RIVA128 datasheet(37/77 Pages) STMICROELECTRONICS

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128-BIT 3D MULTIMEDIA ACCELERATOR

RIVA 128

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7.1

VIDEO SCALER PIPELINE

The RIVA 128 video scaler pipeline performs

stretching of video images in any arbitrary factor in

both horizontal and vertical directions. The video

scaler pipeline consists of the following stages:

1 Vertical stretching

2Filtering

3 Color space conversion

4 Horizontal stretching

Vertical stretching

Vertical stretching is performed on pixels prior to

color conversion. The video scaler linearly interpo-

lates the pixels in the vertical direction using an in-

ternal buffer which stores the previous line of pixel

information.

Filtering

After vertical interpolation, the pixels are horizon-

tally filtered using an edge-enhancement or a

smoothing filter. The edge-enhancement filter en-

hances picture transition information to prevent

loss of image clarity following the smoothing filter-

ing stage. The smoothing filter is a low-pass filter

that reduces the noise in the source image.

Color space conversion

The video overlay pipeline logic converts images

from YUV 4:2:2 format to 24-bit RGB true-color.

The default color conversion coefficients convert

from YCrCb to gamma corrected RGB.

Saturation controls make sure that the conversion

does not exceed the output range. Four control

flags in the color converter provides 16 sets of col-

or conversion coefficients to allow adjustment of

the hue and saturation. The brightness of each

R G B component can also be individually adjust-

ed, similar to the brightness controls of the moni-

tor.

Horizontal stretching

Horizontal stretching is done in 24-bit RGB space

after color conversion. Each component is linearly

interpolated using a triangle 2-tap filter.

Windowing and panning

Video images are clipped to a rectangular window

by a pair of registers specifying the position and

width.

By programming the video start address and the

video pitch, the video overlay logic also supports a

panning window that can zoom into a portion of the

source image.

Video composition

With the color keying feature enabled, a program-

mable key in the graphics pixel stream allows se-

lection of either the video or the graphics output on

a pixel by pixel basis. Color keying allows any ar-

bitrary portions of the video to overlay the graph-

ics.

With color keying disabled and video overlay

turned on, the video output overlays the graphics

in the video window.

Interlaced video

The video overlay can display both non-interlaced

and interlaced video.

Traditional video overlay hardware typically drops

every other field of an interlaced video stream,

resulting in a low frame rate. Some solutions have

attempted to overcome the this problem by de-

interlacing the fields into a single frame. This

however introduces motion artifacts. Fast moving

objects appearing in different positions in different

fields,

when

deinterlaced,

introduces

visible

artifacts which look like hair-like lines projecting

out of the object.

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