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vImage scaling problems

A few weeks ago Eric Raas posted about the vImage framework and that he 
was having problems with the scaling functions such as 
vImageScale_ARGB8888.  I've been using the framework and while I get 
good results with the scaling function in the X direction, I get 
unusably buggy results in the Y direction.

The Tableau example project seems to have similar troubles; it either 
crashes or gives me a blank image when I try to perform a scale.

On the other hand, the vImageAffineWarp family of functions is more 
powerful than a scale and seems to work correctly.  If you're having 
similar troubles, try using those instead.

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Peter
3/2/2004 10:57:06 AM
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Peter Ammon wrote:

> A few weeks ago Eric Raas posted about the vImage framework and that he 
> was having problems with the scaling functions such as 
> vImageScale_ARGB8888.  I've been using the framework and while I get 
> good results with the scaling function in the X direction, I get 
> unusably buggy results in the Y direction.
> 
> The Tableau example project seems to have similar troubles; it either 
> crashes or gives me a blank image when I try to perform a scale.
> 
> On the other hand, the vImageAffineWarp family of functions is more 
> powerful than a scale and seems to work correctly.  If you're having 
> similar troubles, try using those instead.
> 

This is documented at the bottom of 
<http://developer.apple.com/hardware/ve/errata.html>

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Peter
3/2/2004 11:47:07 AM
> > On the other hand, the vImageAffineWarp family of functions is more 
> > powerful than a scale and seems to work correctly.  If you're having 
> > similar troubles, try using those instead.
> > 

Thanks a lot Peter, I did end up going with the vImageAffineWarp
functions - these seem to work reliably and being the most general
form do anything you might want to do in terms of scaling.

Basically the functions I *did* get to work were the convolution and
affine warp ones.  The dedicated scale functions seemed to return bad
results (all zeros) or produce other problems.  The 2D convolution is
*really* fast - I hope Apple fixes the problems because this is a
really useful framework (have filed bug report on Radar).

ER
0
mytvc15
3/2/2004 4:37:35 PM
Eric Raas <mytvc15@hotmail.com> wrote:

> > > On the other hand, the vImageAffineWarp family of functions is more
> > > powerful than a scale and seems to work correctly.  If you're having
> > > similar troubles, try using those instead.
> > > 
> 
> Thanks a lot Peter, I did end up going with the vImageAffineWarp
> functions - these seem to work reliably and being the most general
> form do anything you might want to do in terms of scaling.
> 
> Basically the functions I *did* get to work were the convolution and
> affine warp ones. 

I don't suppose you happen the know the algorithm for Unsharp Mask, do
you? This is something I'd like my app to be able to do... m.

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0
matt
3/2/2004 5:25:24 PM
 matt@tidbits.com (matt neuburg) wrote:

> > Basically the functions I *did* get to work were the convolution and
> > affine warp ones. 
> 
> I don't suppose you happen the know the algorithm for Unsharp Mask, do
> you? This is something I'd like my app to be able to do... m.

<http://www.olympusmicro.com/primer/java/olympusmicd/digitalimaging/unsha
rpmask/>

Regards,
   Tom_E

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0
Thomas
3/2/2004 11:01:40 PM
> 
> I don't suppose you happen the know the algorithm for Unsharp Mask, do
> you? This is something I'd like my app to be able to do... m.


This is the kind of thing made relatively easy by the vImage
framework.   An unsharp mask filter is typically described using three
parameters:  the width (or radius), the amount, and the threshold.

The basic steps are:

1) Generate a copy of the image smoothed using a Gaussian kernel.  The
kernel matrix must be normalized so that the sum of all its values is
one.  The width of the Gaussian you compute is determined by the Width
parameter.  You can use the function vImageConvolve_PlanarF or one of
its siblings for ARGB or 8 bit to do the convolution.  This runs fast
and saves you a lot of work.

2) Subtract the smoothed version from the original image - this
produces an image in which the sharp features are emphasized.  You can
use the AltiVec vsub() function do subtract one buffer of floats from
another (assuming you're working with floats).

3) Set all pixels in the difference image with absolute value less
than your Threshold parameter to zero.

4) Multiply the thresholded difference image by your Amount parameter
and add to the original.  The Amount parameter can be positive or
negative.  If positive the mask will emphasize sharp features - if
negative it will de-emphasize them.


HTH ER
0
mytvc15
3/2/2004 11:59:39 PM
> 4) Multiply the thresholded difference image by your Amount parameter
> and add to the original.  The Amount parameter can be positive or
> negative.  If positive the mask will emphasize sharp features - if
> negative it will de-emphasize them.

Actually you should subtract the difference mask from the original
(Amount parameter is negative in above description) to enhance high
spatial-frequency detail.  Sorry!   ER
0
mytvc15
3/3/2004 2:42:51 AM
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