Re: opacity

At 01:04 PM 5/28/2004 +0100, Patrick Welche wrote:
>On Wed, May 26, 2004 at 12:39:27PM -0400, Glenn Randers-Pehrson <glennrp> wrote:
>> If mask.png is an opaque grayscale image,
>> then composite will use its intensity when it wants opacity.
>> 
>> gm convert skill.eps -matte skill_with_matte.miff
>> gm composite -compose copyopacity null: skill_with_matte.miff mask.png out.png
>
>Thank you for the recipe - now I wonder if there isn't something up with
>gm display..
>
>I enclose out.png which was created with:
>
>        gm convert -matte logo.eps logo.png
>        gm convert mask.eps mask.png
>        gm composite -compose copyopacity mask.png logo.png out.png
>
>and I was confused by the outcome, because gm display out.png shows
>a ragged circular outline. The whole point of using the mask, was
>essentially to use the automatic antialiasing of the mask to get
>semitransparent pixels and avoid seeing the pixelation. I just
>viewed out.png with firefox on a windoze box, and the outline is
>smooth. Bringing up the magnifier in gm display shows cells with
>rgba values like (255,255,255,17) so the image really is semitransparent
>(though I didn't realise at first), and I was also confused by gm
>identify -verbose out.png with
>
>  Colors: 3
>
>      3120: (  0,  0,  0,  0)    black
>       160: (102,102,102,  0)    grey40
>      6720: (255,255,255,255)    #FFFFFFFF
>
>which doesn't give the game away about the non 255 values in the
>alpha channel..
>
>Anyway, it all works in the sense that image is what I wanted.
>(I can now just weep because the most popular browser can't display it)
>
>Cheers,
>
>Patrick
>
>PS could gm display's output have something to do with my X setup?
>
>  depth of root window:    24 planes
>  number of colormaps:    minimum 1, maximum 1
>  default colormap:    0x20
>  default number of colormap cells:    256
>  preallocated pixels:    black 0, white 16777215
>
>seems there are enough colours available..
>
>Attachment Converted: "c:\documents and settings\owner\old_disks\gateway c\eudora\attach\out1.png"

This file looks fine to me under FireFox 0.8 and Mozilla-1.7rc1
When I use a dark background, the circle is clearly nice and round,
antialiased.

I never had much luck with either GM display or IM display and
semitransparent images; they always thresholded the alpha channel
down to binary on my platforms.

Glenn



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