The Reduce Resolution function is available on the Image menu.
When you select this option it shows the following prompt for your confirmation. This is because there is no Undo for this operation.
There is no undo is because this operation is primarily intended for cases when the computer is running low on memory because the image is too large. (You can still use the File->Revert/Reopen menu option to reload the original image from file.)
We have seen many examples where an image has been scanned at an unnecessarily high resolution. For example, 400 dpi when 200 dpi would have been fine. This can have a big impact on the computer memory requirement, because each time the resolution is doubled it makes the image 4 times as big in memory.
For example, consider an A0 (E size) drawing scanned at 400 dpi in 8 bit colour depth (256 colours). This image would be about 48 inches long and 36 inches wide, and with 400 pixels per inch it would have over 276 million pixels. At 8 bit depth there is 1 byte per pixel, so this is a 276 Mb image. Your computer may have 512 Mb of RAM, so you might think that you're OK. But, don't forget that you'll still need another 276 Mb during the Thinning phase of vectorisation because it needs to work on a copy. So, with this image, you'll soon run out of RAM, and that makes the computer run very slowly.
From these figures you can see that halving the image resolution to 200 dpi would make the same image take only 69 Mb of memory, which is no problem for a modern computer.
WinTopo uses a smart algorithm to cope with all types of bit depth image when reducing the resolution. The resulting image will retain as much information as possible. However, you should be aware that this operation will always lose some image detail.
It is advisable to consider the Reduce Palette Colours operation in preference to reducing the resolution, unless it is obvious that the resolution is too high.