Still Image Display
10-15
Next, the host must set the Override B Luma and Chroma Frame Store
Start Addresses. These are the DMA SDRAM Target addresses the host
used for the two stores or the Luma and Chroma Base Addresses for a
decoded picture.
In addition to the address pointers, the host must also program the width
of the image using the Override Picture Width eld. The picture width
register has a resolution of 8 pixels, hence the frame store image width
must be in 8-pixel increments. This picture width register is used by the
Display Controller for accessing subsequent lines of the frame store.
To display a still picture it stored, the host adjusts the main display area
if necessary, sets or clears the Host Top Field First bit as desired, enters
the Decode Stop Command, and sets the Display Override Mode. The
Video Decoder may be left running during the still display.
Display override has two modes, eld and frame. Field Mode is provided
for eld structure pictures where motion between the elds may cause
distortion of the image. In Field Mode, the rst eld is controlled by the
Host Top Field First bit and is output during both eld times. In Frame
Mode, both elds are output to the display.
When enabled, the still image is processed through the horizontal and
vertical lters of the Video Decoder. The override picture width and the
main reads per line are separate registers and allow the exibility of
displaying a portion of the still image frame store (necessary for pan and
scan scaling of a still frame).
When the Display Controller is programmed for still image display, the
data is simply read from the Override Display Frame Store instead of the
frame store indicated by the video decode engine. The parameters for
the Override Display Frame Store are sampled internally at every eld
boundary, allowing the host to change the values in the middle of the
eld.
It is possible to display a still image and continue to decode video in the
background. As long as the decoder is started and freeze is not active,
the Display Controller continues to issue decode signals to the decoder.
When a freeze is issued, the Display Controller temporarily suspends
decoding while the freeze is active. This property can be exploited for
various trick modes that require random access.