(It's not perfect because missing local contrast will not be fixed). The uneven light can be to some degree flattened with high pass filtering. Simple contrast boost will not work because the light is uneven. for subject pixels (which are normally quite different from the background) the difference is far from 0 and they remain visible.for background pixels, the average value of the background around them is removed, so whatever the initial background lightness, the result is close to zero (actually, 50% gray, since Grain extract adds a bias to the result),.The grain extract, which is basically a subtraction subtracts the average lightness of the area from the pixels in the initial image:.With the Gaussian blur a pixel value is replaced by the lightness of the area around it (the blur is assumed to be sufficiently wide to make the influence of local details such as text negligible).Next time you take these pictures, bring a dark sheet of paper (ideally, black) that you insert under the page that you are shooting. The text on the other side of the page shows through and limits a bit your ability to stretch contrast. middle handle adjusted to optimize contrast.left handle to where the histogram seems to cease (anything to its left becomes completely black).right handle slightly left of the middle of the big spike (anything to its right becomes completely white). You can then use the Levels tool comfortably to optimize the result. In the resulting layer, the background is gray (around 50%) but is a more uniform gray. create a new layer with the result: Layer>New from visible.set the top layer to Grain extract mode.apply a Gaussian blur that is sufficient to make the text disappear completely (around 50px on your image).Then you apply the following technique(*) to even the lighting: If you are using Gimp 2.10, you can also set Image>Precision to 32-bit floating point/linear. Adjusting brightness/contrast is not easy due to the uneven lighting.įirst, to avoid color fringes, you work on a grayscale version of the image, either Image>Mode>Grayscale or Color>Desaturate.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |