Azimuth data: Colormap wrapping when performing interpolation

Hello,

I have a problem with interpolating a colormap for azimuth / circular data... Please check the attached images: The colormap (HSV) represents the direction of the vectors (ranging from -180 to +180 degrees). Red is left, cyan ist right, greenish is up and purple is down.

For several reasons, I would like to create a smooothed graphical output. That is why I upscale the dataset using the function 'imresize'.

But the result gives a strong line when the direction changes from -180 to +180 (for reasons that I understand, but I can't think of a workaround...).

Does someone have an idea how to prevent this colormap wrapping...? Thanks!!

Answer 1

[Shunichi Kusano]

As you noticed, phase is discontinuous function. One can convert this to the continuous function, for example, by using cosine and sine functions, for which interpolation works.

%% generate original phase image
x = -1:0.1:1; % original x-coordinate
y = -1:0.1:1; % original y-coordinate
[X, Y] = meshgrid(x,y);
phase = angle(complex(X,Y)) * 180 / pi;
imagesc(phase);
colormap hsv;

% not-correct
figure,imagesc(imresize(phase, [100,100])),colormap hsv;

% phase is converted by cos and sin.
X = cos(phase/180*pi);
Y = sin(phase/180*pi);
% interpolation
X_interp = imresize(X, [100, 100]);
Y_interp = imresize(Y, [100, 100]);
% re-convert to phase
phase_interp = angle(complex(X_interp,Y_interp)) * 180 / pi;
figure, imagesc(phase_interp), colormap hsv;

hope this helps.

Answer 2

[William Thielicke]

ThabkThank you veryvery much for your helphelp!