CODEWITHSUNDEEP
linuxpointersopenglmousexorg
disquisitiones
disquisitiones

Reputation: 197

XOrg server code that draws the mouse pointer

I'm writing an OpenGL application in Linux using Xlib and GLX. I would like to use the mouse pointer to draw and to drag objects in the window. But no matter what method I use to draw or move graphic objects, there is always a very noticeable lag between the actual mouse pointer position (as drawn by the X server) and the positions of the objects I draw with the pointer coordinates I get from Xlib (XQueryPointer or the X events) or reading directly from /dev/input/event*

So my question is: what code is used by the XOrg server to actually draw the mouse pointer on the screen? So I could use the same type of code to place the graphics on the screen and have the mouse pointer and the graphic objects positions always perfectly aligned.

Even a pointer to the relevant source file(s) of XOrg would be great.

Upvotes: 2

Views: 1299

Answers (1)

datenwolf
datenwolf

Reputation: 162367

So my question is: what code is used by the XOrg server to actually draw the mouse pointer on the screen?

If everything goes right no code at all is drawing the mouse pointer. So called "hardware cursor" support has been around for decades. Essentially it's what's being known as a "sprite engine" in the hardware that takes some small picture and a pair of values (x,y) where it shall appear on the screen. At every frame the graphics hardware sends to the display the cursor image is overlaid at the specific position.

The graphics system is constantly updating the position values based on the input device movements.

Of course there is also graphics hardware that does not have this kind of "sprite engine". But the trick here is, to update often, to update fast and to update late.

But no matter what method I use to draw or move graphic objects, there is always a very noticeable lag between the actual mouse pointer position (as drawn by the X server) and the positions of the objects I draw with the pointer coordinates I get from Xlib (XQueryPointer or the X events) or reading directly from /dev/input/event*

Yes, that happens if you read it at integrate it into your image at the wrong time. The key ingredient to minimizing latency is to draw as late as possible and to integrate as much input for as long as long as possible before you absolutely have to draw things to meet the V-Sync deadline. And the most important trick is not draw what's been in the past, but to draw what will be the state of affairs right at the moment the picture appears on screen. I.e. you have to predict the input for the next couple of frames drawn and use that.

The Kalman filter has become the de-facto standard method for this.

Upvotes: 5

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