VS6724 Datasheet, PDF (19/118 Pages) STMicroelectronics – 2 Megapixel single-chip camera module

VS6724
Functional description
2.6.3
The FIFO shown in the figure above is being constantly monitored with fixed break points, if
the FIFO fill state passes one of these break points the squeeze value is increased or
decreased.
The amount of data in the FIFO is dependant on a number of factors
● The squeeze factor set
● The complexity of the scene
● The speed at which the FIFO is being emptied (bOIFClkRatio)
JPEG squeeze controls
The strength of compression or the level of squeeze operated on the image (and therefore
the quality of the final image) can be controlled in two ways via the bJpegSqueezeSettings
register (note each context can be set independently);
bJpegSqueezeSettings0
0x00 SET_USER_SQUEEZE_MODE
0x01 SET_AUTO_SQUEEZE_MODE
In auto squeeze mode, the JPEG compression engine is trying to achieve a target file size
and constantly changes the compression ratio. This is the preferred mode when using the
output JPEG to create a video, or in preview mode.
wJpegTargetFileSize {0x03c3, 0x03c4}: Input required size in KBytes
In user squeeze mode, the JPEG compression engine will use a constant level of squeeze.
This is recommended when trying to capture a high quality image and for snapshot or
flashgun modes. For ease of use, the level of squeeze can be adjusted for each pipe context
between a high, medium or low level. The value of these squeeze settings can be
configured by using the following registers:
bJpegImageQuality0
0x00 High quality. Value set in bHiSqueezeValue {0x2508}
0x01 Medium quality. Value set in MedSqueezeValue {0x250a}
0x02 Low quality. Value set in bLowSqueezeValue {0x250c}
It is also possible to select between the YCbCr formats input to the video compression
engine:
bJpegImageFormat
0x00 YCbCr 422
0x01 YCbCr 420
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