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Re: How must Alpha be managed ?? [message #38550 is a reply to message #38508]

Sun, 23 December 2012 10:03

mirek
Messages: 14267
Registered: November 2005

Ultimate Member

Didier wrote on Thu, 20 December 2012 18:16

Hi all,

I am currently trying to draw buttons that are transparent but they have special shapes.

For rectangles: it is easy, all I have to do is draw a small img in iml designer, set the RGBA values for all points and then use this image to draw in the dest rectangle.

For other shapes (circles for ex), I try to use ImageDraw and Alpha() method and it works ...

Question:
In the iml designer: all points are set to : RGBA(46,46,46,51)
Using Image Draw, I draw a circle with Color(46,46,46) and apply GreyColor(51)
... expecting to get RGBA(46,46,46,51)
But the result is not the same at all !! Sad

Which is the way to go ?? (the color obtained with src image gives me the result expected by client )

Actually, you are doing the right. In fact, ImageDraw uses R channel as alpha.

Anyway, I think the difference you can see MIGHT be because of alpha channel premultiply issue. E.g. RGBA(46, 46, 46, 51) can never be present in Image (it is invalid value). If you have applied 51 alpha to RGB(46, 46, 46), you should get something like RGBA(46, 46, 46, 9) if you read pixel from Image created from ImageDraw.

Mirek

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