Re: [RFC] Gradient fill options

[Pierre <pie...@gm...>]

Hi,

On 2/2/07, Hans-Bernhard Br=F6ker <HBB...@t-...> wrote:
> Ethan Merritt wrote:
>
> > I noted out in my feature request for libgd that it was unclear what
> > sort of interpolation could be done with an indexed colormap. The usual
> > shading interpolation routines assume either RGB or CMY colorspace.
> > But I think you've answered that question.  The gnuplot PM3D grey
> > scale palette routines define a single scale within which interpolation
> > of the index value does make sense.
>
> ... for pm3d it does.  But the same need not apply for GD at large.
>
> PM3D doesn't interpolate colours, it interpolates indices, i.e. it
> interpolates along a polyline through colour space.  The idea being that
> all resulting colours should still be on that polyline.
>
> To give an example, from an RGB palette defined like this:
>
> 0.0: blue
> 0.5: white
> 1.0: red
>
> all colours interpolated by pm3d would currently be pure blue or red at
> various levels of saturation.  A triangle with these three values at its
> corners would have a pure white streak all along the edge from the white
> corner to the center of the opposite edge.
>
> Actual colour interpolation would yield all kinds of violet with pure
> white only around the original white vertex.

I'm not sure to understand the notion of palette in your example.
Which and how many colors do you have in the palette when you start to
render a given triangle?

About GD in general, it supports indexed colors, it is what we define
as palette based image (created by gdImageCreate). The indexes are
sequentially incremented:

> for(i=3D0;i<N;i++) png_smooth_color[i] =3D gdImageColorAllocate(...)

it will create a color from the last existing index.

About what Dan said later (sorry, I subscribed only this morning)
> I assume you are interested only in linear interpolation and not higher o=
rder
> approaches like 2D splines.

I wrote a sample code including the new shaded triangle function in
Ethan's request:

http://bugs.libgd.org/?do=3Ddetails&task_id=3D39

It uses a linear interpolation. Only true color images are supported
now (no palette, "unlimited" colors). I did not test all cases but the
colors on the edges seem to match (see the attached images, a
quadrilateral rendered using two triangles).

> Does gdlib have indexed colormaps?

Yes, see above :)

> If not, a gradient triangle fill shouldn't
> be too difficult to implement.  If we know the resolution of the PNG/JPEG
> then it should be straightforward.

In GD (or any other bitmap based library), the rendering is resolution
independent. It is pixel based. The DPI is  meta information to help
other applications to know the size in inches but the image itself
remains the same.

I have a question about the usage of indexed colors, as I understand
the need of an indexed intensity palette (well known method for
VGA-like devices for example). Is there a reason to keep using them
instead of a true color image?

True color image are faster to render and the image can be saved as
indexed PNG (if size matters). It makes little sense to use indexed
colors if JPEG is the target. JPEG output are true color.

Feel free to post comment in the issues tracker, our mailing lists or
here (privately too but I prefer to keep the process as open as
possible). As Ethan said, we have plenty of time to define our
respective needs.

Thanks for your feedbacks,

--Pierre