The blueprints need some prep work before they can be used for 3D modeling. To start, they need to be cut up into individual views (front, side, top). Each individual image will then be associated with a corresponding view in Blender. Also, most blueprints have been manually scanned from paper and as a result are not entirely straight.
I’ll be using GIMP to manipulate the images.
Cut It Up
The crop tool, in Gimp 2.4
GIMP’s Crop tool is used to extract a portion of a larger image. In my case, I want to extract a single view of the car from a larger sheet containing multiple views. Cropping works like a rectangular selection tool: click and drag to select the region you want to keep, then use those big boxes in the corners of the selected region to adjust the boundaries as needed. When it looks good, double-click inside the box to make the cut. Everything outside the cropping box then gets discarded, and you are left with just the selected region.
Cropping a single view in Gimp 2.4; the dark grey areas will be removed.
Try to get the car roughly centered in the box, and leave some room around the edges.
Now File > Save As… to save the single view to a new file. Important: don’t hit Save and overwrite your full sheet! (I only did that a couple of times - doh!). Save the image in the PNG format, with the default settings.
Reload the main sheet and repeat until each view of the car is in a separate file.
Straighten It Out
Most of the blueprints that I have looked at online are not quite straight, presumably because they were manually scanned from paper. To make everything line up nicely in the modeler they need to get squared off.
Load one of the individual view images into GIMP. You might find it helpful to zoom in a couple of times, which will make any misalignment more visible.
Check to make sure that the image is in “RGB” mode by selecting Image > Mode > RGB from the menu. Images in “indexed” mode will rotate very poorly.
To check the alignment of the image use the Rectangular Selection tool as a straightedge. Place it along straight lines in the blueprint and look to see if it is tipped one way or the other. If so, use the Layer > Transform > Arbitrary Rotation tool to correct the image: positive values turn clockwise, negative values counterclockwise. Just guess a number and see how it turns out, and use [Ctrl+Z] to undo and try again. Learn from my mistake and take your time on this step as it will make your life a lot easier later.
This image required a rotation of -0.4 degrees to straighten.
After you rotate there will be some transparent areas around the outside of the image. Crop the image again to get the edges nice and square. Repeat the process for each of the views.
Flip It
The side view of the car should pointing to the left. The top view should be pointing down. Use the “Flip Vertical” and “Flip Horizontal” tools to flip the orientation of your images if necessary.