Re: [Plplot-devel] x20c.c

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

On Wed, 28 Nov 2001, Andrew Roach wrote:

> >whatever).  Now in order to work with colour images in plplot you do need
> >the appropriate colour pallette defined for cmap1, but the png format, for
> >example, does allow for up to 256 different colours in a pallette.  (png
> >format also has the truecolour alternative, but we must avoid that because
> >plplot would not be able to work with it.)
>
> Is that /really/ true ? I must confess I have not looked at the code yet
> (or for months), but I didn't think that plplot /was/ limited to 256
> colours. my last look at the colourmap stuff made me think it could easily
> cope with more than that. The current PNG driver is limited to 256 because
> the 1.? versions of GD didn't support truecolour, but unless I am mistaken
> plplot should be "open" enough to support more than 256 colours already.
> 16.5 million might be pushing it, but all the same...  Someone please
> correct me if I am wrong.

I agree plplot can cope with almost arbitrary numbers of colours in cmap1 so
it is possible to get rid of the 256 limit in the png driver.  However, I
was talking about the png format palette limit above and not libgd or cmap1.
Sorry for that misunderstanding. For a reference on this 8-bit limit for the
png palette, see http://www.w3.org/TR/PNG-Chunks.html#C.IHDR.

Later, in an update I did talk about libgd where there is a function called
gdImageTrueColorToPalette to convert from truecolor to palette.  It is
largely undocumented except for the header file so I don't know what the
limitations are there on the number of palette indices, but I assume it
is pretty likely it conforms to the 256 png limit above.

> I was sleeping while most of these discussions were going on - you forget,
> I am on the other side of the world from most of you guys :-) I was going
> to have a look at it today and see what was to be done. Let me know what
> progress you're making and let see what happens.

Excellent progress.  It truly is just a small number of lines of code. Gray
scale works fine and I am close on the colour image as well.  See comment I
will make to Joao's question about colour images.

Alan