Ditto/DPI.h

#pragma once
// Definition: relative pixel = 1 pixel at 96 DPI and scaled based on actual DPI.
class CDPI
{
public:
	CDPI() : _fInitialized(false), _dpiX(96), _dpiY(96) { }

	// Get screen DPI.
	int GetDPIX() { _Init(); return _dpiX; }
	int GetDPIY() { _Init(); return _dpiY; }

	// Convert between raw pixels and relative pixels.
	int ScaleX(int x) { _Init(); return MulDiv(x, _dpiX, 96); }
	int ScaleY(int y) { _Init(); return MulDiv(y, _dpiY, 96); }
	int UnscaleX(int x) { _Init(); return MulDiv(x, 96, _dpiX); }
	int UnscaleY(int y) { _Init(); return MulDiv(y, 96, _dpiY); }

	// Determine the screen dimensions in relative pixels.
	int ScaledScreenWidth() { return _ScaledSystemMetricX(SM_CXSCREEN); }
	int ScaledScreenHeight() { return _ScaledSystemMetricY(SM_CYSCREEN); }

	// Scale rectangle from raw pixels to relative pixels.
	void ScaleRect(__inout RECT *pRect)
	{
		pRect->left = ScaleX(pRect->left);
		pRect->right = ScaleX(pRect->right);
		pRect->top = ScaleY(pRect->top);
		pRect->bottom = ScaleY(pRect->bottom);
	}
	// Determine if screen resolution meets minimum requirements in relative
	// pixels.
	bool IsResolutionAtLeast(int cxMin, int cyMin) 
	{ 
		return (ScaledScreenWidth() >= cxMin) && (ScaledScreenHeight() >= cyMin); 
	}

	// Convert a point size (1/72 of an inch) to raw pixels.
	int PointsToPixels(int pt) { _Init(); return MulDiv(pt, _dpiY, 72); }

	// Invalidate any cached metrics.
	void Invalidate() { _fInitialized = false; }

private:
	void _Init()
	{
		if (!_fInitialized)
		{
			HDC hdc = GetDC(NULL);
			if (hdc)
			{
				_dpiX = GetDeviceCaps(hdc, LOGPIXELSX);
				_dpiY = GetDeviceCaps(hdc, LOGPIXELSY);
				ReleaseDC(NULL, hdc);
			}
			_fInitialized = true;
		}
	}

	int _ScaledSystemMetricX(int nIndex) 
	{ 
		_Init(); 
		return MulDiv(GetSystemMetrics(nIndex), 96, _dpiX); 
	}

	int _ScaledSystemMetricY(int nIndex) 
	{ 
		_Init(); 
		return MulDiv(GetSystemMetrics(nIndex), 96, _dpiY); 
	}
private:
	bool _fInitialized;

	int _dpiX;
	int _dpiY;
};