Scaling PCB layout images

[Pakaloabob]

I am having a tough time figuring out how to scale a PCB image, such as http://www.muzique.com/lab/buffer2.gif for example. I would probably use GIMP but I am not sure on what I should base the scaling. Resistor lead spacing, etc.?
I am pretty sure I saw someone else ask this question a while ago but I cannot seem to find the post after searching quite a bit.

[KazooMan]

The easiest way is to just print the layout and compare the result to a component.  An IC socket works great for this since the spacing needs to be pretty much right on whereas other components allow some wiggle room.  Compare the pins of the socket to the pads on the print and estimate how much you need to correct the print.  Then go into your printer setup and select the appropriate scaling factor and print again.  Two or three iterations should allow you to zero in on a perfect spacing.  I often add the scaling factor as text on the layout such as "print at 73%" or you could add the factor to the filename.

Of course you can measure the spacing on the printout and measure the component to derive your scaling factor, but with the small size of the components this doesn't do much better than a good guess.  

You can also resize the original GIF and save it as a new file, but that really doesn't achieve anything more than the printer scaling method.

[Steve Mavronis]

I just loaded the GIF file into GIMP. It is already at 300dpi and when I viewed a grid spaced for 1/10th" it was matching the IC pin spacing. The print size says 300dpi in GIMP also. Just print it straight from GIMP and you should be at actual size with no scaling needed.

[Auke Haarsma]

It's easier than it looks.
1. find the dpi of the source image
2. check the dpi of your destination (printer)
3. do the math

For your example:
1. the dpi is 300 (as mentioned on the picture). If this is not mentioned, often the size of the PCB is mentioned -> this can be used to determine DPI. As a last resort you can use the spacing between pins from an IC or transistor. These use often standard spacing.

2. This is a local setting. I always print at 600 dpi, so I convert all pcb images to 600 dpi before printing. In Gimp you can set this at the print options (I also use Gimp!).

3. If you know the destination IP you can do the math. In my example: source = 300 dpi, dest = 600 dpi, I need to scale the image up to 200% original size.

And yeah: always print on paper first, push some IC(sockets) through the IC locations on the paper. this way you can easily check the spacing of the pins before comitting to transfer of transparent.

Good luck!

[Pakaloabob]

Thanks for the help everyone! I especially like the tip to poke an IC socket through the paper to test.

[Steve Mavronis]

In my example: source = 300 dpi, dest = 600 dpi, I need to scale the image up to 200% original size.

I'm missing something; if you scale up the image in GIMP twice the size just to set it up for 600dpi printing afterwards, won't you degrade the original image quality with some edge dithering?

I'd just stick with 300dpi personally. That's a hell of a lot resolution already.