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From: kens@JARGON.WHOI.EDU (Ken Stewart)
Newsgroups: comp.windows.x
Subject: strip-chart display
Message-ID: <9012281317.AA10469@jargon.whoi.edu>
Date: 28 Dec 90 13:17:33 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 72

Howdy Folks --

	I'm writing a simple strip-chart application to display various
real-time data streams under X. I'm developing under XView 2.0 but using
Xlib calls for display functions. I've tried two approaches to "scroll" the
display: 1) copy an area of the drawable then draw one new line to connect
the new data point; 2) xor the old polyline to clear then xor the updated
polyline.

	The bitblt approach gives the smoothest scrolling. The xor polyline
approach runs about 4 times faster on a SPARC-GX, but flickers annoyingly;
bracketing the drawing calls with server grab/ungrab doesn't help. Relative
speed would depend on the drawable area, but we need reasonably large
displayed areas for good viewable resolution (say, 1024x256).

	I've included skeleton examples below. My questions are: 1) is there
a more efficient approach? 2) does anyone have a similar working application
or can point me to a PD source? 3) are there any comments on efficiency/
portability for other X environments? (our applications run mostly on Suns, 
but must support remote displays on PC's, Mac's, and other workstations).
Thanks for any help.

Ken Stewart

Deep Submergence Laboratory              voice: (508) 457-2000 x2644
Woods Hole Oceanographic Institution       fax: (508) 457-2191
WHOI, Blake 109                       internet: kens@jargon.whoi.edu
Woods Hole, MA  02543                    omnet: k.stewart

**************************************************************************

1) BITBLT
    .
    .
    /* scroll the display right */
    XCopyArea( dpy, xwin, xwin, gc, 0, 0, 
                canvas_width - horiz_incr, canvas_height, horiz_incr, 0 );

    /* clear a small region at left */
    XClearArea( dpy, xwin, 0, 0, horiz_incr, canvas_height, False );

    new_y = ...

    /* connect the new data point */
    XDrawLine( dpy, xwin, gc, horiz_incr, last_y, 0, new_y );

    last_y = new_y;
    .
    .

2) XOR POLYLINE
    .
    .
    XGrabServer( dpy );

    /* clear last displayed polyline */
    XDrawLines( dpy, xwin, xorgc, points, num_points, CoordModeOrigin );

    /* code omitted here to shift y values in array of points */
    .
    . 
    points[0].y = ...

    XDrawLines( dpy, xwin, xorgc, points, num_points, CoordModeOrigin );

    XFlush( dpy );

    XUngrabServer( dpy );
    .
    .

**************************************************************************