# Re: [Gimp-user] "Squaring up" an Image - Perspective Transform Changes Sizes

```Thanks for the quick reply.  The shearing transform works great for what it does, but
it can't help when the opposite sides aren't parallel;  it always moves them together.
Imagine drawing a "square" with straight lines but with none of the corners quite 90
degrees.  Rotate it until the top is level, then use the shear transform to make the
right edge vertical.  Now you have one corner (the lower-left) that isn't quite
correct.  What I need is a way to "drag" that lower-left corner into place.  ```
```
Now consider the case where that lower-left corner is in the correct horizontal
position but too low.  As I move it down into place, everything else in the image
needs to move up too (less so as you get closer to the right and top edges).  But
nothing should need to move to the right or left;  those positions should be OK.  When
I move the lower left corner upward with the perspective tool,  however, it not only
changes the vertical positions, but "squishes" everything to the left horizontally as

I believe that what I need is a 2-D linear transform something like shearing or
scaling that changes the amount of correction linearly as it moves from one edge of
the image to the other, rather than a 3-D transform like the perspective tool.  In
other words, I am not looking at a square map from an angle (in which case the
perspective tool would be perfect), but trying to change a map made with one
projection to another projection.

I don't think that the way the perspective transform works can possibly be the right
way to change between the map projections.  Consider this:  take each section of the
map, run them through the transform tool to make them square, then put them back
together.  If the perspective tool takes the stuff in the center of each section and
squishes it to the left, your scale would ramp in sawtooth fashion (one tooth per
section) as you crossed the newly assembled map.

Steve

At 10:18 PM 1/22/2004 +0100, you wrote:
>On Thu, Jan 22, 2004 at 12:43:18PM -0700, Steve Strobel <[EMAIL PROTECTED]> wrote:
>> I tried using the perspective transform to adjust the corners of the image so the
>> section lines would be horizontal and vertical.
>
>That's actually correctly doing what the perspective transform is doing
>(try to imagine a plane with rectangles on it, in an angle towards you).
>
> From what you are writing, I'd say he shearing transform is *exactly*
>what you need, so what kind of problems are you facing when using a shear
>transform?
>
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