Transparent ContourPlot

[dg]

I have been trying to create a contour plot where the contour shading is transparent, so that I can draw gridlines and see them through the contours. I can't seem to figure this one out....

This produces a mess:

ContourPlot[x y^2 + 2, {x, -2, 2}, {y, -2, 2}, PlotRange -> {-10, 10},
ContourShading -> {Orange, Opacity[0.5]}]

[Bob Hanlon]

Perhaps you want Mesh

ContourPlot[x y^2 + 2, {x, -2, 2}, {y, -2, 2},
PlotRange -> {-10, 10},

Mesh -> 7,
MeshFunctions -> {#1 &, #2 &},
Axes -> False,
ColorFunction -> (Lighter[Orange, #] &)]


Bob Hanlon

--
Bob Hanlon

[Armand Tamzarian]

I'm guessing this might be what you want:

ContourPlot[x y^2 + 2, {x, -2, 2}, {y, -2, 2}, PlotRange -> {-10, 10},
ContourShading -> {{Orange, Opacity[0.5]}}]

{Orange, Opacity[0.5]} are in a list to indicate they apply together.

Mike

[Heike Gramberg]

To draw grid lines on top of the plot, you could use the (undocumented) option
Method->{"GridLinesInFront"->True}, so you could do something like

ContourPlot[x y^2 + 2, {x, -2, 2}, {y, -2, 2},
PlotRange -> {-10, 10},

Method -> {"GridLinesInFront" -> True},
GridLines -> Automatic]

If you still want to have a transparent shading, you probably want to set
Method -> {"TransparentPolygonMesh" -> True} as well which will get rid of the edges of
the individual polygons. You can combine these two options as follows

ContourPlot[x y^2 + 2, {x, -2, 2}, {y, -2, 2},
PlotRange -> {-10, 10},

Method -> {"GridLinesInFront" -> True, "TransparentPolygonMesh" -> True},
ColorFunction -> (Directive[Opacity[.5], ColorData["Rainbow"][#]] &),
ContourShading -> Automatic,
GridLines -> Automatic]

Heike

On 16 Dec 2011, at 11:52, dg wrote:

> I have been trying to create a contour plot where the contour shading is transparent, so that I can draw gridlines and see them through the contours. I can't seem to figure this one out....

[JUN]

Hi,
at first I thought Heike's solution with "TransparentPolygonMesh" ->
True was going to make my day. But unfortunately, when I export the
plot as PDF on Mac, it still shows the ugly mesh lines. So instead of
this solution, I modified another approach that I had settled on:

rasterContourPlot[f_, rx_, ry_, opts : OptionsPattern[]] :=
Module[{img, cont, plotRangeRule, contourOptions, frameOptions},
contourOptions =
Join[FilterRules[{opts},
FilterRules[Options[ContourPlot],
Except[{Background, PlotRangePadding, Frame,
Axes}]]], {PlotRangePadding -> None, Frame -> None,
Axes -> None}];
cont = ContourPlot[f, rx, ry,
Evaluate@Apply[Sequence, contourOptions]];
img = Rasterize[Graphics[
GraphicsComplex[cont[[1, 1]], cont[[1, 2, 1]]],
PlotRangePadding -> None, ImagePadding -> None],
ImageSize -> 2 ((ImageSize /. {opts}) /. {ImageSize ->
CurrentValue[ImageSize]})];
plotRangeRule = FilterRules[AbsoluteOptions[cont], PlotRange];
frameOptions = Join[FilterRules[{opts},
FilterRules[Options[Graphics],
Except[{PlotRangeClipping, PlotRange}]]],
{plotRangeRule, Frame -> True, PlotRangeClipping -> True}];
Show[Graphics[{{FaceForm[], EdgeForm[],
Apply[Rectangle, Transpose[PlotRange /. plotRangeRule]]},
Inset[Show[SetAlphaChannel[img, "ShadingOpacity" /. {opts}
/. {"ShadingOpacity" -> 1}], AspectRatio -> Full],
Scaled[{0, 0}], Scaled[{0, 0}], Scaled[{1, 1}]]},
PlotRangePadding -> None],
Graphics[GraphicsComplex[cont[[1, 1]], cont[[1, 2, 2]]],
PlotRangePadding -> None, ImagePadding -> None],
Evaluate@Apply[Sequence, frameOptions]]
]

Now I'm using this modified contour plot function with grid lines and
a prolog that display a blue disk behind the plot:

rasterContourPlot[x y^2 + 2, {x, -2, 2}, {y, -2, 2},
PlotRange -> {-10, 10}, GridLines -> Automatic, Contours -> 15,
"ShadingOpacity" -> .5, ImageSize -> 600,
Prolog -> {Blue, Disk[{1, 1.5}, 1]}]

The function has one new option that the standard contour plot doesn't
have: "ShadingOpacity" - that's what determines the opacity of the
contour shading. To implement this opacity, I convert the shading to
an image and then apply an alpha channel. The image is also immune to
the mesh lines artifacts that plagued the original exported PDF.

Jens

[Syd Geraghty]

Heike,

I noticed that the mesh function chosen automatically led to a mis-colouring of a small triangle at the origin which can be corrected by using a MaxRecursion of 7.

ContourPlot[x y^2 + 2, {x, -2, 2}, {y, -2, 2}, PlotRange -> {-10, 10},

ImageSize -> 600, MaxRecursion -> 7,

Method -> {"GridLinesInFront" -> True,
"TransparentPolygonMesh" -> True},
ColorFunction -> (Directive[Opacity[.5], ColorData["Rainbow"][#]] &),
ContourShading -> Automatic, GridLines -> Automatic]

Cheers .... Syd

Syd Geraghty B.Sc, M.Sc.

sydge...@me.com

Mathematica 8.0 for Mac OS X x86 (64-bit) (February 23, 2011)
ReleaseID: 8.0.1.0 (2063982, 2063639)
MacOS X V 10.7.1 Lion
MacBook Pro 2.33 GHz Intel Core 2 Duo 3GB RAM