Minimum unique abbreviations are acceptable.
In its simplest use, pnmcomp simply places the overlay file on top of the pnm-input file, blocking out the part of the pnm-input file beneath it. If you specify the alpha-pgmfile, pnmcomp uses it as an alpha mask, which means it determines the level of transparency of each point in the overlay image. The alpha mask must have the same dimensions as the overlay image. In places where the alpha mask defines the overlay image to be opaque, the composite output contains only the contents of the overlay image; the underlying image is totally blocked out. In places where the alpha mask defines the overlay image to be transparent, the composite output contains none of the overlay image; the underlying image shows through completely. In places where the alpha mask shows a value in between opaque and transparent (translucence), the composite image contains a mixture of the overlay image and the underlying image and the level of translucence determines how much of each.
The alpha mask is a PGM file in which a white pixel represents opaqueness and a black pixel transparency. Anything in between is translucent.
In some image file formats (PNG, for example), transparency information (the alpha mask) is part of the definition of the image. In the PNM formats, transparency is always embodied in a separate companion file. The PNM converter programs that convert from an image format such as PNG have options that allow you to extract the transparency information to a separate file, which you can then use as input to pnmcomp.
The output image is always of the same dimensions as the underlying image. pnmcomp only uses parts of the overlay image that fit within the underlying image.
To specify where on the underlying image to place the overlay image, use the -xoff, -yoff, -align, and -valign options. Without these options, the default horizontal position is flush left and the default vertical position is flush top.
The overlay and underlying images may be of different formats (e.g. overlaying a PBM text image over a full color PPM image) and have different maxvals. The output image has the more general of the two input formats and a maxval that is the least common multiple the two maxvals (or the maximum maxval allowable by the format, if the LCM is more than that).