Monthly Archives: December 2007

A “Quick” Cab – and the development of a new Canvas Operation

Still time for one more before Christmas.
A while ago I wrote a Cab builder plugin.
You simply enter a few dimensions for the size, curve height and overhangs and it automatically generates a box with a curved roof.
So that part is easy. I took the dimensions from the plan, as mentioned in the pervious blog post, and filled in the form (dimensions in metres)
Ta-da! A cab with a lovely curved roof in a few seconds.
Then I hit a problem.
Being a bit of a stickler for straight edges, I couldn’t think of an accurate way to move the points that make up the roof edge to match the plan.
Shown below (I applied a translucent material to the roof so the plan could be seen)
Ideally the points needed to be "sheared" or I could shift them by hand – but that could lead to inaccuracies.
Then I had an idea – I vaguely remember back to my days of experimental physics at university. There was a method of finding the best straight line through a set of points of one axis was known accurately, which the X axis (left/right) is in this case.
A bit of a search threw up the Least Squares method
I put this algorithm into a small program that takes a set of selected points, a fixed axis and a variable axis which then processes them into a straight line.
The result is a new operation button for 3D canvas.
From the plan view, I dragged the points, restricting them to Z axis only movement (hold down the Z key)
(I did exaggerate the zig zag for illustrative purposes, normally I would be a bit more accurate when shifting points)
Select the points and run the operation, selecting the "Align Z, X fixed" option.


The result is a mathematically straight line – no human error.

Straightening scanned plans and working out their pixel scale

Looks like I do have a bit of time to do one or two more blogs…
When you scan a plan from a book, 99.9 times out of a 100 they are not perfectly straight.
Using a paint program it is a relatively simple process to fix this.
It is also a good idea to work out the pixel scale for the image.
This can then be used to get unlisted dimensions from plans by using a pixel tool in a 2D paint program.
First load up your image.
My paint tool (the rather ancient Picture Publisher 8) has a pointer tool that lets me mark out a rectangle on an image without drawing anything.



The status bar shows the start coordinates and the width and height of the box.


Most locomotive drawings show the rail top as a line. Using one end of this line as a start point I draw the rectangle to the other end of the rail top line.
In the example below (red rectangle) it shows I have a height of 25 and a width of 2965.
I then start up the most useful application in Windows – the calculator. 
Make sure it is switched to Scientific view.


Divide the height by the width:
25 / 2965 = 0.0084317032040472175379426644182125
Then click Inv and press Tan
Inv Tan  =  0.48308955972084815380076833108458
This is the number of degrees that the image must be rotated by.
I then select the rotate by arbitrary amount option in my paint program and enter 0.483 degrees (anti-clockwise)

Now we have a horizontal plan, I pick a known dimension from the drawing, In this case the 32′ wheelbase.
This is 1532 pixels wide.
This translates to 1523/32  =  47.59 pixels per foot
or the other way up (32 * 12) / 1523 = 0.2521 inches per pixel
and multiply by 2.54 to get to centimetres per pixel = 0.6404
So now, if you measure 10 pixels on the drawing you know it is 10 * 0.2521 = 25.2 inches or 64cm
I usually mark this information directly on the drawing that way it never gets lost – provided you remember to save the altered image file.

Christmas & stuff

Most of my new N gauge train set has arrived and christmas is fast approaching. It’s the season to take time off from the train sim hobby… Not
I’ll try and sneak in a bit more programming and modelling whilst I’m off work.
Here’s one I did yesterday – 7500 polys, for that! My first model using the new extra high poly wheels.
You can see the full size image in the picture gallery.
Anyway if I don’t type anything before, *************** Happy Christmas everyone ***************.


I’ve been busy programming the FSX plugins this week, fixing a few bugs and adding the attachment tool.
I’ve also been reviewing the future of the TS Engineer plugin.
I have changed the materials generated for the wheel rim to make those polygons easily selectible (by material)
I also incorporated a couple of extra faces to the flange profile to make it a bit more accurate.
Here is a sample of the output:
This is starting to build a serious number of polygons – the pacific wheelset, using a 40 sided rim, generated over 9000 polys for the wheels alone.