Smooth animations under XFree86
Billy Biggs <firstname.lastname@example.org>
Mon Apr 7 09:21:11 ADT 2003, Fri Apr 11 16:07:55 ADT 2003
I have been watching the news alot lately, and quite enjoy the
scrolling headlines along the bottom of CNN, al-jazeera, MSNBC, and
other news networks, so I decided to write a news ticker application for
The primary goal of the stock ticker application was smoothness of
display. There are alot of news sources to choose from, and alot of
news occurs in a day, so the text must move rapidly, but also be
legible. Artifacts to avoid are flicker, tearing and judder.
In many applications, the output is drawn multi-pass. For example,
one way to draw a frame is to first clear the window to the background
colour, then draw the text in the new position on the cleared image. If
multiple passes are performed directly on the visible part of the
framebuffer, then a race occurs where the user may see the image in an
intermediate state of drawing. We call this 'flicker'.
Tearing or shearing
Tearing occurs when a window is being updated as the video card is
sending it out to the display. Since writing to video memory is
faster than the speed at which the image is sent to the display,
a user might see images where half is the old frame, and the other half
is the new frame.
To avoid tearing, we have to know what methods we have of controlling
how data is written to the framebuffer.
I have a Matrox G400 and a display running at 75hz. The modeline in
135.00 1280 1296 1440 1688 1024 1025 1028 1066 +hsync +vsync
I wrote a program to time how long the refresh takes, and how long is
spent in the blanking. My timings show that it takes 13.3ms for each
refresh (and one tenth of a millisecond for the gun to return to the top
of the screen). The active area on each scanline is:
1280/1688 = 76%
And the vertical active area is of size:
1024/1066 = 96%
So, this means that for 9.7ms out of every 13ms, something is being
drawn that we can see.
Judder is stuttery motion caused when each frame of the animation
fails to be displayed for the same amount of time on the display device.
This is where refresh rate information, as well as synchronization to
the display, is most important.
There have been many headline applications for linux.
- ticker is a
console-based news ticker. It outputs using slang. Drawing is handled
by the terminal application, or by the console in which it is run. This
method exhibits flicker inside xterm on my display, as slang
clear the text screen and redraws at its new position.
Newsticker is a plugin for the popular GKrellM monitoring
application that displays news.
- gtik was a stock ticker
application that used gdk_draw_rectangle to clear the region,
gdk_draw_string to render the text, and then gtk_widget_draw to update
the image. It is currently an applet shipped with GNOME2.
- Tclticker is
a simple stock ticker application which simply repaints the screen using
the standard tcl calls. On my machine, it flickers badly as it redraws.
Furthermore, there has been some work on smooth animations and judder
reduction under Linux.
The process is as follows:
- wait for the retrace
- generate our frame
- queue a request to page flip or show on the next retrace
X shared memory images (MIT-SHM)
Xv shared memory images (XVIDEO)
Refresh sync with the NVIDIA binary driver
Refresh sync with DRM ioctl