Hi Gary,

edge_dog isn't the most elegant of plug-ins, and behaves more like a script
than most C plug-ins; it creates temporary layers, and to do so, uses
gimp_layer_copy.  (see
http://git.gnome.org/browse/gimp/tree/plug-ins/common/edge-dog.c line
471).  So it inserts a check that the drawable is a layer before it does
anything else.

I expect that careful checks and alternate code paths could alleviate the
issue (copying masks to new layers), or it could be rewritten to do its
operations in a more self-contained way.  I'm not volunteering here, but it
doesn't seem like an insurmountable amount of work for someone motivated.

HTH,

Seth Burgess






On Sat, Jan 21, 2012 at 3:20 PM, Gary Aitken <g...@dreamchaser.org> wrote:

> Hi Alexandre,
>
> Yes, I'm aware that I can work around it as you mention, and have been
> doing so.  But it is a pain in the rear, and unless there is some
> over-riding reason for not allowing one to do this operation, it seems the
> gimp should allow it.
>
> My main reason for posting was to see if it is a known issue and decided
> to be left as-is.  I don't see an open bug for it.  I suspect it may be a
> relic of implementation difficulties and so a capability which may be
> desirable but a bit awkward to implement.
>
> Thanks,
>
> Gary
>
>
> On 1/21/2012 12:39 PM, Alexandre Prokoudine wrote:
>
>> On Fri, Jan 20, 2012 at 11:02 PM, Gary Aitken wrote:
>>
>>> Hello all,
>>>
>>> gimp 2.7.5 on winXP, unfortunately.  New hardware problems on my freebsd
>>> system.  grr.  Anyway...
>>>
>>> Why won't the gimp allow me to use a difference of gaussians on a layer
>>> mask?  This seems like a perfectly fine thing to do for edge detection,
>>> and
>>> indeed, other edge-detect algorithms work:
>>>
>>> For example:
>>>  New layer mask, from grayscale copy of layer
>>>  Filters / edge-detect / difference of gaussians says:
>>>    Execution error for 'Difference of Gaussians':
>>>    Can operate on layers only (but was called on channel or mask).
>>> but
>>>  Filters / edge-detect / Edge...
>>>    works fine
>>>
>>> Is this a bug, or am I missing something?
>>> Any insights would be appreciated,
>>>
>>
>> Hi Gary,
>>
>> Not sure about the technical side of this, but you should be able to
>> perfectly work around that.
>>
>> 1. Enable display of the mask via right-click menu so that the mask is
>> displayed instead of the layers's contents.
>> 2. Layers>  New from Visible.
>> 3. Apply your filter to that layer.
>> 4. Copy all of the layer's contents and paste it into the mask.
>> 5. Remove or disable the extra layer you created.
>>
>> Alexandre Prokoudine
>> http://libregraphicsworld.org
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>>
>>
>>
>>
>>
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