Thursday, July 19, 2007

Finally A Vr Command

Well enough playing around with external files. It's about time to head back to what VrPython was intended for and write a new function. This one in particular allows the user to parallel part of an existing line. This is a good example of how a program evolves (to say it was intelligently designed probably infers too much), because as it started off the user had to digitize the offset distance. Then with the advent of VrArgs in 3.3.? it seemed like a good idea to put in the option to enter an offset as a argument after the user typed parpart. Of course with the opportunity to put in a argument as an offset, it seemed silly to not have a dialog somewhere. The problem was that I didn't want dialog every time, and you already had to Id the line, start point, end point, and offset which was approaching the limits of how much I wanted the user to have to do. Hence the decision that if you got all the way to the point of digitizing the offset and hadn't specified one but hit an "end" or "#" without doing so the program would bring up a dialog to allow typing one in. Well let's take a look

#This has become the standard I use to start each program.
print ' ParallelPartial modified 8:23 AM 7/13/2007'


Copyright 2007 Dennis Shimer, M.A.N. Mapping Services Inc.

Vr Mapping copyright Cardinal Systems, LLC

No warranties as to safety or usability expressed or implied.

Free to use, copy, modify, distribute with author credit.

Inserts a line parallel to the identified line.
Allows for picking start and
end points of new line, and offset.
If a number is passed it as the first
it will be used as an offset.
If the <#> End button is pressed during offset
measurement a
dialog will come up prompting for offset.





# Create all the objects we will use.

while Line.Id() != -1:
In theory you could partially parallel many lines
# though in reality it doesn't happen much, but will do so
# until the user doesn't Id a line which returns -1.

# The undo is inside the Id loop so each line paralleled
# could be undone individually.

....Gui.DspMsg0('Dig start of new line')
# Since GetCoord() isn't very customizable as far as buttons
# and messages, we'll display instructions in the message area.

# And get a coordinate for where the newly paralleled line
# Should begin.

....Gui.DspMsg0('Dig end of new line')
# And the same on the other end.

....if VrArgs:

........print VrArgs

# This whole if statement was added later after the introduction
# of VrArgs. If a string (or multiple strings) are placed after
# the script name, they are passed in as a list of strings. So the
# test would fail if VrArgs was empty (nothing passed) and would
# pass to the else statement. If however something was passed,
# convert it to a real number and assign it to the offset. Bonus
# points for anyone who identifies the huge bug I just noticed in
# this 3 line block.

# In other words if VrArgs is not true (is empty).

........Gui.DspMsg0('Dig offset # End for dialog')
# Same as above, display instructions and read a coordinate
# that represents how far the cursor is from the line.

........Offset=-1.0*(Line.SnapPoint (SnapX, SnapY)[3])

# Use the computed offset. The -1 is just to correct for
# how SnapPoint computes the distance.

........if StatS:
# This comes about if the user hits the end button instead
# of a 1 to cancel the digitize offset.

............PromBox=VrPromBox('Prompt title',20,1)
............PromBox.AddDouble('Enter a real number',.5,2)
............if (PromBox.Display(0) == 0):

# Just a simple prombox to allow for entering the offset if
# it wasn't passed or computed in any other way.

....Num1=Line.SnapPointAdd (NewX1,NewY1,0.0)
....Num2=Line.SnapPointAdd (NewX2,NewY2,0.0)

# We are adding points to the line object that was identified
# this is ok because later we will Rec() instead of ReRec() which
# will create a new line.

....if Num1>Num2:


# At one point DelPoints didn't work properly if the beginning
# point was higher than the end point (it may not still, but doesn't
# really matter if it is checked and swapped if necessary.

....Line.DelPoints (Num1, Num2, 1)
# Mode 1 in effect deletes points outside begin and end rather
# than between them (which would be mode 0)



# Take that line object, offset it by the distance entered or
# computed, record it as a new entity, plot it and there you go.
# Part of the original line has been stored as a new line parallel
# to it.

