[magick-users] gradient to transparency

[Christoph P. Kukulies]

Thanks to all, who gave suggestions.

Pete sent me a line which seems to do what I want:

convert -size 100x100 gradient:red-none grad_rn.png

(btw, which command does this for any direction? left-right, right-left,
bottom-top?)

http://accms04.physik.rwth-aachen.de/~kuku/IMgck/gb.html

Since I actually need only a small strip if this to generate the 
drop shadow edge I tried to 

convert -size 17x17 gradient:#103090-none grad_bluen_17x17.png

and do a background-repeat on it.

But the result doesn't look so nice. Why is there an unsymmetry in the
coloring resulting in that not so smooth looking effect?:
(I would want to have the background repeat seamlessly)

http://accms04.physik.rwth-aachen.de/~kuku/IMgck/grad-blue-none_overbackgnd.gif

(that the background doesn't shine through on this one had another reason and doesn't matter here - it was contained in a <div> that had no background)

I'm only concerned about the inaccuracy. Is it an Imagemagick problem
(precision, rounding)?

Thanks.


--
Chris Christoph P. U. Kukulies 

On Fri, Apr 11, 2008 at 03:58:26PM -0400, Ross Presser wrote:
> I really must remember to use "Reply All" with this list.
> 
> Anyhow, I answered my own question:  add a "-negate" right after the -gamma
> 1.0 expression.  Then the red is opaque and the white is transparent.
>    http://rpresser.googlepages.com/test2.html
> 
> This is a very interesting effect.
> 
> On Fri, Apr 11, 2008 at 3:53 PM, Ross Presser <rpresser at gmail.com> wrote:
> 
> > Following along here ... this did not do what I was expecting.
> >
> > convert \( -size 100x100 gradient:red \) \
> > >        \( +clone -colorspace Gray -gamma 1.0 \) \
> > >        +matte -compose Copy_Opacity -composite out.png
> > >
> >
> > The result of this is that the image is pale red and transparent at the
> > top, slightly darker red and less transparent in the middle, and totally
> > white and opaque at the bottom. I was expecting an image that was opaque and
> > red at the top, and completely transparent at the bottom.
> >
> > I put up a sample HTML page showing this out.png over top of a futurama
> > image:
> > http://rpresser.googlepages.com/test.html
> >
> > This may be what Christoph wanted, I dunno. But how would I do the other
> > thing?  I'm picturing that the red channel value and alpha channel value
> > would be identical throughout.