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pamcomp Updated: 17 July 2004 Table Of Contents NAME pamcomp - composite (overlay) two Netpbm images together SYNOPSIS pamcomp [-align={left|center|right| beyondleft|beyondright}] [-valign={top|middle|bottom| above|below}] [-xoff=X] [-yoff=Y] [-alpha=alpha-pgmfile] [-invert] [-opacity=opacity] [-linear] overlay_file [underlying_file [output_file]] Minimum unique abbreviation of option is acceptable. You may use double hyphens instead of single hyphen to denote options. You may use white space in place of the equals sign to separate an option name from its value. DESCRIPTION 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 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 to- tally blocked out. In places where the alpha mask defines the overlay im- age 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 (translu- cence), 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. (Like any Netpbm program, pamcomp will see a PBM file as if it is PGM). In some image file formats (PNG, for example), transparency information (the alpha mask) is part of the definition of the image. In the Netpbm formats, transparency is usually embodied in a separate companion file. The Netpbm 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 pamcomp. Sometimes, though, people use a version of the PAM format that has an alpha channel built in. But pamcomp doesn't know how to use those. It processes the images just fine, but ignores the built-in alpha channel. The output image is always of the same dimensions as the un- derlying image. pamcomp uses only 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 -align, -valign, -xoff, and -yoff options. Without these op- tions, the default horizontal position is flush left and the default ver- tical 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 over- lay image that appear above the underlying image. It is possible to specify positioning such that none of the overlay image is over the underlying im- age -- 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 for- mats (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 in- put 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 -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 remi- niscent 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 ba- sic 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 ba- sic 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 alpha mask. If you don't specify this option, there is no alpha mask, which is equivalent to having an alpha mask specify total opaqueness every- where. You can specify - as the value of this option and the alpha 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 al- pha 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 an alpha mask (the -alpha option), pam- comp uses the product of the opacity indicated by the alpha 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 un- derlay 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 underlaying image subtracted from it. In either case, pamcomp clips the resulting color component in- tensities 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 underlaying image. See this description of the tech- nique. This option was added in Netpbm 10.6 (July 2002). Be- fore Netpbm 10.15 (April 2003), values less than zero or greater than unity were not allowed. -linear This option indicates that the inputs are not true Netpbm images but rather a non-gamma-adjusted variation. This is relevant only when you mix pixels, using the -opacity option or an alpha mask (the -alpha option). The alpha 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 num- bers that are not linearly proportional to intensity. So pamcomp, by default, performs a calculation on each sample read from its in- put 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 im- ages, but rather a variation in which the sample values are in fact directly proportional to intensity. If so, use the -linear op- tion 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 ppmmix and pnmpaste are simpler, less general versions of the same tool. ppmcolormask and pbmmask can help with generating an alpha mask. pnmcomp is an older program that runs faster, but has less function. pnm 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 (koblas@mips.com). _________________________________________________________________ Table Of Contents * NAME * SYNOPSIS * DESCRIPTION * OPTIONS * SEE ALSO * HISTORY * AUTHOR
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