RE: [png-mng-misc] Test images for proposed PNG sTER chunk

[John B. <jb...@ac...>]

From: Bob Friesenhahn
>Is there a standard PNG chunk to express any bandwidth filtering which 
>has been applied to the image so that the minimum monitor bandwidth 
>necessary to display the image whithout aliasing can be known?  This 
>would avoid the need to pre-filter the image if it has already been 
>sufficiently filtered.
>
>For example, Rec.601 and Rec.709 do specify particular bandwidth 
>limits (and the filter to use) for raster image data.

That's for generating the data isn't it?  (I'm not sure, but I
thought that those standards talked about how to *make* image data,
not how to display it.)

So far as I know once the image data has been prepared there is no way
of recovering the correct result.  I.e. if we get a 1024x768 pixel
PNG and it has been incorrectly filtered there is no way of undoing
this, although it is possible to reduce the sample rate and, potentially,
get a better looking image.  (I.e. a 1024x768 image displayed correctly
on a 640x480 monitor may look a whole lot better than the same image
displayed correctly on a 1024x768 monitor.)

Mathematically the samples in a 1024x768 image contain precisely
the amount of information to reconstruct a 1024x768 image, so there
is a real original image corresponding to almost any data (the
checkerboard pattern is an exception).  There are fairly obvious
tricks to removing commonly introduced aliases - such as filtering
out higher but valid frequencies.  However if the image is prepared
correctly that will remove useful visual information.

John Bowler <jb...@ac...>