[clutter] Jitter in Bspline behaviour

[Steve Smith]

Hi,

I'm playing around with the bspline behaviour and seeing something
odd.  I can define a 4-point path that causes an object to curve down
the stage.  However while it starts off smooth it gradually becomes
more jittery until at the end it is jumping about a considerable
degree.  Is this a problem with the bspline implementation or
something in the way I am using it?  I've attached a python test case
below.

Thanks,
Steve


import sys, math, colorsys
import clutter, cairo
from cluttercairo import CairoTexture

WIDTH  = 1000
HEIGHT = 800
GRIDWIDTH = 10
GRIDHEIGHT = 8
BLOCKWIDTH = WIDTH/GRIDWIDTH
BLOCKHEIGHT = HEIGHT/GRIDHEIGHT

##############################################################################
# Drawing ops

def clear(ctx):
    ctx.set_operator (cairo.OPERATOR_CLEAR)
    ctx.paint()
    ctx.set_operator(cairo.OPERATOR_OVER)

def drawlayout(tex):
    ctx = tex.cairo_create()    
    clear(ctx)

    # Axis'
    ctx.set_line_width(1)
    ctx.set_source_rgba(1, 1, 1, 1)

    for x in range(0, WIDTH, int(BLOCKWIDTH)):
        ctx.move_to(x+.5, 0)
        ctx.line_to(x+.5, HEIGHT)
        ctx.stroke()

    for y in range(0, HEIGHT, int(BLOCKHEIGHT)):
        ctx.move_to(0,     y+.5)
        ctx.line_to(WIDTH, y+.5)
        ctx.stroke()


class BlockTexture(CairoTexture):
    def __init__(self, name, colour):
        CairoTexture.__init__(self, width=BLOCKWIDTH, height=BLOCKHEIGHT)
        ctx = self.cairo_create()
        tenpc = BLOCKWIDTH * 0.1

        ctx.set_source_rgba(*colour)
        ctx.move_to(tenpc, tenpc)
        ctx.line_to(BLOCKWIDTH-tenpc, tenpc)
        ctx.line_to(BLOCKWIDTH-tenpc, BLOCKWIDTH-tenpc)
        ctx.line_to(tenpc, BLOCKWIDTH-tenpc)
        ctx.close_path()
        ctx.fill()

        ctx.set_source_rgba(1,1,1,1)
        ctx.set_font_size(16)
        x_bearing, y_bearing, width, height = ctx.text_extents(name)[:4]
        ctx.move_to(BLOCKWIDTH / 2 - width / 2, 
                    BLOCKHEIGHT / 2 + height / 2)
        ctx.show_text(name)

######################################################################

if __name__ == '__main__':
    stage = clutter.Stage()
    stage.connect('destroy', clutter.main_quit)
    stage.connect('button-press-event', clutter.main_quit)
    stage.connect('key-press-event', clutter.main_quit)
    stage.set_size(WIDTH, HEIGHT)
    stage.set_color(clutter.Color(0x0, 0x0, 0x0, 0xff))

    if False:
        background = CairoTexture(width=WIDTH, height=HEIGHT)
        drawlayout(background)
        background.set_position(0,0)
        background.show()
        stage.add(background)

    bt = BlockTexture("Test", (0,0,1))
    bt.show_all()
    bt.set_position(0,0)
    stage.add(bt)

    ## Define behaviour
    timeline = clutter.Timeline(num_frames=int(240), fps=30)
    timeline.set_loop(True)

    knots = ((0,     0),
             (WIDTH, 0),
             (WIDTH, HEIGHT-BLOCKHEIGHT),
             (0,     HEIGHT-BLOCKHEIGHT),)

    alpha = clutter.Alpha(timeline, clutter.ramp_func)
    behave = clutter.BehaviourBspline(alpha=alpha, knots=knots)
    behave.apply(bt)
    timeline.start()

    stage.show_all()
    clutter.main()