PNX1302EH NXP Semiconductors, PNX1302EH Datasheet - Page 217

• Manufacturer Part Number: PNX1302EH
• Manufacturer: NXP Semiconductors

Specifications of PNX1302EH

• Lead Free Status / RoHS Status: Not Compliant

Philips Semiconductors
3. Multi-tap filtering, where the new in-between pixels
The more general case is where the output image reso-
lution is not an integral multiple or sub-multiple of the in-
put image resolution, such as converting from 640 x 480
to 1024 x 768. In this case, the output pixels have differ-
ing positions relative to the input pixels in the horizontal
or vertical dimensions. In converting from 640 to 1024,
the first output pixel on a line corresponds to the first in-
put pixel. The second output pixel is at 640/1024 of the
distance between the first and second input pixels. The
third output pixel is at (2*640)/1024 of the distance =
1280/1024 = 1+ 256/1024 = 256/1024 of the distance be-
tween the second and third input pixels, etc. The output
pixels shift with respect to the input pixel grid as you
move along the line in the horizontal or vertical dimen-
sions. This is shown in
New pixels are generated by interpolation or filtering of
the original pixels. Interpolation is the weighted average
of the input pixels adjacent to the output pixel. Filtering
extends interpolation to include input pixels beyond the
input pair adjacent to the output pixel. The number of pix-
els used to generate the output defines the filter type. In-
terpolation is a 2-tap filter. A 4-tap filter would use the two
pixels to the left and the two pixels to the right of the out-
put pixel. A 5-tap filter identifies the single pixel nearest
the output as the center pixel, and uses this pixel plus
two to the left and two to the right to generate the output.
If the ratio of the output pixel count per line (in H or V) to
input pixel count per line is the ratio of small integers,
there is a repeating pattern in these relative positions of
input to output pixel locations. For example, for 640 to
1024, the ratio is 8/5. The pattern repeats for every 8 out-
put and every 5 input pixels. If the ratio is not a ratio of
small integers, the pattern will take a long time to repeat.
The worst case would be 640 to 641, for example. There
would be no exact repetition for the whole line.
The interpolator or filter coefficients must be weighted
according to the relative position of the new pixel relative
to the old pixels. The weighting factor is between 0.0 and
1.0, corresponding to the relative position of the new pix-
el with respect to the old pixel grid. With a repeating pat-
tern, fewer weighting factors are needed, and therefore
fewer coefficients in the linear interpolator or filter gener-
ating the new pixels, since you can reuse them each time
the pattern repeats. A filter with a repeating pattern is
Figure 14-8. 640 to 1024 upscaling example
Output Pixels
Input Pixels
are multi-pixel filtered version of the adjacent input
pixels. This approach results in the best image.
1
1
Figure
2
14-8.
2
3
4
3
called polyphase, indicating a repeating pattern in the
phase (offset position) of the output pixels relative to the
input pixels.
Generating the output pixels: relating the output grid to the
input grid
Scaling is a pixel transformation in which an array of out-
put pixels is generated from an array of input pixels. The
value of each pixel on the output pixel grid is calculated
from the values of its adjacent pixels on the input grid. To
find these adjacent pixels, you overlay the output grid on
the input grid and align the starting pixels, X0Y0, of the
two grids. To identify the adjacent input pixels for a given
output pixel, you divide the output pixel X (pixel number
along the output line) and Y (pixel line number within win-
dow) by their corresponding scaling factors:
Note that the resulting X
numbers because the output pixels will usually fall be-
tween the input pixels. The fractional portion indicates
the fractional distance to the next pixel. To calculate the
output pixel value, you use the value for the nearest pixel
to the left and above and combine it with the value of the
other adjacent pixel(s). For example, horizontal interpo-
lation uses the starting pixel to the left interpolated with
the next pixel to the right, with the fractional value used
to determine the weighting for the interpolation.
ICP scaling output resolution
In the ICP, scaling is forced to have a repeating pattern
by limiting the resolution of the new pixel position to 1/32;
the new position is forced to be at a location n/32 in H
and V relative to the position of the original pixel grid.
This results in a worst case error of approximately 1.5%
in amplitude relative to calculations using exact output
pixel positions. This is comparable to the errors caused
by quantizing the amplitude of the pixels. The additional
quantization noise can be avoided by choosing an appro-
priate scale factor which, when inverted, results in frac-
tional values which are expressed in 32
8/5 scaling factor in the 640 to 1024 example above. A
diagram of the input to output pixel relationship and the
PRELIMINARY SPECIFICATION
5
X
Y
in
in
= X
= Y
where: horizontal scaling factor =
output length / input length
where: vertical scaling factor =
output height / input height
4
out
out
6
/ (horizontal scaling factor)
/ (vertical scaling factor)
7
in
and Y
5
Image Coprocessor
in
8
values will be real
nd
s, such as the
1
1
14-7