Graph rendering using 3D acceleration

Question

We generate graphs for huge datasets. We are talking 4096 samples per second, and 10 minutes per graph. A simple calculation makes for 4096 * 60 * 10 = 2457600 samples per lineg

Answer 1

I'd like to comment on your assertion that you cannot omit samples, on the back of tgamblin's answer.

You should think of the data that you're drawing to the screen as a sampling problem. You're talking about 2.4M points of data, and you're trying to draw that to a screen that is only a few thousand points across (at least I assuming that it is, since you're worried about 30fps refresh rates)

So that means that for every pixel in the x axis you're rendering in the order of 1000 points that you don't need to. Even if you do go down the path of utilising your gpu (eg. through the use of opengl) that is still a great deal of work that the gpu needs to do for lines that aren't going to be visible.

A technique that I have used for presenting sample data is to generate a set of data that is a subset of the whole set, just for rendering. For a given pixel in the x axis (ie. a given x axis screen coordinate) you need to render an absolute maximum of 4 points - that is the minimum y, maximum y, leftmost y and rightmost y. That will render all of the information that can be usefully rendered. You can still see the minima and maxima, and you retain the relationship to the neighbouring pixels.

With this in mind, you can work out the number of samples that will fall into the same pixel in the x axis (think of them as data "bins"). Within a given bin, you can then determine the particular samples for maxima, minima etc.

To reiterate, this is only a subset that is used for display - and is only appropriate until the display parameters change. eg. if the user scrolls the graph or zooms, you need to recalculate the render subset.

You can do this if you are using opengl, but since opengl uses a normalised coordinate system (and you're interested in real world screen coordinates) you will have to work a little harder to accurately determine your data bins. This will be easier without using opengl, but then you don't get the full benefit of your graphics hardware.