[Interest] Plotting 2d line with color depending on value (like colormap)

Massimiliano Maini maxmaini at gmail.com
Thu Jun 21 12:48:25 CEST 2018


Thanks for the reply Giuseppe !

You're right, the graph in the picture does not show exactly what I need to
do: I do
need a 3rd independent variable for the color of each point.

And if I understood you right, the solutions you mentioned won't fit my
need, right ?


On Thu, 21 Jun 2018 at 12:41, Giuseppe D'Angelo via Interest <
interest at qt-project.org> wrote:

> On 21/06/18 11:51, Massimiliano Maini wrote:
> > I'd need to do something like this: https://imgur.com/MLAEuad
> >
> > I have a set of (x,y,z) data points and want to plot them on a usual 2d
> > plot ( x and y values), but with the line color depending on the z
> > values (colormap).
> >
> > How can this be achieved in Qt ?
>
> I'm not sure if I understood correctly; the graph in the picture isn't
> using a third independent variable for the color of each point, rather
> it's just using the y (in other words: the color of each point in the
> line is just a function of its y coordinate.)
>
> Anyhow, short of using a complete graphing solution, you can easily
> generate a vertical gradient using QLinearGradient (with the number of
> stops you need), and the curve as a QPainterPath. Then you have a couple
> of easy options:
>
> 1) set the curve as a clip path on a QPainter, and fill a rectangle with
> the gradient (=> only the path will be filled), or
>
> 2) create a transparent QImage of suitable size, draw the path in some
> solid color, and draw a rectangle filled with the gradient over the path
> using some composition mode, e.g. SourceIn. Then draw the image on your
> chart.
>
> HTH,
> --
> Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
> KDAB (France) S.A.S., a KDAB Group company
> Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
> KDAB - The Qt, C++ and OpenGL Experts
>
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