NAME
pamcomp - composite (overlay) two Netpbm images togetherSYNOPSIS
pamcompDESCRIPTION
This program is part of Netpbm(1).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).
ARGUMENTS
The overlay_file argument is the name of the file containing theoverly image, while underlying_file is the name of the file
containing the underlying image. For either, you may specify '-'
to indicate Standard Input, and underlying file defaults to Standard
Input. Make sure you aren't specifying (or defaulting) Standard Input as
both. Note that there may be a third input file, identified by an -alphafile option. The output_file argument is the name of the file to which
pamcomp writes the output, creating or truncating it first. You may
specify '-' to indicate Standard Output, in which
case pamcomp does not truncate it. Note that pamcomp is
unusual among Netpbm programs, as a historical accident, in having an output
file argument; Netpbm programs normally write to Standard Output only.
OPTIONS
In addition to the options common to all programs based on libnetpbm (most notably -quiet, seeCommon Options ), pamcomp recognizes the following command line options:
- -align=alignment
- This option selects the basic horizontal position of the overlay image with respect to the underlying image, in syntax reminiscent of HTML. left means flush left, center means centered, and right means flush right. The -xoff option modifies this position. beyondleft means just out of frame to the left -- the right edge of the overlay is flush with the left edge of the underlying image. beyondright means just out of frame to the right. These alignments are useful only if you add a -xoff option. These two values were added in Netpbm 10.10 (October 2002). The default is left.
- -valign=alignment
- This option selects the basic vertical position of the overlay image with respect to the underlying image, in syntax reminiscent of HTML. top means flush top, middle means centered, and bottom means flush bottom. The -yoff option modifies this position. above means just out of frame to the top -- the bottom edge of the overlay is flush with the top edge of the underlying image. below means just out of frame to the bottom. These alignments are useful only if you add a -yoff option. These two values were added in Netpbm 10.10 (October 2002). The default is top.
- -xoff=x
- This option modifies the horizontal positioning of the overlay image with respect to the underlying image as selected by the -align option. pamcomp shifts the overlay image from that basic position x pixels to the right. x can be negative to indicate shifting to the left. The overlay need not fit entirely (or at all) on the underlying image. pamcomp uses only the parts that lie over the underlying image. Before Netpbm 10.10 (October 2002), -xoff was mutually exclusive with -align and always measured from the left edge.
- -yoff=y
- This option modifies the vertical positioning of the overlay image with respect to the underlying image as selected by the -valign option. pamcomp shifts the overlay image from that basic position y pixels downward. y can be negative to indicate shifting upward. The overlay need not fit entirely (or at all) on the underlying image. pamcomp uses only the parts that lie over the underlying image. Before Netpbm 10.10 (October 2002), -xoff was mutually exclusive with -valign and always measured from the top edge.
- -alpha=alpha-pgmfile
- This option names a file that contains the transparency mask. If you don't specify this option, there is no transparency mask, which is equivalent to having a transparency mask specify total opaqueness everywhere. You can specify - as the value of this option and the transparency mask will come from Standard Input. If you do this, don't specify Standard Input as the source of any other input image.
- -invert
- This option inverts the sense of the values in the transparency mask, which effectively switches the roles of the overlay image and the underlying image in places where the two intersect.
- -opacity=opacity
- This option tells how opaque the overlay image is to be, i.e. how much of the composite image should be from the overlay image, as opposed to the underlying image. opacity is a floating point number, with 1.0 meaning the overlay image is totally opaque and 0.0 meaning it is totally transparent. The default is 1.0. If you specify a transparency mask (the -alpha option), pamcomp uses the product of the opacity indicated by the transparency mask (as modified by the -invert option, as a fraction, and this opacity value. The -invert option does not apply to this opacity value. As a simple opacity value, the value makes sense only if it is between 0 and 1, inclusive. However, pamcomp accepts all values and performs the same arithmetic computation using whatever value you provide. An opacity value less than zero means the underlay image is intensified and then the overlay image is "subtracted" from it. An opacity value greater than unity means the overlay image is intensified and the underlay image subtracted from it. In either case, pamcomp clips the resulting color component intensities so they are nonnegative and don't exceed the output image's maxval. This may seem like a strange thing to do, but it has uses. You can use it to brighten or darken or saturate or desaturate areas of the underlay image. See this description(1) of the technique. This option was added in Netpbm 10.6 (July 2002). Before Netpbm 10.15 (April 2003), values less than zero or greater than unity were not allowed.
- -mixtransparency
- This option controls what pamcomp does where both the underlying and overlay image are non-opaque. By default, the output image has the same transparency as the underlying image and the transparency of the underlying image has no effect on the composition of color. But with this option, pamcomp composes the image according to a plastic transparency metaphor: the underlying and overlay images are plastic slides. The output image is the slide you get when you stack up those two slides. So the transparency of the output is a combination of the transparency of the inputs and the transparency of the underlying image affects the underlying image's contribution to the output image's color. Unlike the metaphorical slide, a PAM pixel has a color even where it is completely transparent, so pamcomp departs from the metaphor in that case and makes the output color identical to the underlying image. This option was new in Netpbm 10.56 (September 2011). Before that, the output is always opaque and the pamcomp ignores the transparency of the underlying image.
- -linear
- This option indicates that the inputs are not true Netpbm images but rather a light-intesity-proportional variation. This is relevant only when you mix pixels, using the -opacity option or a transparency mask (the -alpha option). The transparency mask and -opacity values indicate a fraction of the light intensity of a pixel. But the PNM and PNM-equivalent PAM image formats represent intensities with gamma-adjusted numbers that are not linearly proportional to intensity. So pamcomp, by default, performs a calculation on each sample read from its input and each sample written to its output to convert between these gamma-adjusted numbers and internal intensity-proportional numbers. Sometimes you are not working with true PNM or PAM images, but rather a variation in which the sample values are in fact directly proportional to intensity. If so, use the -linear option to tell pamcomp this. pamcomp then will skip the conversions. The conversion takes time. And the difference between intensity-proportional values and gamma-adjusted values may be small enough that you would barely see a difference in the result if you just pretended that the gamma-adjusted values were in fact intensity-proportional. So just to save time, at the expense of some image quality, you can specify -linear even when you have true PPM input and expect true PPM output. For the first 13 years of Netpbm's life, until Netpbm 10.20 (January 2004), pamcomp's predecessor pnmcomp always treated the PPM samples as intensity-proportional even though they were not, and drew few complaints. So using -linear as a lie is a reasonable thing to do if speed is important to you. Another technique to consider is to convert your PNM image to the linear variation with pnmgamma, run pamcomp on it and other transformations that like linear PNM, and then convert it back to true PNM with pnmgamma -ungamma. pnmgamma is often faster than pamcomp in doing the conversion.
SEE ALSO
pammixmulti.html(1) mixes together two or more images of the same size, in various ways. ppmmix(1) and pnmpaste(1) are simpler, less general versions of the same tool. ppmcolormask(1) and pbmmask(1), and pambackground(1) can help with generating a transparency mask. pnmcomp(1) is an older program that runs faster, but has less function. pnm(1)HISTORY
pamcomp was new in Netpbm 10.21 (March 2004). Its predecessor, pnmcomp, was one of the first programs added to Netpbm when the project went global in 1993.AUTHOR
Copyright (C) 1992 by David Koblas ( [email protected]).DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation is at13 August 2011 | netpbm documentation |