i wouldnt call it a bug. its just that all available renderers work different and have different kind of pros and cons...

thats what it says in the size() reference

Rendering graphics requires tradeoffs between speed, accuracy, and general usefulness of the available features. None of the renderers are perfect, so we provide multiple options so that you can decide what tradeoffs make the most sense for your project. We'd prefer all of them to have perfect visual accuracy, high performance, and support a wide range of features, but that's simply not possible.

Thanks cedrickiefer for your response.

I know they work differently.
I would like to have a better understanding of these pros & cons,
so I will be able to make more educated decisions and choices with my future programming.
Is there an explanation for the question above?

Cheers.

Without following the links, my answer might not be accurate...
But well, convolution and other operations like filtering, blending, fuzzing, etc. are operations on a bunch of pixels.
P2D and P3D are based on pixel arrays, so they are quite good at manipulating them. Java2D works more on abstractions of these pixels (rect, ellipse) and the internal Java buffer must be converted to PImage, so it can be a bit slower on such operations.
OpenGL is even more abstract, as it delegates the operations to the GPU, and if you do pixel manipulations, you probably need to pull the raster data from the GPU's memory, which is slow too. It is much better at showing a triangle in 3D space than updating a single pixel...

Not sure if it answers your question, but it can help you in the educated decisions, I hope.

Is there a particular reason to use OpenGL in this instance?

If you want to use OpenGL for convolution, you  should probably use the GLGraphics library and do it as a filter instead. Will be MUCH much faster.

kb, http://www.riftgame.com/

Thanks of your answer phi.lho, that helps a lot.

Kevin,
There is no particular reason in this instance.
However I imagine that there mighty be a time when
I will want to use OpenGL and have pixel manipulation within the program.
Thanks for your suggestion, I will have to read up on it.

Thank you guys for your time.
D