In the past few weeks, I discussed the new HG2 axes Backdrop and Baseline properties with their associated ability to specify the transparency level using a fourth (undocumented) element in their Color.
In other words, color in HG2 can still be specified as an RGB triplet (e.g., [1,0,0] to symbolize bright red), but also via a 4-element quadruplet RGBA, where the 4th element (Alpha) signifies the opacity level (0.0=fully transparent, 0.5=semi-transparent, 1.0=opaque). So, for example, [1, 0, 0, 0.3] means a 70%-transparent red.
This Alpha element is not documented anywhere as being acceptable, but appears to be supported almost universally in HG2 wherever a color element can be specified. In some rare cases (e.g., for patch objects) Matlab has separate Alpha properties that are fully documented, but in any case nowhere have I seen documented that we can directly set the alpha value in the color property, especially for objects (such as plot lines) that do not officially support transparency. If anyone finds a documented reference anywhere, please let me know – perhaps I simply missed it.
Here is a simple visualization:
xlim([1,5]); hold('on'); h1a = plot(1:5, 11:15, '.-', 'LineWidth',10, 'DisplayName',' 0.5'); h1b = plot(1.5:5.5, 11:15, '.-', 'LineWidth',10, 'DisplayName',' 1.0', 'Color',h1a.Color); % 100% opaque h1a.Color(4) = 0.5; % 50% transparent h2a = plot(3:7, 15:-1:11, '.-r', 'LineWidth',8, 'DisplayName',' 0.3'); h2a.Color(4)=0.3; % 70% transparent h2b = plot(2:6, 15:-1:11, '.-r', 'LineWidth',8, 'DisplayName',' 0.7'); h2b.Color(4)=0.7; % 30% transparent h2c = plot(1:5, 15:-1:11, '.-r', 'LineWidth',8, 'DisplayName',' 1.0'); % 100% opaque = 0% transparent legend('show','Location','west')
Transparent HG2 plot lines
Now for the fun part: we can make color-transition (gradient) effects along the line, using its hidden Edge property:
>> h2b.Edge.get AlignVertexCenters: 'off' AmbientStrength: 0.3 ColorBinding: 'object' ColorData: [4x1 uint8] ColorType: 'truecoloralpha' DiffuseStrength: 0.6 HandleVisibility: 'off' HitTest: 'off' Layer: 'middle' LineStyle: 'solid' LineWidth: 8 NormalBinding: 'none' NormalData: [] Parent: [1x1 Line] PickableParts: 'visible' SpecularColorReflectance: 1 SpecularExponent: 10 SpecularStrength: 0.9 StripData: [1 6] Texture: [] VertexData: [3x5 single] VertexIndices: [] Visible: 'on' WideLineRenderingHint: 'software' >> h2b.Edge.ColorData %[4x1 uint8] ans = 255 0 0 179
The tricky part is to change the Edge.ColorBinding value from its default value of ‘object’ to ‘interpolated’ (there are also ‘discrete’ and ‘none’). Then we can modify Edge.ColorData from being a 4×1 array of uint8 (value of 255 corresponding to a color value of 1.0), to being a 4xN matrix, where N is the number of data points specified for the line, such that each data point along the line will get its own unique RGB or RGBA value. (the data values themselves are kept as a 3xN matrix of single values in Edge.VertexData).
So, for example, let’s modify the middle (30%-transparent) red line to something more colorful:
>> cd=uint8([255,200,250,50,0; 0,50,250,150,200; 0,0,0,100,150; 179,150,200,70,50]) cd = 255 200 250 50 0 0 50 250 150 200 0 0 0 100 150 179 150 200 70 50 >> set(h2b.Edge, 'ColorBinding','interpolated', 'ColorData',cd)
HG2 plot line color, transparency gradient
As you can see, we can interpolate not only the colors, but also the transparency along the line.
Note: We need to update all the relevant properties together, in a single set() update, otherwise we’d get warning messages about incompatibilities between the property values. For example:
>> h2b.Edge.ColorBinding = 'interpolated'; Warning: Error creating or updating LineStrip Error in value of property ColorData Array is wrong shape or size (Type "warning off MATLAB:gui:array:InvalidArrayShape" to suppress this warning.)
Markers
Note how the markers are clearly seen in the transparent lines but not the opaque ones. This is because the markers have the same color as the lines in today’s example. Since the lines are wide, the markers are surrounded by pixels of the same color. Therefore, the markers are only visible when the surrounding pixels are less opaque (i.e., lighter).
As a related customization, we can control whether the markers appear “on top of” (in front of) the line or “beneath” it by updating the Edge.Layer property from ‘middle’ to ‘front’ (there is also ‘back’, but I guess you won’t typically use it). This is important for transparent lines, since it controls the brightness of the markers: “on top” (in front) they appear brighter. As far as I know, this cannot be set separately for each marker – they are all updated together.
Of course, we could always update the line’s fully-documented MarkerSize, MarkerFaceColor and MarkerEdgeColor properties, in addition to the undocumented customizations above.
