[Kstars-devel] questions re: gathering images

[Luciano Montanaro]

On Thursday 07 December 2006 16:11, Luciano Montanaro wrote:

> Wait...
> For monochrome images, you could use the image as an alpha channel
> (inverted, maybe -- I never remember if the alpha value 255 means
> transparent or opaque) the RGB channels could just be set to white. I think
> this would give a smoother transitione between the bluish background and
> the image. Maybe this didn't work in Qt3 times, but it should work well
> with Qt4.
>
> I imagine it could work for color images as well -- you just have to use a
> B/W copy of the image as the alpha channel.
>
> Luciano

Actually, alpha works with Qt3 as well, in this case.

Using this image:
http://www.cirulla.net/listing/kde_stuff/kstars/m42-alpha.png

which I obtained from the kstars m42 image with these steps:

Convert image to RGB
Create a mask from a grayscale copy of the image (Layer->Mask->Add Layer Mask)
From the layers dialog, select the mask (it should be already selected)
Adjust color levels, with Tools->Color Tools->Levels
 (I have used the "auto" function to find the minnimum, and set the mask to 
opaque for values above 128, so luminous objects are visible, and the rest of 
the images blends gradually to the background)
Apply the layer mask from the popup menu in the layers dialog
Save.

This is the resulting image in KStars:
http://www.cirulla.net/listing/kde_stuff/kstars/orion-nebula-alpha.png

Compared to this:
http://www.cirulla.net/listing/kde_stuff/kstars/orion-nebula-no-alpha.png

I'd say it is an improvement.

The only problem is that the new image is three times as large as the 
original. I'm not sure if this is a problem. Maybe the images using the alpha 
channels could be given as an alternative to the smaller images as a 
different KHotnewstuff source.

Luciano

-- 
Luciano Montanaro //
                \X/ mikelima at cirulla.net