How would you calculate the point size of a font necessary to best fit a predefined area?

Question

I have an interesting problem. I'm almost there but am curious how others would tackle it. I want to display some multi-line text in a predefined area. I don't know what the text will be or how big the area will be so the function would have to be written generically. You can assume a standard font is always used but the point size is what must change.

Assume you have a function that will draw text that is passed to it in a string parameter. The function has a form object to draw in, and is also passed a rectangle object defining the bounding area of the text on the form. The function needs to display the text on the form in the given rectangle in as large a font as will fit. The challenge for me was is in calculating the size of the font to use to have the text fit as best it can, in the rectangle with minimal white space.

These 2 equations might be useful:

float pixels = (points *dpi)/72f;
float points = (pixels*72f)/dpi);

Also:

float dpi = CreateGraphics().DpiY;

Answer 1

binary search over point sizes: Start with the biggest available point size. If it doesn't fit, try half of that, ...

Answer 2

Well, it is tricky. Calculating a point size directly isn't going to work, the width of the text is dependent on the font metrics. Binary search is an obvious strategy but it cannot work in practice. True-type hinting and word wrapping conspire to destabilize it.

I'd recommend you start with binary search, setting hi and lo to reasonable defaults like 72 and 6. Then when the range narrows down to, say, 5 points, start testing each individual point size until you find the largest one that fits. When you write the algorithm, do make sure you count on a size N that fits but a size N-1 that doesn't fit.