CPpopup and Gmask will not always work

As mentioned before, if CPpopup tells you a CP that doesn't work, you might be able to figure out the CP still. 'Slick Ron' showed one way in archive 6. =)
If CPpopup says "undecodable", then there isn't much you can do but ask around.
In any case, if an image is masked, you will almost always be able to find an unmasked version of the same image elsewhere, usually at an even higher quality. This is because masked images are almost always found in websites, and when found in websites, the images tend to be reduced versions of the originals.
In my experience, CPpopup and Gmask work the vast majority of the time (even though it might not seem like it psychologically).

Flmask supports jpeg and bmp file formats.

If you have another format, you can often copy the image from another application and paste it into Flmask.

Q0 masks

Most Blue masks fall into this category and are known as Q0 masks. Usually a Q0 mask is a combination of a Negation, a hgl (horiz. glassy effect), and a vgl. If you ever find an image you cannot unmask, fool around with all the effects, including FL. That's how I learned all that I have written here.

Meko masks

This is a very rare type of mask. You can recognize it by the fact that its masking blocks appear much larger than the usual 8x8.

Download Links

Sometimes new versions come out or links get broken, so if a link here doesn't work, see if you can search for it using a search engine like http://www.excite.com.

Urls for Flmask

Win95 English

ftp://ftp.vector.co.jp/pack/win95/art/graphics/edit/flmsk30e.exe
ftp://ftp.igat.com/pub/program/flmsk30e.exe
http://www.martec.net/isg/flmask/bin/flmsk30e.exe
http://www.igat.com/flmask/bin/flmsk30e.exe

Win 3.1 English

ftp://ftp.vector.co.jp/pack/win31/art/graphics/edit/flmsk27e.exe
ftp://ftp.igat.com/pub/program/flmsk27e.exe
http://www.martec.net/isg/flmask/bin/flmsk27e.exe
http://www.igat.com/flmask/bin/flmsk27e.exe

Win95 Japanese

ftp://ftp.vector.co.jp/pack/win95/art/graphics/edit/flmsk30j.exe

Win 3.1 Japanese

ftp://ftp.vector.co.jp/pack/win31/art/graphics/edit/flmsk27j.exe

Urls for Gmask

http://village.infoweb.or.jp/~furumizo/gmsk160e.lzh (English)
http://village.infoweb.or.jp/~furumizo/gmask160.lzh (Japanese)
http://www.raysco.com/patches/zip/GMask.zip (English)
http://village.infoweb.ne.jp/~fwga3304/italy160.lzh (Italian)
http://village.infoweb.ne.jp/~fwga3304/cn160.lzh (Chinese Add-on?)

The first two links are lzh files, so pkunzip won't open unless you get the lha addon. The reason I don't link to self-extracting executables is not that they aren't necessarily reliable links, just that I don't want to encourage people to run .exe files that they download since they can easily be viruses. If you're smart enough to know when a download is safe and when it isn't, then I assume you are knowledgeable enough to also know how to unzip .lzh files in the first place. ;)

Gmask webpage:

http://village.infoweb.ne.jp/~fwga3304/gmaskd.htm

Url for CPPopup and other decoders

http://www.rr.iij4u.or.jp/~cosack/archive/cppopup102e.exe

A program similar to CPpopup

http://www1.t3.rim.or.jp/~p-lolita/___/neko.zip

The thanks goes to the professor for putting this up. Unfortunately, I have not gotten it to work, and the instructions are in Japanese. ;)

Mac Users

Mac Users
For Mac users, try Mozkiller. It is in Japanese, however.

http://members.aol.com/uchino4/mozkiller.sit

http://members.aol.com/uchino4/mozkiller.sit.hqx

Url Notes

If one of these urls does not work, notify me so that I can update this FAQ. Usually the reason why links die out is because new versions of the programs come out.
All URLs were found using the excite search engine and tested out. If you want to search for things yourself but have trouble with search engines, try FAQ 4. If I can do it, you can too. =)

The rest you'll have to do yourself. ;)

Closing Comments

Most people I know rarely unmask images anymore. Often it is not worth the trouble. This is because you can often find better quality as well as higher quantity on newsgroups (even of the same images). In fact, Bluebird images are considered of such high quality that they are rarely ever masked, and so they typically appear either in newsgroups or on quality webpages (Bluebirds are scans of Japanese models, typically of the same quality and content as Playboy is in the US).

Then again, many casual surfers who do not use news will probably find unmasking to be very useful. ;)

-Ramses

I write these FAQs in hopes it will benefit and educate you, so as always, feel free to correct me and add your own suggestions. You may remain anonymous or allow me to credit you with the suggestion. Also feel free to add any short jokes or comments you want.

References

Online Flmask Documentation