Next week I will describe how we can customize plot line markers in ways that you never thought possible. So stay tuned 
Related posts:
- Plot markers transparency and color gradient Matlab plot-line markers can be customized to have transparency and color gradients. ...
- Accessing hidden HG2 plot functionality In HG2, some of the plot functionality is hidden in undocumented properties. ...
- Plot LineSmoothing property LineSmoothing is a hidden and undocumented plot line property that creates anti-aliased (smooth unpixelized) lines in Matlab plots...
- Draggable plot data-tips Matlab's standard plot data-tips can be customized to enable dragging, without being limitted to be adjacent to their data-point. ...
- Performance: scatter vs. line In many circumstances, the line function can generate visually-identical plots as the scatter function, much faster...
- Plot LimInclude properties The plot objects' XLimInclude, YLimInclude, ZLimInclude, ALimInclude and CLimInclude properties are an important feature, that has both functional and performance implications....
Have you managed to get the same working for markers? I can’t set the alpha component for MarkerFaceColor, and setting it for Color doesn’t even affect the marker edge:
I’m using 2013b, maybe it’s different in later versions?
@Paul – I plan to cover Marker customizations (including transparency) in next week’s post. Stay tuned
That’s absolutely splendid, @Yair!
All those years I used a 2×N mesh in order to make a gradient plot, and it turns out to be so simple in HG2. Keep up the good work!
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Is it possible to set the transparency of 2D contourf plot fills? I used to be able to do this before 2014b, but the latest update broke what I was using. I used allchild to get handles of the subobjects and set the FaceAlpha to less than 1
Great, many thanks for this. When I try that code, hFaces comes out as 0×0. When I insert a pause(0.05) before the hFaces = hContour.FacePrims line it works. Strange.
I assume that drawnow will also work…
Yes, it does. Thanks again.
This is great, but I find that when i set an alpha for a line color, the line smoothing goes away and i get the old fashioned pixelated lines. Have you noticed that too or is it just me?
It seems that the workaround doesn’t work on 2014a edition!
I receive “Attempt to reference field f non-structure array.”
Any comments?
Of course it doesn’t – as stated in the top paragraph, this article (which is part of a series) only discusses HG2, in other words R2014b and later. R2014a still uses the previous version of HG, which is radically different.
I’ve been able to make some beautiful figures using HG2 thanks to your help. Unfortunately, I am now unable to find a way to export to any sort of vector format that maintains my transparent lines (among other issues). I have tried export_fig, and exporting to .svg. When I do the export, I simply get the figure without my edited properties like alpha and color interpolation. Have you found any way around this? It’s driving me crazy and, from an academic standpoint, appears to make many of the advances of HG2 frustratingly useless. At this point I would put up with just about anything that would get me a super-high resolution image out.
@James – try using the opengl (rather than painters) renderer (the ‘-opengl’ parameter in
export_fig). Note that this might take a long time, consume much memory, and generate a huge output file.Also try using bitmap output with a large resolution (the -r# parameter) – it would not be vectorized, but at least it might have a large-enough resolution to be publication quality.
Hi Yair,
Thanks for the quick response. The opengl renderer works for .png or .bmp format, yes, but when I change the resolution the output changes from my pretty figure with transparency, etc. to essentially the plot before I edit any of the color and alpha properties (still not working for vector outputs).
What I’ve gathered so far is that this may be related the necessity of using the drawnow command in between plotting the line and getting the line marker handles. For example, if I do not use drawnow after plotting the line, I am unable to access the line.MarkerHandle properties. I do not need the drawnow to do to things such as changing the grid lines (so documented properties related to the axes rather than the line). When I try to render with vector graphics or .bmp with increased resolution, I get only the changes I made that don’t rely on the drawnow (so it renders the grid lines, but not the color and alpha values based on line.MarkerHandle.EdgeColorData).
I have no current idea how to fix this. I wonder if it has to do with the way the exporter “re-renders” the figure for export, and if it’s as simple as getting it to add another drawnow in there somewhere so that it reads the MarkerHandle changes. I don’t know if export_fig would be able to deal with such a change. I guess any further advice, tips or even a solution would be greatly appreciated.
Thanks again!
@James – I suspect that you will find the same behavior with Matlab’s built-in print function, and in this case there is not much that export_fig can do since it uses print internally.
Thanks. I guess these hidden properties are unfortunately still useless when it comes to vector graphics (or even high quality non-vector formats unless you have a high-res display and effectively take a screenshot). Hopefully this gets updated soon.
Quick comment: on my Computer (Win7, Matlab 2015a) the above syntax only works for lines up to exactly 25000 data points. Also, changing the ColorBinding and setting the ColorData is only possible, if the line has already been drawn. Therefore, it is necessary to issue a drawnow before setting these parameters:
Hi Yair,
I tried to use the code for varying the transparency and for some reason the following example code does not work as expected:
I expected first 5 points to have transparency of 200 and then 5 with transparency of 100. Can you explain why the result is with same transparency all over ?
Thanks,
-Moshe.