Re: [Plplot-devel] x20c.c

[Alan W. I. <ir...@be...>]

On Wed, 28 Nov 2001, [iso-8859-1] Jo=E3o Cardoso wrote:

> You want to write a tutorial; I just want to show that it can be
> done. You want to write code that users can modify to theirs needs; I
> just want to show them that it can be done.
>
> I profoundly disagree (as you said) on puting library dependencies on
> demos. It is not a question of being nice to have color images, but a
> question of software development.

Note, I am leaving in your gray-scale image reading since it now works, and
I am simply adding another page for a colour image.  We can
straightforwardly configure out the colour image reading in x20c.c for the
case where the user does not have libgd installed.  Thus, I don't believe
there is any cause for your concern here since your work is essentially
left unaffected.

> ....(lots of example code from octave deleted)
> To continue, reducing the palete to 256 colors, I need a definition
> of what a "near color" to other color is.

I am running into a problem with the plimage colour image that may be
relevant to your question.  The problem is the index range is effectively
stored as a floating point number and converted inside plimage to the range
0. --> 1, then a call to plcol1 is made. That is all well and good, but I a=
m
just starting to review the plcol1 code now to see how the 0. to 1. range i=
s
mapped to the colour palette index range. For colour images that index
mapping has to be exact. If there is an off-by-one error in the resulting
index it makes very little difference to gray-scale images since the colour
palette is continuous in that case. However, such an error would make a hug=
e
mess for non-continuous colour palettes that occur for most colour images. =
I
am getting such a colour mess at the moment (poor Lena has surreal colours!=
)
while the identical set of calls to libgd and plimage work fine if the
lena.png image is gray-scale. That is why I am looking for off-by-one error=
s
in plcol1.

Alan