Create mask from top-left pixel color?

[peravoj]

Just want to know what "Create mask from top-left pixel color" actually does? And what are it's uses in general.
I tried to find some explanation in the help and manual but no luck.
Thank you

[Gintaras]

Uses color of top-left pixel of the image file as a transparent color. When searching, does not compare pixels of this color.

[peravoj]

Ok i understand. But how and why would i use this? Are there any benefits or downsides in using this option?

[Gintaras]

Rarely used. The main purpose is reliability.

Use when you want to compare PART of the image file, not the whole rectangle. For example, in mspaint you can easily draw a mask on a captured image. Draw parts that don't want to compare with the color of the top-left pixel. You can choose any color.

Also: Often you capture an icon or other small image that is drawn in a dialog or some other background color. The background can change (changed theme, Windows version etc). In most cases color of top-left pixel is background, not the visible part of the icon. If you use this flag, you make the searching independent of the background color. However it will not work with icons that have alpha channel (partially transparent parts), unless you edit the image.

[peravoj]

Thank you for that explanation it made it clear a lot And i already have use for it!
Thank's again! This awesome software is just full of wonderful surprises