[gdal-dev] Proposal: gdal_muladd.py

[Greg Coats]

I would prefer to obtain in one step the absolute value of the difference between two pixels, in part because I do not see the meaning of a negative pixel value. In my example, I do not see meaning to a signed byte value for a pixel channel being red=-51. Greg

On Feb 1, 2010, at 3:08 AM, Antonio Valentino wrote:

> Hi Greg,
> 
> Il giorno Sun, 31 Jan 2010 14:58:19 -0500
> Greg Coats <gregcoats at mac.com> ha scritto:
> 
>> Regarding the diff function
>> "diff": computes the difference between 2 raster bands (b1 - b2)
>> Since the result of diff could be a negative number, for example
>> 99-150=-51, I suggest that there be an additional function that
>> provides the absolute value of diff, which would be 51 in this
>> example. Greg
> 
> Of course, it would not be a problem to include but my original idea
> was to provide only base functions and use them to get complex
> expressions.
> 
> In this case, what you need can be obtained using two stacked virtual
> files:
> - diff (using appropriate band data type and SourceTransferType)
> - mod
> 
> Is it a reasonable approach for you?
> 
> Even more interesting for common tasks should be to compute 
> 
> abs((b1-b2)/(b1+b2))
> 
> 
>> On Jan 31, 2010, at 12:18 PM, Antonio Valentino wrote:
>> 
>>> Hi Matt,
>>> 
>>> Il giorno Mon, 25 Jan 2010 20:57:58 -0600
>>> Matt Klaric <matt at klaric.com> ha scritto:
>>> 
>>>> On Mon, Jan 25, 2010 at 8:38 PM, Frank Warmerdam
>>>> <warmerdam at pobox.com>wrote:
>>>> 
>>>>> I don't think the VRT approach supports actually adding
>>>>> different images together, only overlaying and rescaling.
>>>>> 
>>>>> 
>>>> However, pixel functions could be used in a VRT file to accomplish
>>>> the desired outcome.
>>>> 
>>>> --matt
>>> 
>>> Indeed, it would require a pixel function for sum and the ScaleRatio
>>> property of ComplexSource.
>>> 
>>> I have just submitted a patch 
>>> 
>>> http://trac.osgeo.org/gdal/ticket/3367
>>> 
>>> that provides a set of standard pixel functions, including sum (of 2
>>> or more raster bands).
>>> 
>>> Using this set of functions it should be quite easy to write a
>>> python program that performs generic bands arithmetic.
> 
> 
> best regards
> 
> -- 
> Antonio Valentino