Computer Graphics with Photoshop - how to blur a selected area

Author

how to blur a selected area

Greg N.

2007-01-10, 6:20 pm

Look at the sample pic at http://coolhaus.de/misc/how2blur.jpg

The leftmost square shows the source image, the middle square shows the
result of blurring (which I don't like), the rightmost shows the result
I'd like to achieve, but I don't know how to.

I want to blur the blue parts of the figure.

I arrived at the middle figure by selecting the blue square (minus the
inner white circle) and using the gaussian blur filter. This results in
some color from outside the selection (in this case, white) to bleed
into the blurred area.

That is not what I want. I want the blurring to occur only based on the
selected area, like shown in the rightmost square.

Can this be done somehow (in CS2)?

--
Gregor mit dem Motorrad auf Reisen
http://hothaus.de/greg-tour/

ronviers@gmail.com

2007-01-10, 6:20 pm

Greg N. wrote:

> Can this be done somehow (in CS2)?
>
> --
> Gregor mit dem Motorrad auf Reisen
> http://hothaus.de/greg-tour/

Hi Greg,
Knocking out the hole and background will stop the bleeding. Because
the hole and background is high key the extraction of the square is
especially simple. There are many ways to go about it, maybe the
easiest would be to use the 'select' 'color' then drag the
mouse across the colored area - you can get away with a very fuzzy
setting in this case. You can then use the inverted selection either as
a mask or to delete the area completely.

Good luck,
Ron

Derek Fountain

2007-01-10, 6:20 pm

> I want to blur the blue parts of the figure.

If I understand correctly, the simplest way would be to select the blue
bit you want to blur and make it onto a layer of its own (layer via
copy). Then lock the transparent pixels on that new layer, then run the
blur filter.

--
Derek Fountain on the web at http://www.derekfountain.org/

Derek Fountain

2007-01-10, 6:20 pm

Derek Fountain wrote:
>
> If I understand correctly, the simplest way would be to select the blue
> bit you want to blur and make it onto a layer of its own (layer via
> copy). Then lock the transparent pixels on that new layer, then run the
> blur filter.

Um, only that also produces a halo, because although the transparency
lock prevents the blur going out into the white circle, the transparent
area is used in the blur so the inner edge of the blue area still fades
out. Let me have another try, having thought about it a bit better:

1) Select the blue area and make it into its own layer.
2) Select the white area and make that into another layer.
3) Make the white hole the top layer, the blue bit the second layer, and
leave the original image at the bottom as the background layer.
4) Select the blue area layer and Ctrl-click it to select the blue pixels.
5) Select->Modify->Expand by about 3 pixels
6) Now run the blur filter

This leaves the blue area blurred and bleeding into the hole on its
layer. This hole (and hence the bleeding) is covered by the white circle
on the top layer. Most of the halo effect is diminished by expanding the
selection before blurring (i.e. the halo is moved in and under the white
circle), but what there is you can't see because where the blurred edge
is semi-transparent the original background shows through.
--
Derek Fountain on the web at http://www.derekfountain.org/

Greg N.

2007-01-10, 6:20 pm

Derek Fountain wrote:

I tried your approach, it works satisfactorily.

However, I don't quite get this:

> Most of the halo effect is diminished by expanding the
> selection before blurring

This does not reduce the halo effect in the selected layer, because
expanding the selection now includes some blank pixels along the edge of
the selection, which still bleed into the original selection.

However, the key is, as you suggested, to place the object to be blurred
onto its own layer. Blurring then produces a transparent halo which is
invisible.

So, I think the procedure is simply:

1. select, copy, and paste the area to be blurred
3. reselect
3. blur

--
Gregor mit dem Motorrad auf Reisen
http://hothaus.de/greg-tour/

pixel_a_ted@yahoo.com

2007-01-10, 6:20 pm

Greg N. wrote:
> However, the key is, as you suggested, to place the object to be blurred
> onto its own layer. Blurring then produces a transparent halo which is
> invisible.
>
> So, I think the procedure is simply:
>
> 1. select, copy, and paste the area to be blurred
> 3. reselect
> 3. blur
>

I've been trying to accomplish the same thing so I am very interested
in this. I don't quite understand your 3 steps. I have tried it as
follows: copy the area to be blurred and then create a new layer from
copy. What is in this new layer is the area to be blurred and the rest
is transparency. Then I select the area to be blurred and blur it. As
you say, a transparent halo is created inside the selected area. Why do
you say that it is invisible? The transparency halo reveals some of the
original, unblurred parts of the selected area so this reduces the
blurring effect where the halo is.

Please elaborate if you think the above 3 steps work. Thanks.

Greg N.

2007-01-10, 6:20 pm

pixel_a_ted@yahoo.com wrote:

> ... a transparent halo is created inside the selected area. Why do
> you say that it is invisible? The transparency halo reveals some of the
> original, unblurred parts of the selected area so this reduces the
> blurring effect where the halo is.

You're right, the area along the edge gets a bit less blurred than the
rest of the selection. But what's important is that there is no color
from outside the selection that bleeds into the halo.

Maybe I should have said I find the halo barely visible.

--
Gregor mit dem Motorrad auf Reisen
http://hothaus.de/greg-tour/