Re: Diagonal Stripes in Resizes

Jess Fertudei wrote:
[snip]
> Whenever I resize a photo (usually 50% to make it 1024x768)
> and it has power lines or, more importantly railroad tracks...
> whatever the long straight object is... it distorts from
> perfectly sharp to having a 'candy cane' or 'barbers pole'
> type of diagonal stripe effect all along it. I could blur it,
> but that would defeat the entire projects (I need to do this
> often).
[snip]

When I zoom in to examine your sample at 5:1, I see horizontal, not 
diagonal, stripes representing the slanted lines on my monitor's 
horizontal grid of pixels. Similarly, short vertical lines represent 
the yellow handrails on the front of the locomotive.

Would you care to post a crop of the original "perfectly sharp" 
image showing the rails and the handrails? I expect that it works 
the same, but contains enough pixels to keep those short lines below 
the resolution of our eyes.

In short, what you see is due to representing the scene with too few 
pixels, not to the processing involved in reducing the size. The 
resizing algorithm does not know that a collection of pixels 
represents a slanting line in the outside world.

Try shooting at 1024 x 768. I expect you will see the same effect. 
Your camera needs to represent the slanting rail on the horizontal 
grid of its sensor. Its internal processing probably gives slightly 
different results than the bilinear reduction algorithm of PSP 
7--maybe better, maybe worse.

That said, you can experiment with different resizing algorithms in 
PSP and in Irfanview. They do vary a little in the trade-off between 
smoothing and sharpness. One might suit your taste better than 
others.
-- 
Fred Hiltz,  fhiltz at yahoo dot com