Poor performance with DrawText on Win7 x64

Question

I noticed in an MFC application I'm developing that while dragging the scroll bar to smoothly scroll down the document, the framerate drops to choppy levels when a block containing about a paragraph of text is on screen, but silky smooth when it's offscreen. Investigating the performance, I found the single CDC::DrawText call for the paragraph of text responsible. This is in an optimised release build.

I used QueryPerformanceCounter to get a high-resolution measurement of just the DrawText call, like this:

QueryPerformanceCounter(...);
pDC->DrawText(some_cstring, some_crect, DT_WORDBREAK);
QueryPerformanceCounter(...);

The text is unicode, lorem-ipsum style filler, 865 characters long and wraps over 7-and-a-bit lines given the rectangle and font (Segoe UI, lfHeight = -12, a standard body text size). From my measurements, that call alone takes on average 7.5 ms, with the odd peak at 21ms. (Note to keep up with a 60Hz monitor you get about 16ms to render each update.)

I tried making some changes to improve the performance:

• Removing the DT_WORDBREAK improves performance to about 1ms (about 7 times faster), but given only one line of text is making it to the screen, and there were just over 7 lines with word breaking, this seems to suggest to me the bottleneck is elsewhere.
• I was drawing text in transparent mode (SetBkMode(TRANSPARENT)). So I tried opaque mode with a solid background fill. No improvement.
• I thought ClearType rendering might be to blame. I changed the font lfQuality from CLEARTYPE_QUALITY to NONANTIALIASED_QUALITY. It looked like crap with sharp edges and all, and no improvement.
• As per a comment suggestion, I was using a CMemDC, but I got rid of it and did direct drawing. It flickered like mad, and no improvement.

This is running on a Windows 7 64-bit laptop with an Intel Core 2 Duo P8400 @ 2.26 GHz and 4 GB RAM - I don't think it counts as a slow system.

I'm calling DrawText() every time it draws and this obviously hammers the performance with such a slow function, especially if several of those text-blocks are visible at once. It's enough to make the experience feel sluggish. However, Firefox can render a page like this one in ClearType with much more text, and seems to cope just fine. What am I doing wrong? How can I get around the poor performance of an actual DrawText call?

Answer 1

Drawing the text at every refresh is wasteful. Use double buffering, that is, draw in an offscreen bitmap and just blit it to the screen. Then, for scrolling, just copy most of the bitmap up or down or sideways as necessary, then draw only the invalidated area (before blitting the result to the screen).

If even that turns out to be too slow, keep also the drawn text in an off-screen bitmap, and blit instead of draw.

Cheers & hth.,