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Making Bitmaps from Vector Fields: avf2ppm

The avf2ppm utility converts a collection of vector field files (e.g., .omf, .ovf) into color bitmaps suitable for inclusion into documents or collating into movies. The command line arguments control filename and format selection, while plain-text configuration files, modeled after the mmDisp configuration dialog box, specify display parameters.

The avf2ppm launch command is:

tclsh oommf.tcl avf2ppm [standard options] [-config file] [-f] \
   [-filter program] [-format <P3|P6|B24|PNG>] [-ipat pattern] \
   [-opatexp regexp] [-opatsub sub] [-v level] [infile ...]
-config file
User configuration file that specifies image display parameters. This file is discussed in detail below.
Force overwriting of existing (output) files. By default, if avf2ppm tries to create a file, say foo.ppm, that already exists, it generates instead a new name of the form foo.ppm-000, or foo.ppm-001, ..., or foo.ppm-999, that doesn't exist and writes to that instead. The -f flag disallows alternate filename generation, and overwrites foo.ppm instead.
-filter program
Post-processing application to run on each avf2ppm output file. May be a pipeline of several programs.
-format <P3|P6|B24|PNG>
Specify the output image file format. Currently supported formats are the true color Portable Pixmap (PPM) formats P3 (ASCII text) and P6 (binary), the uncompressed Windows BMP 24 bits-per-pixel format, and the compressed Portable Network Graphics (PNG) format. Conversion to the PNG format requires either Tk 8.6+, or earlier Tk plus the Img package. The default format is P6.
-ipat pattern
Specify input files using a pattern with ``glob-style'' wildcards. Mostly useful in DOS.
-opatexp regexp
Specify the ``regular expression'' applied to input filenames to determine portion to be replaced in generation of output filenames. The default regular expression is: (\.[^.]?[^.]?[^.]?$|$)
-opatsub sub
The string with which to replace the portion of input filenames matched by the -opatexp regexp during output filename generation. The default is .ppm for type P3 and P6, .bmp for B24, and .png for PNG file output.
-v level
Verbosity (informational message) level, with 0 generating only error messages, and larger numbers generating additional information. The level value is an integer, defaulting to 1.
infile ...
List of input files to process.

The file specification options require some explanation. Input files may be specified either by an explicit list (infile ...), or by giving a wildcard pattern, e.g., -ipat *.omf, which is expanded in the usual way by avf2ppm (using the Tcl command glob). Unix shells (sh, csh, etc.) automatically expand wildcards before handing control over to the invoked application, so the -ipat option is not needed (although it is useful in case of a ``command-line too long'' error). DOS does not do this expansion, so you must use -ipat to get wildcard expansion in Windows.

As each input file is processed, a name for the output file is produced from the input filename by rules determined by handing the -opatexp and -opatsub expressions to the Tcl regsub command. Refer to the Tcl regsub documentation for details, but essentially whatever portion of the input filename is matched by the -opatexp expression is removed and replaced by the -opatsub string. The default -opatexp expression matches against any filename extension of up to 3 characters, and the default -opatsub string replaces this with the extension .ppm or .bmp.

If you have command line image processing ``filter'' programs, e.g., ppmtogif (part of the NetPBM package), then you can use the -filter option to pipe the output of avf2ppm through that filter before it is written to the output file specified by the -opat* expressions. If the processing changes the format of the file, (e.g., ppmtogif converts from PPM to GIF), then you will likely want to specify a -opatsub different from the default.

Here is an example that processes all files with the .omf extension, sending the output through ppmtogif before saving the results in files with the extension .gif:

tclsh oommf.tcl avf2ppm -ipat *.omf -opatsub .gif -filter ppmtogif
(On Unix, either drop the -ipat flag, or use quotes to protect the input file specification string from expansion by the shell, as in -ipat '*.omf'.) You may also pipe together multiple filters, e.g., -filter "ppmquant 256 | ppmtogif".

