you can get a black silhouette on a white background.
Note: Threshold is a plug-in. There are several.
The one that I propose to you is at this address. After extracting the files from
the Zip archive, place the EdHarvey. Effects.dll file
into C:\Program Files\Paint.NET\Effects
into C:\Program Files\ Paint.NET\ FileTypes
(attention : "Program Files" may be named "Programs" in Vista and Windows 7+).
Sometimes, the model itself has very clear parts that are likely to be eliminated. Before applying Threshold, let's draw the inner contour of these zones with a black line 10 pixels thick (approximately), after having disabled the antialiasing (antialiasing means that, seen at 100%, a line must present frank, not blurred “stair steps”). The line has four handles that allow it to be moved or deformed, for example to follow a curve. If it is missed, delete it by Esc. You may also use Undo (Ctrl + Z) to go back. When the line suits your needs, validate by Enter.
On the other hand, if there are any black spots in the future background, don’t worry: they will be eliminated by the process, provided they don't touch the main silhouette.
Finally, we get what was already called a mask in the times of the analog photo laboratory. Let’s verify by magnifying the image that white pixels (due to reflections on chromes for example) do not remain encrusted in the black silhouette. In fact, the whole mask must be well filled.
The mask, including the ground
Let’s select the mask with the Magic Wand, then reverse the selection (Edit / Invert Selection menu, or more simply Ctrl + I). So the future “sky” is now selected.
The selection of the “sky”, ready to receive the gradient