Histogram function plots different colours from those requested

2022 3 28
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更新时间：2022 3 29
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I"m trying to color-code each of 4 histograms using a predefined color scheme. Here's a minimal working example:
colours = [0 32 96;
192 0 0;
71 207 255;
255 143 143;
] / 255;
for i=1:4
x = randn(1,100);
subplot(1,4, i)
values = histogram(x, 'FaceColor', colours (i, :));
end
However, in the image I'm getting, the colors are actually (slightly) different, for instance for the first histogram I get (102,121,160) instead of (0,32,96):

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Steven Lord
Steven Lord 2022-3-28
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Ran in:
The histogram object has a FaceAlpha property, so that if you overlay two histogram objects you can see the one underneath. If you change that FaceAlpha property the color appears slightly different and may be closer to what you want.
x = randn(1, 100);
figure
h = histogram(x, 'FaceColor', 'r');
title("Histograph with FaceAlpha " + h.FaceAlpha)
figure
h = histogram(x, 'FaceColor', 'r', 'FaceAlpha', 1);
title("Histograph with FaceAlpha " + h.FaceAlpha)

更多回答（1 个）

Scott MacKenzie
Scott MacKenzie 2022-3-28
编辑：Scott MacKenzie 2022-3-28
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1 个投票

Ran in:
The histogram function uses a face alpha of 0.6 by default. That's why the colors appear a bit lighter than the values from your eye dropper app. If you set the face alpha to 1.0 (see below), then the displayed colors will agree with the eye dropper values.
colours = [0 32 96;
192 0 0;
71 207 255;
255 143 143;
] / 255;
for i=1:4
x = randn(1,100);
subplot(2,4, i)
values = histogram(x,'FaceColor', colours(i, :));
values.FaceAlpha = 1.0; % NOTE: default is 0.6
values.FaceColor * 255
end
ans = 1×3
0 32 96
ans = 1×3
192 0 0
ans = 1×3
71 207 255
ans = 1×3
255 143 143

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z8080
z8080 2022-3-28
Thanks Scott. I should have metioned that the pop-up in my screenshot was from a system-tray Windows programme called Instant Eyedropper, which gives the RGB coordinates of the color currently under the mouse cursor. According to that, the facecolor inside the bar is differnt from the color coordinates specified in Matlab. Other plots that I make in Matlab (e.g. with gramm) do follow the stated color scheme, according to the same Instant Eyedropper tool, which is what made me think this problem is peculiar to the histogram function.
Scott MacKenzie
Scott MacKenzie 2022-3-28
编辑：Scott MacKenzie 2022-3-28
@z8080, hmm, right your are. I dug a bit deeper and discovered why there is a discrepancy between the set colors and the eye dropper colors. See my modified answer for the details.
Seems as I was modifying my answer, @Steven Lord posted an answer. Same story.

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