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Typical Tasks Civil Engineers do - any help appreciated

I work at a firm that uses large images for all kinds of things, especially with AutoCad.
Took me years to find GIMP, as there are so many free image prog's, but most do not handle large images. Happy I found it.
Here are my main tasks that I would like help on with GIMP:
1) convert a color image to black and white - in a special way. In Corel PP, there is a tool that lets you adjust a threshold slider to determine what pixels go to black and white. It also uses a "lineart" filter that seems to do well with ignoring noise (scattered pixels) at the same time. I am looking for a similar tool that allows control of what goes to black and white.

2) merge an image into another, by pasting the image in, then scaling, rotating, translating to line up with say two points in common. I can do it in several steps, but it would be nice to specify a pt1 from, and pt1 to, then pt2 from and pt2 to. It would handle the three transformations for me and line up the image.

So anything available would be great, or letting me know what API is best for these level tasks would be helpful. I do C# so can dig into that or use the scripting mechanism I noticed.

Are the images you speak of in #2 - photographs of land, like aerial images? Or pictures of drawings, like tiff files, etc? Because in order for this to be an effective script for this sort of merge it would require the images be at identical scale. Just curious.

well, the images could be from anywhere. Sometimes I start with an aerial photo, then want to paste in contour linework from a USGS topo map. I would want to bring in the usgs topo as a layer, then rotate and scale that so two points on the topo line up with two points on the aerial photo.

You never mentioned using any other software, just thought I would mention this in case you haven't heard of it. You might want to give it a look.

Did you find a solution for your 2. issue? The transformation? I'm seeking since Month for a easy solution. No success... I can do all the math, but don't know how to implement it into the gimp interface: i.e. I don't know how to hand over cooridantes of the vectors in both images to get in exact overlaping into script-fu functions. Once I know that, i can create a new image with two layers and do all the transformation needed.

I actually noodled around w/this the other day but did not get anywhere. I though that maybe if you drew two paths (from pointAlayer1->pointAlayer2, and pBl1->pBl2), you could deduce the amount of rotation and scale needed. BUT, it's been so long since I've taken trig that I even had to look up the Pythagorean theorem to double-check. (Ouch.) If someone is willing to throw out some basic equations for the transforms above, I'd give it another shot...

1. 'Colors - Threshold' menu item
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