NAME¶
pnmcomp - composite (overlay) two portable anymap files together
SYNOPSIS¶
pnmcomp [
-xoff=X |
-align={left,center,right}]
[
-yoff=Y |
-valign={top,middle,bottom}]
[
-alpha=alpha-pgmfile] [
-invert]
overlay [
pnm-input] [
pnm-output]
Minimum unique abbreviations are acceptable.
DESCRIPTION¶
pnmcomp reads two images and produces a composite image with one of the
images overlayed on top of the other. The images need not be the same size.
The input and outputs are PNM format image files.
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).
OPTIONS¶
- -invert
- This option inverts the sense of the values in the alpha
mask, which effectively switches the roles of the overlay image and the
underlying image in places where the two intersect.
- -xoff X
- -yoff Y
- These options position the overlay image with respect to
the underlying image. X and Y are the horizontal and
vertical displacements of the top left corner of the overlay image from
the top left corner of the underlying image, in pixels. A positive value
means right or down; a negative value means left or up. The overlay need
not fit entirely (or at all) on the underlying image. pnmcomp uses
only the parts that lie over the underlying image.
- -align=[left,center,right]
- This option is an alternative to -xoff, in the style
of HTML. It selects the horizontal position of the overlay image so that
it is flush left, centered, or flush right on the underlying image.
- -valign=[top,middle,bottom]
- This option is an alternative to -yoff, in the style
of HTML. It selects the vertical position of the overlay image so that it
is flush top, centered, or flush bottom on the underlying image.
SEE ALSO¶
ppmmix(1) and
pnmpaste(1) are simpler, less general versions of
the same tool.
pnm(5),
pbmmask(1)
AUTHOR¶
Copyright (C) 1992 by David Koblas (koblas@mips.com).