Pixel selection/marquee from centre point

[matt.baker]

Is it possible to create a pixel selection from a centre point and contract/expand around this. In the shape mode, the Ctrl key modifier does this (as shown below), but doesn't seem to work for marquees in the pixel persona.

 

[MEB]

Hi mattb5906,
This is a known issue. Use the vector shape tools to create a shape the same way you would create the selection then press and hold CTRL (⌘ (cmd) if you are using a Mac) and click the thumbnail of the shape layer in the Layers panel to create a selection from its contents.

[Too_Bad]

I may be way off topic but what's described in this topic sounds sort of like what I want to do. Take something like a round watch face. Put the cursor in the centre (the axis the hands rotate around) and create a round marquee so I can cut out the face of the watch. It's such a simple idea yet I can't find it anywhere. There has to be some modifier or something that says I'm in the centre, make the circle marquee bigger or whatever. All I can do now is create a rough outline and spend 40-60 minutes going pixel to pixel erasing everything that doesn't look like part of the watch face. Actually, I think MS Paint might be able to do this... :)

[lepr]

.

[firstdefence]

I don't see the "creating a shape and cmd + clicking the layer" as a workaround, I just see it as an alternative way to make selections, you can adjust a shape more easily than you can using the Selection Tool, just drop the opacity and tweak around say your watch, build complex shapes add them and then cmd + click to get your selection. You can also group shapes and get a selection from the group instead of adding the shapes together.

It's just one more way to get the selection you want.

[carl123]

Draw 3 guidelines as shown below, 1 vertical on the centre line of the watch and 2 horizontal lines at the points where you want your cut out to be.

Select the round (ellipse) marquee tool, hold shift key down and starting at point A (below) drag to the left and downwards simultaneously until the marquee snaps to the bottom horizontal guide (if snapping is switched on)

Then use the right arrow key to slide the marquee selection over the face of the watch.

PS You don't really need the vertical guideline, just create your round selection between the 2 horizontal guidelines and slide it across.  I just added it to show where, vertically,  to start drawing the marquee from.

 

[Too_Bad]

The guidelines method works perfectly. It's not as easy as just drawing a circle from the centre would be, but it works and that's all that matters. I wonder though why the "centre" method does not exist. The math looks easier than the current method.

Anyway, thanks a bunch.

 

[R C-R]

1 hour ago, Too_Bad said:

I wonder though why the "centre" method does not exist.

It does exist but the only center a pixel or image layer has is the center of the layer, or of the document if the layer & document are not the same size.

IOW, while you see a watch or whatever other object might be in a photo, the app just sees a collection of pixels. That's why you have to supply the guides or use some other method to define the center. Even if an object is dead center in a photo & part of it is perfectly round, it might be tilted at an angle relative to the camera, so you might have to use an ellipse & convert it to a slightly egg-shaped curve to compensate for the perspective to get all of it in the selection.

[Alfred]

1 hour ago, Too_Bad said:

I wonder though why the "centre" method does not exist. The math looks easier than the current method.

 

1 minute ago, R C-R said:

It does exist but the only center a pixel or image layer has is the center of the layer, or of the document if the layer & document are not the same size.

I think you’ve misunderstood what the OP is asking about! The point is that when you create a shape such as an ellipse you can choose to create it from the centre instead of a corner, but when you’re creating a “marching ants” elliptical selection there is no such option.

[R C-R]

7 minutes ago, αℓƒяє∂ said:

I think you’ve misunderstood what the OP is asking about! The point is that when you create a shape such as an ellipse you can choose to create it from the centre instead of a corner, but when you’re creating a “marching ants” elliptical selection there is no such option.

Sorry about that. 

I was trying to explain why using MEB's workaround or the guides method would still be necessary to find the center, whether or not any selection tool supported drawing the selection outward from its center. That is what I meant by "it does exist" but with perfect 20/20 hindsight I see that I could have been a lot clearer about that..

[Too_Bad]

As I said, the guides method does work well. Of interest is that I tried the same thing using a different package, Corel PaintShop (I usually avoid anything Corel,) and got a surprise when I went to do the Shift-Drag. That itself does the draw from the centre without any other messing around. I guess it's more a paint thing rather than a photo processing thing although as much as nature abhors straight lines, it loves circles so I was surprised that photo processing wouldn't have it. But, as was explained, while nature loves circles, photos rarely will contain perfect circles and even if one does, the centre will not usually be conveniently marked.

Thanks to all for the explanations.

[JimmyJack]

9 hours ago, Too_Bad said:

..... and even if one does, the centre will not usually be conveniently marked.

Ya don't really need to find the center.... a la @carl123's method.

I've got a slight twist.
Two tangential guides > Draw out a constrained marquee from the corner. Should fit perfectly. No need to slide into place.

But yeah.... kinda mind boggling that we can't draw marquees from the center. But as MEB said, it's a known "issue".