pciGrabber-4x4
114
PHYTEC Messtechnik GmbH 2008 L-720e_0
Examples:
field-based digitizing
A quadratic image of the size
256 x 256 pixels should be digitized with a resolution and
proportion corresponding to the TV picture.
(a)
Resolution and Scaling
For digitization a field (288 lines) is sufficient. In order to obtain a
correct width/height ratio, the resolution in X-direction has to be
reduced to the half (since field = half height).
Therefor 720:2 = 360.
The result is :
ppl
=360,
lines
=288
(b)
Size of the section
The image should have a quadratic size of 256 x 256 pixels.
The result is :
hsize
=256,
vsize
=256
(c)
Positioning
It is advisable to center the section of the image.
In x-direction only 256 pixels of 360 pixels are displayed in the
window. At the edge 360 – 256 = 104 pixels remain, which are
divided in equal parts to both sides, which result in 52 pixels on
both sides.
hpos
is the size of the left edge, so
hpos
=52.
Correspondingly in Y-direction: (288-256):2=16;
vpos
=16.
Note:
It would be wrong to set
ppl
=256 and
lines
= 256. In this case the
width/height ratio (TV-Norm 4:3) would be changed to 1: 1 and
the image would be distorted. It would be possible to set
lines
= 256 and to compute
ppl
by the width/height ratio. In this
case we would gain the optimal height of the image.
