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openglvector-graphics

Rendering Vector Graphics in OpenGL?

Is there a way to load a vector graphics file and then render it using OpenGL? This is a vague question as I don't know much about file formats for vector graphics. I know of SVG, though.

Turning it to raster isn't really helpful as I want to do real time zooming in on the objects.

Solution

  • Let me expand on Greg's answer.

    It's true that Qt has a SVG renderer class, QSvgRenderer. Also, any drawing that you do in Qt can be done on any "QPaintDevice", where we're interested in the following "paint devices":

    • A Qt widget;
    • In particular, a GL-based Qt widget (QGLWidget);
    • A Qt image

    So, if you decide to use Qt, your options are:

    1. Stop using your current method of setting up the window (and GL context), and start using QGLWidget for all your rendering, including the SVG rendering. This might be a pretty small change, depending on your needs. QGLWidget isn't particularly limiting in its capabilities.
    2. Use QSvgRenderer to render to a QImage, then put the data from that QImage into a GL texture (as you normally would), and render it any way you want (e.g. into a rectangular GL_QUAD). Might have worse performance than the other method but requires the least change to your code.

    Wondering what QGLWidget does exactly? Well, when you issue Qt rendering commands to a QGLWidget, they're translated to GL calls for you. And this also happens when the rendering commands are issued by the SVG renderer. So in the end, your SVG is going to end up being rendered via a bunch of GL primitives (lines, polygons, etc).

    This has a disadvantage. Different videocards implement OpenGL slightly differently, and Qt does not (and can not) account for all those differences. So, for example, if your user has a cheap on-board Intel videocard, then his videocard doesn't support OpenGL antialiasing, and this means your SVG will also look aliased (jaggy), if you render it directly to a QGLWidget. Going through a QImage avoids such problems.

    You can use the QImage method when you're zooming in realtime, too. It just depends on how fast you need it to be. You may need careful optimizations such as reusing the same QImage, and enabling clipping for your QPainter.