display of video images, threads

[Enrico Costanza <ec142 york ac uk>]

Dear All,

I am trying to write a (simple) application to get images from video4linux 
(process) and display them in a window.

I have inherited from a Gtk::Window, and in this I have a button and a 
DrawingArea (in fact a class inherited from it).

In "myDrawingArea" I have overridden the expose_event_impl: so that it uses 
gdk_draw_rgb_image to draw an image buffer.

On the button click the program initializes the video4linux device, grabs an 
image, puts it the imagebuffer and then it calls queue_draw().
This works fine.

The problems start if I insert a loop so that on button click the program 
grabs and displays N images. At first I tried to simply loop grabbing and 
queue_draw(), but the application only displayed the last frame acquired. 
I do not understand why: can anyone help?

Then I used gdk_draw_rgb_image directly in the click callback function, and it 
works, BUT I have some (refresh?) problems: 
at 640x480 the part of the image below a line at 2/3 is delayed (or at least 
this is the impression it gives me)
at 320x240 I have a similar effect, but rather than having a fixed part 
delayed I have a line scanning throuh the image
at 160x120 the display is fine

The video is noticeably delayed. e.g. pointing the camera at someone talking 
the images on the computer go out of synch with reality.

How can I improve the situation? (I already use double buffering for 
acquisition, so I suspect the problems come from display)

Shall I use threads so that I start displaying while I am still capturing?
If so where can I find some documentation about this? 

I have seen threads in Java, but never in c++. Is it common to use threads 
with gui, for example to start a process and at the same time keep the gui 
responsive?

Thank you very much in advance.

Enrico Costanza