In a .ico file, there is no such thing as partial transparency (translucency). Where the PGM mask says completely opaque, the icon will be opaque. Everywhere else, the icon will be transparent. Note that as with any Netpbm program, you can use a PBM image for the transparency mask and ppmtowinicon will treat it like a PGM.

The and mask is like a transparency mask, except for what it signifies in the "not opaque" areas. In the usual case, the foreground image is black in those areas, and in that case the areas are fully transparent -- the background shows through the icon. But in general, a not opaque pixel signifies that the background and foreground should be merged as follows: The intensities of the color components in the foreground and background are represented as binary numbers, then corresponding bits of the background and foreground intensities are exclusive-or'ed together. So there is a sort of reverse video effect.

If you don't want this special effect and instead want straightforward transparency, use the -truetransparent option. This causes ppmtowinicon to make the base image black everywhere your transparency mask says transparent, regardless of what color your input image is at that location.

If you don't specify -andmask, ppmtowinicon puts all-opaque and masks into the .ico file.