MATLAB: How to change an arrow in a quiver plot

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How can I change the color/line width of a single arrow in a quiver plot?

Best Answer

This functionality does not currently exist in "quiver" in MATLAB R2015b, but the same effect can be achieved by creating two separate quiver plots and enforcing equal scales between them.

Turning off the 'AutoScale' property of "quiver" prevents "quiver" from rescaling vectors according to the data. By manually scaling the data, the same scale can be kept between two quiver plots. Refer to the following example which creates a quiver plot of a vector field and colors one of the vectors differently.

stepX = 0.2;
stepY = 0.2;
[x,y] = meshgrid(0:stepX:2,0:stepY:2);
x = x(:);
y = y(:);
u = cos(x).*y;
v = sin(x).*y;
%manually scale u and v
scale = max( stepX/max(u), stepY/max(v) );
u = scale*u;
v = scale*v;
I = 52; %choose an arrow to change
figure
a = quiver(x(I),y(I),u(I),v(I),'AutoScale','off','Color','red','MaxHeadSize',Inf);
%remove point from the data
x(I) = [];
y(I) = [];
u(I) = [];
v(I) = [];
hold on
quiver(x,y,u,v,'AutoScale','off','Color','blue')

Related Solutions

MATLAB: How to use functions “fitgeotrans” and “transformPointsForward” instead of “cp2tform” and “tformfwd”

It is possible to use the function "cp2tform" to get a 'tform' structure and use it as an input to the function "tformfwd". However, the documentation for the function "cp2tform" recommends using the function "fitgeotrans" instead. The function "fitgeotrans" returns a 'tform' object, which is not a valid input to the function "tformfwd". This issue can be resolved by using the function "transformPointsForward" instead of "tformfwd".

The following is an example of how one can use the function "fitgeotrans" followed by "transformPointsForward" instead of "cp2tform" followed by "tformfwd". The only additional step required in the new workflow is to create a 'projective2d' object from the 'tform' object returned by the function "fitgeotrans". This 'projective2d' object is a valid input to the function "transformPointsForward".

Example:

Please refer to the following example that shows how to use the function "cp2tform":

% Transform an image, use the cp2tform function to return the transformation, and 
% compare the angle and scale of the TFORM to the angle and scale of the original 
% transformation:
I = checkerboard;
J = imrotate(I,30);
fixedPoints = [11 11; 41 71];
movingPoints = [14 44; 70 81];
cpselect(J,I,movingPoints,fixedPoints);
t = cp2tform(movingPoints,fixedPoints,'nonreflective similarity');
% Recover angle and scale by checking how a unit vector 
% parallel to the x-axis is rotated and stretched. 
u = [0 1]; 
v = [0 0]; 
[x, y] = tformfwd(t, u, v); 
dx = x(2) - x(1); 
dy = y(2) - y(1); 
angle = (180/pi) * atan2(dy, dx);
scale = 1 / sqrt(dx^2 + dy^2);

In this example, an image is transformed and the function "cp2tform" is used to return the transformation as a 'tform structure'. Subsequently, the angle and scale of the 'tform' is obtained using the function "tformfwd", and compared to the angle and scale of the original transformation.

The MATLAB script "usingFITGEOTRANS.m" demonstrates how the same example can be rewritten using functions "fitgeotrans" and "transformPointsForward".

Please note the following key differences in these two examples:

(1) Using the function "fitgeotrans" instead of "cp2tform":

% Using "cp2tform"
t = cp2tform(movingPoints,fixedPoints,'nonreflective similarity');
% Using "fitgeotrans"
t = fitgeotrans(movingPoints,fixedPoints,'nonreflectivesimilarity');

(2) Creating a 'projective2d' object from the 'tform' object, and using it with the function "transformPointsForward" instead of "tformfwd":

% Using "tformfwd"
[x, y] = tformfwd(t, u, v); 
% Using "transformPointsForward"
projectiveObj = projective2d(t.T);
[x, y] = transformPointsForward(projectiveObj, u, v);

MATLAB: How does QUIVER scale the vectors it plots

There are two independent scaling operations that occur when making a Quiver plot. First, the lengths of the arrows are scaled to fit within the plot area without overlapping. Next, scaling is performed as determined by the 'AutoScale' and 'AutoScaleFactor' properties.

Unfortunately, there is no way to retrieve the scale factor that is applied internally in the first scaling operation of the QUIVER command.

As a workaround, it is possible to create a Quiver plot with arrows scaled by a known amount by turning off the 'AutoScale' option. The U- and V- data can be scaled manually as necessary to improve the aesthetics of the plot. By performing this scaling manually, a known scale factor can be used. For example,

x = [0 0]
y = [0 10]
u = [10 0]
v = [0 10]
quiver(x,y,u,v)

produces arrows with lengths of about 6.364. To get better control over these lengths, manually scale the vectors:

scale_factor = 0.5;
quiver(x,y,u*scale_factor,v*scale_factor,'AutoScale','off')

The commands produce arrows of length 5.

Best Answer

Related Solutions

MATLAB: How to use functions “fitgeotrans” and “transformPointsForward” instead of “cp2tform” and “tformfwd”

MATLAB: How does QUIVER scale the vectors it plots

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