Optimize Code for Getting and Setting Graphics Properties

Automatically Calculated Properties

MATLAB® automatically calculates the values of some properties based on the values of other properties. For example, axis limits affect the values used for axis ticks, which, in turn, affect the axis tick labels.

Mode properties can also affect when a property is calculated. Mode properties are paired with specific properties of an object, and their names end with the word "Mode". For example, axes objects have an XTick property and a XTickMode property. If the XTickMode property has a value of "auto", MATLAB updates the XTick property when conditions (such as the x-axis limits) change. If the XTickMode property has a value of "manual", the XTick property is preserved regardless of other property values.

MATLAB can also calculate the values of certain properties based on other objects. For example, plotting functions automatically create an axes with axis limits, tick labels, and a size appropriate for the plotted data and the figure size.

When you query a calculated property, MATLAB performs an implicit drawnow to ensure all property values are up to date before returning the property value. The query causes a full update of all dependent properties and an update of the screen.

MATLAB performs a full update, if necessary, before returning a value from a calculated property to ensure the returned value is up to date.

This table lists some of the more commonly calculated properties.

ObjectPropertiesWhen MATLAB Calculates these Properties

Axes

CameraPosition, CameraTarget, CameraUpVector, CameraViewAngle

When the corresponding mode property is set to "auto"

 

Position, InnerPosition,

When the PositionConstraint property is set to "outerposition"

 OuterPosition

When the PositionConstraint property is set to "innerposition"

 TightInset

Always

 

XLim, YLim, ZLim

When the corresponding mode property is set to "auto"

 

XTick, YTick, ZTick, XMinorTick, YMinorTick, ZMinorTick

When the corresponding mode property is set to "auto"

 

XTickLabel, YTickLabel, ZTickLabel, TickDir

When the corresponding mode property is set to "auto"

 

SortMethod

When the corresponding mode property is set to "auto"

Text

Extent

Always

 

Position

When the text object is used as an axes title, subtitle, axis label, or secondary label and the PositionMode property is set to "auto"

 

FontSize, FontWeight

When the text object is used as an axes title, subtitle, axis label, or secondary label and the corresponding mode property is set to "auto"

Inefficient Cycles of Sets and Gets

When you set property values, you change the state of the graphics model and mark it as needing to be updated. When you query a calculated property, MATLAB needs to perform an update if the graphics model and graphics hardware are not in sync.

When you get and set properties in the same loop, you can create a situation where updates are performed with every pass through the loop.

  • The get causes an update.

  • The set marks the graphics model as needing an update.

The cycle is repeated with each pass through the loop. It is better to execute all property queries in one loop, then execute all property sets in another loop, as shown in the following example.

This example gets both the Position and Extent properties of a text object and then sets the Position property.

Code with Poor PerformanceCode with Better Performance
h = gobjects(1,500);
p = zeros(500,3);
for ix = 1:500
   h(ix) = text(ix/500,ix/500,num2str(ix));
end
drawnow

% Gets and sets in the same loop,
% prompting a full update at each pass
for ix = 1:500
   pos = get(h(ix),'Position');
   ext = get(h(ix),'Extent');
   p(ix,:) = [pos(1)+(ext(3)+ext(1)), ...
              pos(2)+ext(2)+ext(4),0];
   set(h(ix),'Position',p(ix,:))
end
drawnow
h = gobjects(1,500);
p = zeros(500,3);
for ix = 1:500
   h(ix) = text(ix/500,ix/500,num2str(ix));
end
drawnow

% Get and save property values
for ix=1:500
   pos = get(h(ix),'Position');
   ext = get(h(ix),'Extent');
   p(ix,:) = [pos(1)+(ext(3)+ext(1)), ...
              pos(2)+ext(2)+ext(4),0];
end

% Set the property values and 
% call a drawnow after the loop
for ix=1:500
   set(h(ix),'Position',p(ix,:));
end
drawnow

This code performs poorly because:

  • The Extent property depends on other values, such as screen resolution, figure size, and axis limits, so querying this property can cause a full update.

  • Each set of the Position property makes a full update necessary when the next get of the Extent property occurs.

The performance is better because this code:

  • Queries all property values in one loop and stores these values in an array.

  • Sets all property values in a separate loop.

  • Calls drawnow after the second loop finishes.

Using Text Extent to Rotate Labels

In cases where you query the text Extent property to rotate axes labels, it is more efficient to use the axes properties XTickLabelRotation, YTickLabelRotation, and ZTickLabelRotation.