Configuration files
The details of the conversion process are specified by plain-text configuration files, in the same format as the mmDisp configuration file.

Each of the configurable parameters is an element in an array named plot_config. The default values for this array are read first from the main configuration file

followed by the local customization file
if it exists. Lastly, any files passed as -config options on the command line are input. Each of these parameters is interpreted as explained in the mmDisp documentation, except that avf2ppm ignores the misc,defaultwindowwidth and misc,defaultwindowheight parameters, and the following additional parameters are available:
If 1, then each pixel along the edge of an arrow is drawn not with the color of the arrow, but with a mixture of the arrow color and the background color. This makes arrow boundaries appear less jagged, but increases computation time. Also, the colors used in the anti-aliased pixels are not drawn from the arrow or pixel colormap discretizations, so color allocation in the output bitmap may increase dramatically.
Width of a colored outline around each arrow; this can improve visibility of an arrow when it is overlayed against a background with color similar to that of the arrow. Default value is zero, meaning no outline. A value of 1 produces an outline with a recommended width, and other positive values are scaled relative to this.
If arrow,outlinewidth is positive, then this is the color of the arrow outline.
Placement of the bounding polygon, either back or front, i.e., either behind or in front of the rendered arrows and pixel elements.
Specifies the width, in pixels, of a mat (frame) around the outer edge of the image. The mat is drawn in front of all other objects. To disable, set matwidth to 0.
Color of the mat.
misc,width, misc,height
Maximum width and height of the output bitmap, in pixels. If misc,crop is enabled, then one or both of these dimensions may be shortened.
If disabled (0), then any leftover space in the bitmap (of dimensions misc,width by misc,height) after packing the image are filled with the background color. If enabled (1), then the bitmap is cropped to just include the image (with the margin specified by misc,margin). NOTE: Some movie formats require that bitmap dimensions be multiples of 8 or 16. For such purposes, you should disable misc,crop and specify appropriate dimensions directly with misc,width and misc,height.

The default configuration file can be used as a starting point for user configuration files. You may also use configuration files produced by the File|Write config... command in mmDisp, although any of the above avf2ppm-specific parameters that you wish to use will have to be added manually, using a plain text editor. You may omit any entries that you do not want to change from the default. You may ``layer'' configuration files by specifying multiple user configuration files on the command line. These are processed from left to right, with the last value set for each entry taking precedence.

array set plot_config {
    arrow,status       1                misc,background    #FFFFFF
    arrow,antialias    1                misc,drawboundary  1
    arrow,outlinewidth 0.0              misc,boundarywidth 1
    arrow,outlinecolor #000000          misc,boundarycolor #000000
    arrow,colormap     Red-Black-Blue   misc,boundarypos   front
    arrow,colorcount   100              misc,matwidth      0
    arrow,quantity     z                misc,matcolor      #FFFFFF
    arrow,colorphase   0                misc,margin        10
    arrow,colorreverse 0                misc,width         640
    arrow,autosample   1                misc,height        480
    arrow,subsample    10               misc,crop          1
    arrow,size         1                misc,zoom          0
    arrow,viewscale    1                misc,rotation      0
                                        misc,datascale     0
    pixel,status       1                misc,relcenterpt   {0.5 0.5 0.5}
    pixel,colormap     Teal-White-Red
    pixel,colorcount   100              viewaxis            +z
    pixel,opaque       1                viewaxis,xarrowspan {}
    pixel,quantity     x                viewaxis,xpixelspan {}
    pixel,colorphase   0                viewaxis,yarrowspan {}
    pixel,colorreverse 0                viewaxis,ypixelspan {}
    pixel,autosample   1                viewaxis,zarrowspan {}
    pixel,subsample    0                viewaxis,zpixelspan {}
    pixel,size         1
Figure 16.1: Contents of default configuration file avf2ppm.config. (description)

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OOMMF Documentation Team
September 30, 2022