Another factor I've read is that while doing the compression JPEG looks at
'blocks' of eight pixels, so if possible you want the end resolution to come
out even when divided by eight... i.e. 400, 480, but not 450... I don't
remember where I read this, but it can't hurt, I think...
--
Jim Brokaw
OM-'s of all sorts, and no OM-oney...
on 1/26/03 1:05 AM, Moose at olymoose@xxxxxxxxxxxxx wrote:
> The electro-optical mechanism of the scanner can only scan at one dpi,
> determined by the actual sensors in the mechanism. Any scanning at less
> than that dpi means the driver is downsampling and any higher dpi means
> it is interpolating up. To avoid artifacts and generally goofy results
> (some a bit and some a lot goofy, depending on specific dpi chosen), you
> should always scan at a dpi which a whole number and which is the result
> of division of the native hardware dpi by a whole number. Thus, with a
> 600 dpi native resolution, acceptable dpis would be 300, 200, 150, 120,
> 100, 75, 60, etc.
>
> If you ask it to downsample to 66 dpi, you are pretty close to 600/9 =
> 66.666..., whereas as 168 dpi, you aren't anywhere close to a number
> that allows simple downsampling. That may be a the reason the supposedly
> higher scan resolution gives poorer sharpness. The same rule applies
> when doing one's own downsampling in an image editor.
>
> Moose
>
> Brian Swale wrote:
>
>> Hi folks,
>>
>> This might possibly open a can of worms, but anyway, here goes.
>>
>> I've been playing with a print, about 8 x 12 inches, and scanned it at a
>> variety
>> of resolutions. Mostly this was due to my scanner misbehaving horribly, and
>> swapping parts of the image around, and I tried several scanning resolutions
>> to find if that would get around the misbehaving.
>>
>> In the process, I found this.
>>
>> When I scanned at 66 pixels/inch (25.98 px/cm) I got an image that was
>> (height = 20.01 cm / 520px / 7.87 in.)
>> (width = 29.56 cm / 768 px / 11.6 in.) as a pdd file
>>
>> I presume the jpeg I converted it to is the same.
>> I adjusted colour and brightness, and did one simple sharpen. I did not re-
>> size. The image has come out quite sharp with none of the over-sharp
>> artifacts one sometimes sees, including none of the white halo effect.
>>
>> When I scanned at 168 px / inch then after adjusting colour, brightness and
>> contrast, resized and sharpened in that order (I think) I just could not get
>> it
>> as sharp no matter what method I used, and sharpening artifacts appeared
>> quite quickly.
>>
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