Digital BW, The Print

> ----- Original Message -----
> From: "Kevin Gulstene" <kevin@...>
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Sent: Sunday, March 16, 2003 10:47 PM
> Subject: [Digital BW] Self Inflicted Posterization
>
>
>> Aaargh-
>>
>> I've been having problems with posterization introduced into a PS7
>> file, that is very slightly visible in the print.
>>
>> The image has very similar tones around each of the four edges of a
>> greyscale image.  To help with the center of focus I want to darkening
>> the edges.  To do this I make a rectangular selection, invert the
>> selection and then feather it by 50 pixels.  When a curve is applied 
>> to
>> the selection the feathered edge has 'bands' in it.  It is not a 
>> smooth
>> looking transition but steps in concentric bands from tone to tone --
>> I'm calling this posterization.  It is not that obvious but once I 
>> know
>> it is really distracting.
>>
>> Adding a small amount of noise to the image mostly removes the
>> posterization but (and it may just be my imagination) leaves a less
>> smooth texture to the image.
>>
>> So, is there a better way to make the selection without introducing 
>> the
>> banding/posterization?  Is it better to go around the edge with a big
>> soft brush and paint it in the selection?  Is there a setting 
>> somewhere
>> that will introduce some dithering or noise in the feathered area?
>>
>> I tried making the curves on a 16 bit image after loading the
>> selection(s) from the adjustment layers of an 8 bit version but got 
>> the
>> same results for the most part.
>>
>> Any suggestions are appreciated.
>>
> Kevin,
>
> I have encountered the same thing occassionally. Especially in smooth 
> areas
> like the sky portion of landscapes. Generally when I want a selection 
> with a
> blurred edge I make a selection with the lasso tool (you could paint 
> the
> selection as well) and save it as a new alpha channel, turn the 
> selection
> off, apply gaussian blur to the alpha channel and then use that to 
> create a
> selection which has a blurred edge.
>
> You could also create a duplicate file, resample it to a much lower
> resolution and then create your blurred edge mask. Resample back up to 
> the
> original resolution and copy the mask or alpha channel into your file.
> Resampling up seems to decrease any banding.
>
> You can also work with very large soft brushes. You can try these 
> techniques
> in combination.
>
> I wouldn't add any noise to the image since you will lose detail as you
> noted.
>
> Martin Wesley
>

Martin-

Thank you for an excellent answer.  Your first suggestion worked like a 
treat.

Your suggestion also helped me fix a similar problem in another image 
that used a gradient as layer mask.  In this case I created a separate 
alpha channel from the initial gradient, then, adding noise to the 
alpha channel ( instead of the image ) removed the banding without 
changing the texture of the image.  Blurring the channel worked as well 
but changed the 'shape' of the mask more.

Thanks again,

Kevin