FontSizeFromHeight implementation and sample code.

FontSize.cpp

#include <stdio.h>
#include <algorithm>

#include <Application.h>
#include <Window.h>
#include <View.h>
#include <Rect.h>


float FontSizeFromHeight(BFont font, float height)
{
	const int maxIterations = 16;
	const float precision = 1.0e-2f;

	float lo = 0.0f, hi = 2.0f * height;
	for (int i = 0; ; i++) {
		float m = (lo + hi) / 2.0f;
		font.SetSize(m);
		font_height fh;
		font.GetHeight(&fh);
		float val = fh.ascent + fh.descent;
		if (!(i < maxIterations) || std::abs(val - height) < precision)
			return m;
		if (val < height)
			lo = m;
		else
			hi = m;
	}
}

class TestView: public BView
{
public:
	TestView(BRect frame, const char *name): BView(frame, name, B_FOLLOW_NONE, B_FULL_UPDATE_ON_RESIZE | B_WILL_DRAW | B_SUBPIXEL_PRECISE)
	{
	}

	void Draw(BRect dirty)
	{
		BRect rect = Frame().OffsetToCopy(B_ORIGIN);
		float targetHeight = rect.Height() + 1;
		float fontSize = FontSizeFromHeight(be_plain_font, targetHeight);
		SetFontSize(fontSize);
		font_height fh;
		GetFontHeight(&fh);
		printf("targetHeight: %g\n", targetHeight);
		printf("size: %g\n", fontSize);
		printf("asc+dsc: %g\n", fh.ascent + fh.descent);
		DrawString("This is a test.", BPoint(0, fh.ascent));
	}
	
};

class TestWindow: public BWindow
{
private:
	TestView *fView;
public:
	TestWindow(BRect frame): BWindow(frame, "TestApp", B_TITLED_WINDOW_LOOK, B_NORMAL_WINDOW_FEEL, B_ASYNCHRONOUS_CONTROLS)
	{
		fView = new TestView(frame.OffsetToCopy(B_ORIGIN), "view");
		fView->SetResizingMode(B_FOLLOW_ALL);
		AddChild(fView, NULL);
	}

	void Quit()
	{
		be_app_messenger.SendMessage(B_QUIT_REQUESTED);
		BWindow::Quit();
	}

};

class TestApplication: public BApplication
{
private:
	TestWindow *fWnd;
public:
	TestApplication(): BApplication("application/x-vnd.Test-App")
	{
	}

	void ReadyToRun() {
		fWnd = new TestWindow(BRect(0, 0, 512, 64).OffsetByCopy(64, 64));
		fWnd->Show();
	}
};


int main()
{
	TestApplication app;
	app.Run();
	return 0;
}