My team and I have noticed some strange issues when affecting the overall interpolation of a .psd via scaling, cropping, or canvas resize.
Original .psd is even x even overall pixel dimensions.
Original vector and raster smart object assets are all proportionally scaled with width x height percentages equal.
Preferences: "Snap vector tools and transforms to pixel grid" is turned off
Resize .psd via Image > Image Size palette.
Scale styles, constrain proportions selected.
Every interpolation style tested.
Resize the .psd to 50%
Result: All smart objects (both types) no longer are proportionately scaled. Usually off by a minor amount under 1/10th of a percent.
Some objects' center point shift a half pixel to the right or left.
This also occurs if we use the crop tool or resize the canvas.
Is this a known bug and if so, is there:
1) any setting to truly lock proportions?
2) any resolution being pursued currently?
Though the shift and the disproproptions seem minor, the overall quality of items such as icons, hairlines, and other things that should be more crisp become far more degraded in CS6 vs CS5.
We have over 100 users and need to evaluate if we should drop back down to CS5, where we weren't having any of these issues, as our work demands a lot of precision, so you can see why this is worrisome.
I'm not going to say anything about the abysmal rendering of Vector Smart Objects in CS6 because it'll be beating a dead horse but I can explain the issues you see with measurements after scaling.
The slightly disproportionate scaling and half-pixel shift of the centre is unavoidable when it happens because the resulting Smart Object/Vector Smart Object must measure an integer number of pixels in height and width and be perfectly aligned to the document pixel grid. That's true in CS5 and CS6.
Say a SO/VSO originally measures 102 x 103 pixels and you scale it by 50%. The Options bar may say the result is 51 x 51.5 px but the result will really be 51 x 52 px, and therefore the new height will really be 50.49% of the old height.
The original SO/VSO is an even number of pixels wide and an odd number of pixels high, therefore its centre will be halfway down a pixel. The 50%-scaled SO/VSO is an odd number of pixels wide and an even number of pixels high, therefore its centre will be halfway across a pixel.
We thought that would be the issue, since this seems to be an issue for slices and interpolation for export. However, we have perfectly even x even pixel smart objects, transformed to whole number percentages that are divisible by 2, placed the smart objects in the main document on even x even pixel locations, with the xy coordinates as the center point as well as in the top left, and yet the scale is still disproportionate in a manner that makes no sense to the original smart object values.
The method you mention above actually is something that was true in CS5, but doesn't appear to apply to the issue we're seeing in CS6. We're getting values such as 80% original smart objects, when scaling the .psd at 50%, becoming w: 41.17% h:39.94% objects.