Question

I have many circles positioned randomly in my processing program. Each one is initiated as so:

ellipse(x, y, radius, radius);

I was wondering if I could somehow specify an z-index value for each circle. Circles of higher z-indices display over those of lower ones.

Answer 1

I cannot test this right now but I believe you should look at the fill() method. The fourth parameter in this function specifies opacity (AKA alpha transparency). It is described at Processing.org/learning/color.

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EDIT: This thread on drawing order seems to ask the same question as yours. Playing around with the image transparency is suggested, but it seems like a better answer is to utilize the Processing Layar Library. Here are some examples – the first one appears to be along the lines of your question.

Quoting the Advanced section of the Layers examples:

Dynamically Adding, Removing, or Reordering Layers

It is possible to dynamically add, remove, or reorder layers during a
sketch’s execution, but great care should be taken when doing so. The
list of layers managed by the AppletLayers object can be manipulated
by obtaining a ListIterator object. The semantics of how a list can be
manipulated via an iterator should be studied carefully by reading the
Javadoc for java.util.ListIterator. The reason it is difficult to
manipulate a list during sketch execution is because the list of
layers is being iterated through in order to render them. The best
place to manipulate the list of layers is in the main sketch’s draw()
method and not in a layer’s draw() method or in mouse or keyboard
event processing.