Quick and dirty solution:
Create a sublass of the original LookAndFeel class. Then, overwrite the following function:
virtual void drawRotarySlider ( Graphics & g,
int x,
int y,
int width,
int height,
float sliderPosProportional,
float rotaryStartAngle,
float rotaryEndAngle,
Slider & slider );
with something like:
void LookAndFeelCustom::drawRotarySlider ( Graphics & g,
int x,
int y,
int width,
int height,
float sliderPosProportional,
float rotaryStartAngle,
float rotaryEndAngle,
Slider & slider )
{
Image myStrip;
myStrip = ImageCache::getFromMemory (BinaryData::knobmid_png, BinaryData::knobmid_pngSize);
const double fractRotation = (slider.getValue() - slider.getMinimum()) / (slider.getMaximum() - slider.getMinimum()); //value between 0 and 1 for current amount of rotation
const int nFrames = myStrip.getHeight()/myStrip.getWidth(); // number of frames for vertical film strip
const int frameIdx = (int)ceil(fractRotation * ((double)nFrames-1.0) ); // current index from 0 --> nFrames-1
const float radius = jmin (width / 2.0f, height / 2.0f) ;
const float centreX = x + width * 0.5f;
const float centreY = y + height * 0.5f;
const float rx = centreX - radius - 1.0f;
const float ry = centreY - radius /* - 1.0f*/;
g.drawImage( myStrip, // image
(int)rx, (int)ry, myStrip.getWidth(), myStrip.getWidth(), // dest
0, frameIdx*myStrip.getWidth(), myStrip.getWidth(), myStrip.getWidth()); // source
}
Voila! (this only supports a specific size, you'll have to play around with values, but you probably get the idea)