Can you explain a little more about what kind of program you're writing here?

PApplet is never instanced more than once. What you're looking for is most easily done using a PGraphics instance as your "engine". See the createGraphics example in the Learning sections for how to do this.

Any class that is descended from PGraphics can be created, so you could conceivably use P2D, Java2D, P3D or even PDF depending on your needs. Creating an off-screen PGraphicsOpenGL instance is not possible as far as I know, but the GLGraphics library has a class for off-screen OpenGL rendering.

Yes, I would like to do something outside the model of "display loop". Essentially, I want to use Processing like the Java graphics to draw objects when I want to without using the display loop. Is this possible?

For example, if I want to draw a square composed of 4 lines. I want to be able to do this by calling engine four different times, each time drawing a specified line. Now say, later after some work has been done by my algorithm I decide to add more lines or remove a line or two, I want to be able to do that.

The problem is that my algorithm has to work withing "draw()" for my drawing to be done as specified above.

Instead I want my algorithm to be separate and my drawing to be separate, that is I want to draw when required.

Is this at all possible?

Thanks.

Yes, that's exactly what createGraphics() is for. Did you look at the createGraphics example I linked to? It shows how to set up a PGraphics canvas and draw to it.

Simply call createGraphics() to create a PGraphics canvas to draw into (your "engine"). Then call PGraphics.beginDraw() to begin drawing to that canvas, execute your drawing commands and then call PGraphics.endDraw() to complete the drawing. You need begin/endDraw(), otherwise the drawing commands aren't rendered to the canvas.

To draw the contents of your canvas just use the image() function. PGraphics is a subclass of PImage, hence it's drawable to other PGraphics objects.

Yes, checked the page that you pointed to in your response. Looks like that will do the trick. Thanks for your help.