Cyclops wrote:So do you carefully select around the edge of the spider and grass stem then cut and paste that into a new layer?
No, each layer always contains the whole image. The mask selects what shows. I construct the mask with a combination of select-and-fill, and painting with a brush.
ChrisR wrote:look at the leg of the spider which is furthest upper left in the picture. There's a light halo- line to the left. Check the animated gif and you'll see it isn't there in the original.
That appears to be a lightness/darkness sharpening effect.
Another sort of overlap you can get is where the mask didn't quite fit. Look at the leg furthest lower right, and you'll see a bit of dark background on the animated gif which has got sharpened, which shouldn't have done.
Actually look harder at the very end of that leg and you'll see a light halo, with a dark halo beyond it, along the side of the leg.
The final pic is slightly different.
Excellent analysis, and completely correct -- good eyes, Chris!
The light halo-line is due to sharpening an edge that was already quite strong -- black spider against light tan grass. (Is there a Photoshop filter now that sharpens without creating such halos?) I look for such things, and if I find any that bother me, I'll do something to get rid of them. A bit of cloning with a tiny soft brush at high magnification does wonders, but it's tedious. In this case, I wasn't bothered, so I left it.
The sharpened background that shows when it shouldn't is of course due to the mask not being precisely fit to the subject. It's easy to see that this is the case by looking at the purple mask. There are "large" areas of background between the leg and the silk strand that are not masked at all. This is a matter of how much time I was willing to take. For a souvenir photo, I decided to not take more time tuning up the mask.
The light halo with a dark halo beyond it may be due to what is often the most troublesome problem with this technique. The problem is that in the blurred image, regions near the subject are not pure background but rather incorporate some colors and tones from the subject. The blurring filter doesn't know what's what, of course, so it just mixes everything together. What this means is that if the mask fits too closely to the subject, then you get another form of halo. So it's a bit of dilemma: fit the mask too close and you get this halo, fit it too loosely and you see sharpened background. To attack this problem, it is possible to "paint out" the subject on the layer to be blurred, before doing the blur. Someplace in the forum archives is a posting by someone else who describes this technique. I've never cared enough to take the time to do it.
"The final pic is slightly different." Yes -- great eyes! Because I was originally preparing only a souvenir photo, I was even sloppier than you see in the presentation above. When I cropped and enlarged the image for presentation in this thread, I saw some problems that were so blatant as to be distracting. (There was, for example, a large hole in the mask, right over a dark area of the spider's abdomen.) So I fixed those problems. The fixes are incorporated into the illustrations of the mask and the animation, but not into the "final" image, which is actually re-displayed from several weeks ago at the bottom of the posting. I thought about replacing that image with a current final, but since it was very late at night, I opted not to. I'm glad now that I did not -- it makes a nice point for discussion.
--Rik
"I have made this letter longer than usual, because I lack the time to make it short (Je n'ai fait celle-ci plus longue parceque je n'ai pas eu le loisir de la faire plus courte)" ~
Blaise Pascal, Lettres Provinciales (1656-1657), no. 16. The discussion at
quoteland was interesting to me.