Scaling layers and loosing resolution

[grit]

When I scale down and rotate layers in 2.9.8/9, resolution is decreased and layer is blurred.  I remember this wasn't like that before.
 
Is that a bug?

[Blighty]

(02-03-2018, 06:06 AM)grit Wrote: When I scale down and rotate layers in 2.9.8/9, resolution is decreased and layer is blurred

That is just the way it is with bitmaps. Applies to all bitmap editors. The more you scale the more the quality is reduced.

[Ofnuts]

(02-03-2018, 06:06 AM)grit Wrote: When I scale down and rotate layers in 2.9.8/9, resolution is decreased and layer is blurred.  I remember this wasn't like that before.
 
Is that a bug?

When you scale down, pixels are interpolated so you lose definition, even if this is not as visible as when scaling up. In some bad cases you can be hit by aliasing and get moirés and jagged edges, in which cas the solution is to pre-blur the picture before scaling down. In 2.8 the rule is to apply a gaussian blur of N pixels when you downscale by N.

Pixels are also interpolated when you rotate. This isn't very visible on the first rotation, but if you rotate things several times blur shows up around th the third rotation. If you want several copied of an object with varying angles, always produce them from the initial object and do not obtain copy N+1 from copy N.

Last, all geometric transforms (except the flips and the rotations of multiple of 90°) interpolate pixels, so the fewer you do, the better it is. Gimp 2.9 introduced a combined transform tool that has the big advantage that you can apply scaling, rotation and shear in one single operation (so you interpolate pixels only once and avoid the blurring caused by repeated interpolations.

[rich2005]

(02-03-2018, 06:06 AM)grit Wrote: When I scale down and rotate layers in 2.9.8/9, resolution is decreased and layer is blurred.  I remember this wasn't like that before.
 
Is that a bug?

Obviously depends on the image sizes and amount of scaling/rotating.

There is a difference between 2.8 and 2.9 - I think less of a bug, more a decision by the developers.

The Gimp 2.8 sinc(Lanczos3) algorithm replaced by 2 new methods.

LoHalo method: when you downscale an image thats less than a half of the original size
NoHalo method: when you do not reduce the size much (rotate, shear or something)

This does mean that for small images there is a less sharp 'feel' - edge artifacts are not there. You need to zoom in for this comparison, but differences are there.

   

You might need to sharpen, depending on requirements. Gimp 2.9 does now come with wavelet sharpen. Usual advice remains, leave any sharpening to the very final procedure.