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This program is part of Netpbm. pamcomp reads two images and produces a composite image with one of the images overlayed on top of the other, possible translucently. The images need not be the same size. The input and outputs are Netpbm format image files.
In its simplest use, pamcomp simply places the image in the file overlay_file on top of the image in the file underlying_file, blocking out the part of underlying_file beneath it.
If you add the -alpha option, then pamcomp uses the image in file alpha-pgmfile as a transparency mask, which means it determines the level of transparency of each point in the overlay image. The transparency mask must have the same dimensions as the overlay image. In places where the transparency 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 transparency 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 transparency 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 transparency mask is a PGM file in which a white pixel represents opaqueness and a black pixel transparency. Anything in between is translucent. (Like any Netpbm program, pamcomp will see a PBM file as if it is PGM).
If the overlay image is a PAM image of tuple type RGB_ALPHA or GRAYSCALE_ALPHA, then the overlay image contains transparency information itself and pamcomp uses it the same way as the transparency mask described above. If you supply both an overlay image that has transparency information and a transparency mask, pamcomp multiplies the two opacities to get the opacity of the overlay pixel.
Before Netpbm 10.25 (October 2004), pamcomp did not recognize the transparency information in a PAM image -- it just ignored it. So people had to make appropriate transparency masks in order to have a non-opaque overlay. Some Netpbm programs that convert from image formats that contain transparency information are not able to create RGB_ALPHA or GRAYSCALE_ALPHA PAM output, so you have to use the old method -- extract the transparency information from the original into a separate transparency mask and use that as input to pamcomp.
The output image is always of the same dimensions as the underlying image. pamcomp uses only parts of the overlay image that fit within the underlying image.
The output image is a PAM image. Its tuples are color, grayscale, or black and white, whichever is the "highest" format between the two input images. The maxval of the output is the least common multiple of the maxvals of the input, up to the maximum possible PAM maxval, 65535.
The output has an opacity channel if and only if the underlying image does, and then the opacities are as described under the -mixtransparency option. Before Netpbm 10.56 (September 2011), the output never has an opacity channel.
To specify where on the underlying image to place the overlay image, use the -align, -valign, -xoff, and -yoff options. Without these options, the default horizontal position is flush left and the default vertical position is flush top.
The overlay image, in the position you specify, need not fit entirely within the underlying image. pamcomp uses only the parts of the overlay image that appear above the underlying image. It is possible to specify positioning such that none of the overlay image is over the underlying image -- i.e. the overlay is out of frame. If you do that, pamcomp issues a warning.
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).
The overlay_file argument is the name of the file containing the overly 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.
In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common Options), pamcomp recognizes the following command line options:
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.
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.
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.
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.
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.
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 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.
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.
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.
pammixmulti.html mixes together two or more images of the same size, in various ways.
ppmmix and pnmpaste are simpler, less general versions of the same tool.
ppmcolormask and pbmmask, and pambackground can help with generating a transparency mask.
pnmcomp is an older program that runs faster, but has less function.
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.