Re: Projective image transformations

Keith Packard <[email protected]> writes:

> > Edge conditions are definitely the hard part of the specification; consider
> > a straight scale of a  solid rectangle from 100x100 pixels by a ratio
> > of 75/100. The desired result is obvious ... a 75x75 pixel solid square
> > with hard edges.
> 
> Both nearest neighbor and bilinear interpolation will yield this result.

This example and my last mail were making things too complicated.
Lets take a *very* simple example - scaling a 1x1 solid colored
constant square to 3x3 with bilinear interpolation and compositing it on 
a 3x3 destination image.

Originally we have alpha values:

  1

When scaled to 3x3 with bilinear interpolation, using
the "all pixels outside the image are transparent rule",
we get:

 0.44 0.66 0.44 

 0.66   1  0.66 

 0.44 0.66 0.44 

I don't think this is a terribly useful result. 

(You don't have to get as complicated as my partial-convolution 
suggestion to fix this ... a simpler rule is to simply clamp 
source  pixel coordinates to the bounds of the original image.)

Regards,
                                        Owen