6. Image decomposition
Until now weāve tried to modulate the sharpening into the same, high frequency range (edges, texture). But an image usually contains several different frequencies: higher ones correspond to ļ¬ner detail (hair, for instance), lower ones to large, smoother tonal transitions (like the cheeks in a portrait). Now it's time to try to target those different frequency ranges with appropriate, different sharpening. Pyramid decomposition is just... a way to decompose an image into several frequency ranges, and that makes easier to achieve our end.
Very simple math lies under image decomposition. Iāll use GB ļ¬lter (so Iāll build a Gaussian Pyramid of 3 levels, but you can extend it to as many levels as you like); being O the Original picture, GBn(O) the GB ļ¬lter applied to O with radius increasing with n:
U0 = O; (Eq. 6.1)
U1 = GB1(O); (Eq. 6.2)
U2 = GB2(O); (Eq. 6.3)
U3 = GB3(O); (Eq. 6.4)
We deļ¬ne differences Dn as follows:
D1 = U0 - U1; (Eq. 6.5)
D2 = U1 - U2; (Eq. 6.6)
D3 = U2 - U3; (Eq. 6.7)
So the image can be decomposed into:
O = U3 + D1 + D2 + D3; (Eq. 6.8)
In fact, substituting all the elements we get:
O = U3 + U0 - U1 + U1 - U2 + U2 - U3 = U0 = O; (Eq. 6.9)
Letās switch to Photoshop and try to build this pyramid. We only need to perform GB ļ¬ltering and subtractions, something we should be familiar with, by now.
The three GB radius will deļ¬ne the frequency range: Iāve chosen small radii (i.e. 1px, 5px, 15px) because the original picture is quite small; depending on the resolution of yours, they may vary. I can suggest you to select the smaller radius as the one you would use when applying conventional USM ļ¬lter, the larger one corresponding to HiRaLoAm radius, and the middle one, guess what, somewhere in between. Here are the three U1, U2, U3 and the D1, D2, D3 as we deļ¬ned them (Iām using SS2):
(Fig. 6.1) U1, U2 and U3, i.e. GBn(O).
(Fig 6.2) D1 = U0 - U1.
(Fig 6.3) D2 = U1 - U2.
(Fig 6.4) D3 = U2 - U3.
Having all the elements, itās now time to compose the pyramid on the Layers palette.You should make a new Set which contains bottom-up: the most blurred version U3, then D1, D2, D3, all the three LL mode, 100% opacity (remember, I used SS2). Here is how should look like your Channels and Layers palette:
(Fig 6.5) Channels and Layers palettes after ļ¬ltering, subtractions and recomposing.
Donāt feel afraid by the mess here, itās possible to automate everything (ToDo list +1) and end up with a nice tidy Channels palette. Switching on and off the Set should make no difference - and hence the decomposition works: great! So what?
| Comments |
|
|
|
|
|
|
|
|
|
|
|
A simple yet powerful Photoshop tool to effectively boost the local contrast of an image, by means of a smart algorithm which avoids the clipping issues typical of methods like shadows/highlights. The user interface is simple and clean, with just one slider, allowing the user to customize the effect in a straightforward way.
In 2000 Imacon, now Hasselblad, created a new proprietary format for the Imacon scanners, the Flexible File Format or fff. 3F has determined the biggest revolution in the field of original analog scanning since the introduction of the drum scanner.
First of all forgive us the word play, which might come off as presumptuous. However, we felt like we came up with something that perfectly summarizes our philosophy in this arena by uniting the concepts of high resolution and quality reproduction.