ClearType in Windows 7

One of my big pet peeves with ClearType prior to Windows 7 was that it only anti-aliased horizontally with sub-pixels. This is great for small fonts, because at such a small scale traditional anti-aliasing has a smudging effect, reducing clarity and increasing the font’s weight. For large fonts however, it introduces some very noticeable aliasing on curves, as best seen in the ‘6′ and ‘g’ here:

"" rendered with GDI

You’ve probably noticed this on websites everywhere, but have come to accept it. Depending on your browser and operating system, you can probably see it in the title here. This problem is solved in Windows 7 with the introduction of DirectWrite, which combines ClearType’s horizontal anti-aliasing with regular vertical anti-aliasing when using large font sizes:

"" rendered with DirectWrite

Of course, DirectWrite affects more than just Latin characters. Any glyphs with very slight angles will see a huge benefit, such as hiragana:

"まこと" rendered with GDI and DirectWrite

Unfortunately, this isn’t a free upgrade. For whatever reason, Microsoft didn’t make all the old GDI functions use DirectWrite’s improvements so to make use of this, all your old GDI and DrawText code will need to be upgraded to use Direct2D and DirectWrite directly, so an old WM_PAINT procedure like this:

HDC hdc = BeginPaint(hwnd, &ps);

HFONT font = CreateFont(-96, 0, 0, 0, FW_NORMAL,
                        0, 0, 0, 0, 0, 0, 0, 0, L"Calibri");

SelectObject(hdc, (HGDIOBJ)font);

RECT rc;
GetClientRect(hwnd, &rc);

DrawText(hdc, L"", 9, &rc,

EndPaint(hwnd, &ps);

Will turn into this:

ID2D1Factory *d2df;

   __uuidof(ID2D1Factory), 0, (void**)&d2df);

IDWriteFactory *dwf;

   __uuidof(IDWriteFactory), (IUnknown**)&dwf);

IDWriteTextFormat *dwfmt;

dwf->CreateTextFormat(L"Calibri", 0, DWRITE_FONT_WEIGHT_REGULAR,
   96.0f, L"en-us", &dwfmt);


RECT rc;
GetClientRect(hwnd, &rc);

D2D1_SIZE_U size = D2D1::SizeU(rc.right - rc.left,
                               rc.bottom -;

ID2D1HwndRenderTarget *d2drt;

   D2D1::HwndRenderTargetProperties(hwnd, size), &d2drt);

ID2D1SolidColorBrush *d2db;


D2D1_SIZE_F layoutSize = d2drt->GetSize();
D2D1_RECT_F layoutRect = D2D1::RectF(0.0, 0.0,
   layoutSize.width, layoutSize.height);

d2drt->DrawText(L"", 9, dwfmt, layoutRect, d2db);

This is no small change, and considering this API won’t work on anything but Vista and Windows 7, you’ll be cutting out a lot of users if you specialize for it. While you could probably make a clever DrawText wrapper, Direct2D and DirectWrite are really set up to get you the most benefit if you’re all in. Hopefully general libraries like Pango and Cairo will get updated backends for it.

DirectWrite has other benefits too, like sub-pixel rendering. When you render text in GDI, glyphs will always get snapped to pixels. If you have two letters side by side, it will choose to always start the next letter 1 or 2 pixels away from the last—but what if the current font size says it should actually be a 1.5 pixel distance? In GDI, this will be rounded to 1 or 2. This is also noticeable with kerning, which tries to remove excessive space between specific glyphs such as “Vo”. Because of this, most of the text you see in GDI is very slightly warped. It’s much more apparent when animating, where it causes the text to have a wobbling effect as it constantly snaps from one pixel to the next instead of smoothly transitioning between the two.

DirectWrite’s sub-pixel rendering helps to alleviate this by doing exactly that: glyphs can now start rendering at that 1.5 pixel distance, or any other point in between. Here you can see the differing space between the ‘V’ and ‘o’, as well as a slight difference between the ‘o’s with DirectWrite on the right side, because they are being rendered on sub-pixel offsets:

"Volcano" close-up comparison with GDI and DirectWrite

The difference between animating with sub-pixel rendering and without is staggering when we view it in motion:

"Volcano" animation comparison with GDI and DirectWrite

Prior to DirectWrite the normal way to animate like this was to render to a texture with monochrome anti-aliasing (that is, without ClearType), and transform the texture while rendering. The problem with that is the transform will introduce a lot of imperfections without expensive super-sampling, and of course it won’t be able to use ClearType. With DirectWrite you get pixel-perfect ClearType rendering every time.

Apparently WPF 4 is already using Direct2D and DirectWrite to some degree, hopefully there will be high-quality text integrated in Flash’s future. Firefox has also been looking at adding DirectWrite support, but I haven’t seen any news of Webkit (Chrome/Safari) or Opera doing the same. It looks like Firefox might actually get it in before Internet Explorer. Edit: looks like Internet Explorer 9 will use DirectWrite—wonder which will go gold with the feature first?

Direct2D and DirectWrite are included in Windows 7, but Microsoft has backported them in the Platform Update for Windows Server 2008 and Windows Vista so there’s no reason people who are sticking with Vista should be left out. Are there people sticking with Vista?

Posted on November 04, 2009 in C++, ClearType, Coding, Direct2D, DirectWrite, Fonts, GDI, Microsoft, Windows, Windows 7

Related Posts