why does cs/6 resize jpegs while cropping but doesn not resize pngs?

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Hi, I’m not a graphics expert by any means but I use Photoshop to clean up and crop scanned paper drawings that are then imported into AutoCad for use in drafting revised cad drawings. I was just upgraded to CS/6 and I’ve found that the crop tool does not work as it did in CS/2. I worked with users in the AutoCad forums and discovered that if my graphic image was a png that my cropping would be fine but that if my graphic image was a jpeg a crop would change the physical size of the image. I will try to attach a comparison image for review. The 2 large images are fullsize scans imported into autocad at 1:1. The top is a jpeg, the bottom is a png. Next to these images are cropped version of the same images. The top is a cropped jpeg, the bottom is a cropped png. Note that the jpeg has been resized while the png has not. What causes the jpeg to be resized? It did not work this way in CS/2.
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Posted 4 years ago

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Chris Cox

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The type of file has no bearing on how the crop tool works. The only things that matters are the settings that you specify in the crop tool.
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Steve Sprengel, Champion

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If PS-CS6 was resizing the images and you were doing things by hand (rather than an automated process) it would be readily apparent that the image was already resized in PS-CS6 immediately after the crop while still in PS-CS6.

Since you're screenshots are in AutoCAD (apparently) it doesn't prove where the resizing is happening, only that you're noticing it in AutoCAD.

If you want to prove that PS-CS6 is actually resizing the images and that it is occurring as part of your crop operation, then show us screenshots of PS-CS6 immediately before and after the crop operation at 100% zoom.
If both the PNG and JPG are produced from the same cropped image in PS-CS6 and the PNG is ok, then obviously PS-CS6 is NOT resizing the file during the crop, and the difference in size is caused by something else.

A wild guess would be that somewhere the PPI/DPI setting is being changed and AutoCAD is using that to resize the image on the way in, and the PNG vs JPG being different is because AutoCAD can't see a PPI/DPI setting stored in the PNG but it can from the JPG.

For example, suppose AutoCAD has a default PPI/DPI setting of 96 PPI like the screen could have, but the PPI stored in the JPG is 300 PPI, then AutoCAD might resize the input image by a ratio of 96/300 or a little over three times smaller, but when AutoCAD imports the PNG version of the image is doesn't see any PPI setting so assumes it is the same as what AutoCAD is set for, hence no resize.

You can use PS to change the PPI of an image by using Image / Resize and unchecking Resample and type a new number into the PPI area.