LAB channels don't behave during threshold

[tom_dl]

I'm having trouble getting the result I want while using ImageMagick 6.8.9-5 Q16 x64 on Windows 7 64bit via Cygwin.

I want to take an input image, convert it to LAB colorspace, apply a threshold on the 'A' channel, and output this as a grayscale (more specifically black&white, as a threshold should be). I am using the following line:

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convert input.png -colorspace lab -channel G -separate -threshold 50% +channel ouput.png

The result I get is not grayscale at all, but rather a 'greenscale' with 126 colours (according to identify), rather than the 2 I'm expecting. I've tried using this line:

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convert input.png -colorspace lab -channel G -separate -threshold 50% -channel RB -evaluate set 0 +channel output.png

to zero channels L and B (i.e. R and B) as explained in the online documentation, but that's still an awful green mess. Can someone explain where I'm going wrong please?

[tom_dl]

Turns out I needed to -separate again:

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convert input.png -colorspace lab -channel G -separate -threshold 50% -channel G -separate output.png

Thought I'd leave this here for anyone else confused by separating channels.

[snibgo]

convert input.png -colorspace lab -channel G -separate -threshold 50% ouput.png

There are two problems with that command. The first is that PNG can't store Lab, so IM converts it back to sRGB. The cure is to lie to IM and tell it that the image is actually sRGB so there is no need to convert it. Do this with "-set colorspace sRGB".

The second problem is that "-channel G" remains in effect until cancelled with another "channel" command. Use "+channel" to get back to the default, which is RGB.

So we get:

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convert in.png -colorspace Lab -set colorspace sRGB -channel G -separate +channel -threshold 50% out.png

[tom_dl]

@Snibgo - thanks very much for your answers. I didn't realise PNG didn't support Lab, so tricking imagemagick didn't cross my mind. Great solution!