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Re: Converting 8-bit image + pallete to 24 bit image with alpha channel [message #19304]

Fri, 10 March 2000 00:00

Struan Gray
Messages: 178
Registered: December 1995

Senior Member

Ricardo Fonseca, zamb@physics.ucla.edu writes:

> AlphaImage = BytArr(4, s[1], s[2])
> AlphaImage[0,*,*] = rr(Data[*,*])
> AlphaImage[1,*,*] = gg(Data[*,*])
> AlphaImage[2,*,*] = bb(Data[*,*])
> AlphaImage[3,*,*] = 128

It is often faster to construct the array directly than to
construct and empty array and fill the planes:

TVLCT, rr, gg, bb, /get
data = bytscl(dist(200))
s = Size(Data)

alphachannel = make_array(size=s, value=128b)
AlphaImage2 = [rr(Data), gg(Data), bb(Data), alphachannel]
alphaimage2 = reform(alphaimage2, s[1], 4, s[2], /overwrite)
alphaimage2 = transpose(alphaimage2, [1,0,2])

It is not as easy to see what is going on here, but on my machines
it is three to four times faster.

The last transpose step is necessary because of the way IDL orders
array elements in memory (as is the order of the dimensions in the
reform line). With 3-channel images you can usually avoid the
transpose step if you correctly use the TRUE or INTERLEAVE
keyword/properties of plotting routines or image objects. I haven't
used alpha channels much, and the help files are opaque, so I don't
know if you can set an interleave for a four channel image - if you
can, the transpose step can be omitted here too.

Struan

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		Re: Converting 8-bit image + pallete to 24 bit image with alpha channel By: Struan Gray on Fri, 10 March 2000 00:00
		Re: Converting 8-bit image + pallete to 24 bit image with alpha channel By: davidf on Thu, 09 March 2000 00:00

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