potential bug with extracting alpha channel IM 6.4.3-7 beta

[fmw42]

IM 6.4.3-7 beta Q16 Mac OSX (Monday beta 3:42 AM)

Trying to extract the alpha channel from:

image.png



convert -verbose info:
...
Depth: 16/8-bit
Channel depth:
red: 1-bit
green: 1-bit
blue: 1-bit
alpha: 8-bit
Channel statistics:
red:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: -0 (-0)
green:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: -0 (-0)
blue:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: -0 (-0)
alpha:
min: 0 (0)
max: 255 (1)
mean: 55.0361 (0.215828)
standard deviation: 103.896 (0.407436)
Alpha: none #0000000000000000


Test:

convert image.png -channel o image_o.png

The above seems fine

Anthony mentioned that -channel a is now different from -channel o. But either it is not working or I am premature in testing it.
convert image.png -channel a image_a.png

This is not different from -channel o

convert image.png -fx "u.a" image_fx_a.png

This is not correct as it should not be transparent. The majority (background) should be black and the letters white.

(get same result with convert image.png -channel rgba -fx "u.a" image_fx_a.png)

identify -verbose image_fx_a.png
...
Depth: 16/8-bit
Channel depth:
red: 8-bit
green: 8-bit
blue: 8-bit
alpha: 8-bit
Channel statistics:
red:
min: 0 (0)
max: 255 (1)
mean: 55.0361 (0.215828)
standard deviation: 103.896 (0.407436)
green:
min: 0 (0)
max: 255 (1)
mean: 55.0361 (0.215828)
standard deviation: 103.896 (0.407436)
blue:
min: 0 (0)
max: 255 (1)
mean: 55.0361 (0.215828)
standard deviation: 103.896 (0.407436)
alpha:
min: 0 (0)
max: 255 (1)
mean: 55.0361 (0.215828)
standard deviation: 103.896 (0.407436)
Alpha: none #0000000000000000


Am I doing something wrong here?


Likewise:

convert image.png -fx "u.o" image_fx_o.png

This is not correct as it should not be transparent. The majority (background) should be white and the letters black.

Again, I apologize if I am testing prematurely

Fred

[magick]

With the latest patch from the Subversion this command works:

• convert image.png -channel rgba -fx "u.a" -alpha deactivate image_fx_a.png

[fmw42]

With the latest patch from the Subversion this command works:

• convert image.png -channel rgba -fx "u.a" -alpha deactivate image_fx_a.png

Yes, the above does work as you suggest, even in the current beta.

But would you clarify please.

Why is -alpha deactivate necessary?

I guess I can understand why -channel rgba is needed if -fx by default only works on rgb. Therefore without it, one would get a perfectly opaque white image as -fx would create an opaque alpha to extract. So I guess one needs to add -channel rgba in order for -fx to get the real alpha channel and not generate an opaque one. Is this correct?

BUT, seems to me that -fx "u.a" is very clearly explicit that all it wants is the alpha channel as output as a grayscale image! Adding -alpha deactivate seems a bit strange and potentially more than should be necessary.


Also, what about:

convert image.png -channel a image_a.png

It gives the same results as

convert image.png -channel o image_o.png

Am I premature in testing that?

[magick]

An alpha channel can be overloaded. Sometimes it contains something other than alpha. If the alpha channel is activated it is treated as alpha when resizing or drawing or whatever (alpha-blending). If its deactivated it is treated just like red or green or blue are treated. That is why the user must explicitly state their intentions with -alpha and why we introduced -alpha to begin with.