Reminder that as with all other scripts, you can either copy and paste what you see here (substituting your prefered indentation for the holding characters "....", or find the original at

Tuesday, July 10, 2007

Life Outside Vr Wrap Up (almost)

Before going back to writing functions for Vr let's wrap up a couple of things. First binary files, I have recently begun to like the idea of persistent parameters for several of my programs. I really need to create some basic generic parameter code to paste in. In the end I'll probably settle on ascii files with keywords, or at least comments. But what if you want to use binary, or what if you want to read a binary file that you know (or could guess) the structure of. An example is the old .COO coordinate files used by orientp3 in pre-Vr days. We still use it and often want to work directly with the .COO files for various things. But they are binary, here are some basics working with something similar to the parameter file we used before. This will be plain python, you can copy and paste this, download from my source code website, then either run it from a command line like
or open or paste it into pyedi and run it inside Vr. The only requirement is that you will need to have python installed because it uses two standard modules. By this time I am going to assume some python knowledge and ability to read the code. I am thrilled to answer any questions that come up, but I will post this code in a very plain, line at a time manner rather than using multiple methods on a single line. I'm also going to use my favorite method of accessing binary data since as is common in python there are probably dozens of ways of doing it. You could probably do this in about 8 lines but the point is to explain what is going on. I'll also assume that anyone interested in this would take the time to look up the modules listed to see how many cool things they will do... So here goes

If you were to run this script 3 consecutive times it would give 3 different responses. First that the parameter file was created but without any values read. Second it would read the default values, tell you what the values are and modify them. Third it will read the modified values print them out and tell you that it found modified values and not the defaults.

import os,struct
# We'll use os to test for the existence of the file, an struct to
# interact with the binary data.

The format string tells several parts of the program what we are working
with in regards to binary data. The format string listed above represents
a 10 character string, 2 double precision real numbers, and an integer.
Same as before but I added the int to show by example. Note that it is
just a string with letters representing different kinds of data.

# The file by this name will show up in the directory the script is run in.

print 'Bytes occupied by binary data',struct.calcsize(FormatString),'\n\n'
Sometimes you need to know how many bytes to read from a file in order
to get one set of data when multiple sets exist, a good example goes
back to the coordinate file mentioned earlier. In order to read and process
one point at a time you can only read the necessary bytes. the calcsize
method in struct will read a format string and tell how many bytes are
represented. You can go into a python interpreter, import struct then
evaluate some single item format strings like the following for a
float, a double, an int, and a 1 character string.
>>> struct.calcsize('f')
>>> struct.calcsize('d')
>>> struct.calcsize('i')
>>> struct.calcsize('s')

if not os.path.isfile(ParFileName): # If the file doesn't exist.
...# Then open it in write (create or erase) mode with a binary modifier

...The pack method in the struct module uses the format string to process
...the data that comes next into the bytes which represent them in binary
...form. It is important that the amount and type of data following the
...format string match, notice I am passing in a string (less than 10
...characters allowed, 2 real numbers and an integer.

...# We have a binary file and binary data so just and write and close it.

...print 'Program run with no existing file'
...print 'Data set to defaults.'
...print 'testbinary.par should now exist.'
...# If it does exist open for reading + writing with binary modifier.
...This time since the file exists we are going to get the binary data
...right out of the file with a read. Since we want the whole file there no reason to use calcsize to tell it how many bytes to read.

...# Going the other direction, lets use unpack to use the format string
...# to convert the binary data into a list of values.

...print 'Data from file, note it is a list\n ',DataList
...# and print the list.,0)
...Now assuming that there are new values that need to be used to update
...the parameter file. The first thing we need to do is go back to the
...beginning of the file, read up on seek, but this basically moves you
...0 bytes from the beginning. If in the coordinate file example I had a structure that require 24 bytes and wanted to get the 3rd value
...from the file I could to the 48th byte and read 24. This way you can, write, and overwrite data in place within a file.

...print 'File position after seek ',ParFile.tell()
...# Every open file has a tell method which gives the current position.

...# Same idea as above, just using the changed data (wherever it came from)

...if DataList[0].split('\x00')[0]=='Updated':
......print 'You have seen it all.'
......print 'Remove testbinary.par to start over'
......There may be a better way to convert this 10 character string to
......what you normally think of as a string, but it comes back padded
......with the 0 bytes and this works simply enough. Think of it this
......way 'Updated\x00\x00\x00' has 3 extra bytes, if you split on that
......character you get back ['Updated', '', '', ''] of which 'Updated' the first or [0] element. Ugly but I've never really look into
......a better way.

......print 'Program run with an existing file'
......print 'Data was updated run again to see change'
......# 2nd run string doesn't equal 'Updated' so print appropriate message.

...# Good form to explicitely close files.

I need to wrap it up for now, I'll have to save the parsing command for next time.

For anyone interested in trying VrPython for the first time or if you are early in the game, I suggest going to the earliest posts and working forward. I use VrPython every day for many wonderful things, needless to say it will change and could potentially damage a file. Any risk associated with using VrPython or any code or scripts mentioned here lies solely with the end user.

The "Personal VrPython page" in the link section will contain many code examples and an organized table of contents to this blog in a fairly un-attractive (for now) form.