I have made good use of Photoshop and Photoshop elements and GIMP for drawing comparisons. They can import most raster formats and interpret PDF for conversion to raster.

Because the comparison is done with a raster image, the image parts can be moved, rescaled, deskewed, despeckled, or thickened or thinned to account for differences between the versions. For example, a typical drawing change is to move a view from one sheet to another; this is something typical comparison software cannot deal with. In addition, differences can be visibly amplified so the user doesn't need extraordinary care to spot them.

The comparison result is easily saved in any number of low level formats (.gif, .png), so that a third party can verify the check was done using almost any imaging tool (even a web browser) and, because Photoshop/GIMP is a drawing program, annotations can be made - text and graphics highlighting any changes. Since Photoshop and GIMP are layer based, a single file can retain comparisons for any number of versions, if desired.

I tried messaging, but that doesn't work now. And I'm not under maint, so I can't comment on or vote for ideas.

Drawing Change Checking

Using Photoshop or GIMP to find and emphasize differences between versions of images - drawing sheets for example.

Basic steps
1) Take original image and convert to a layer
2) Fill the markings with a solid color (red) and set transparency to 60%
3) Take newer image and convert to a layer
4) Fill the markings with a solid color (green) and set transparency to 40%
5) Create solid underfill layer - black.
6) Optionally create a new layer, select by all-layer color black, and yellow-green, then expand the selection by 10 or more pixels and fill with new color (bright blue) to high-light even small differences.

Old material without match will be red; new material without match will be green, and matching material will be yellowish-green..

Unlike other comparison methods, the use of a bitmap editor allows non-significant changes to be recognized and cancelled out. It may be necessary to shift elements to get moved items to line up. It is suggested to move the older material so that comparison in the future will be based on the un-moved latest version.

If line weight (thickness, width) has changed, one can select and expand the selection for the thinner version to match the weight of the thicker version to cancel out spurious indication.

Refs:

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