Canon Camera, Руководство хакера

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C H A P T E R 4 Teach Your Camera New Tricks
The Edge Overlay function allocates some internal memory. When an
overlay is no longer needed, it is possible to regain the memory with the
function
Free internal memory . Normally, this is only necessary on cameras
where internal memory is scarce.
4.8 More video options
The video functions of small cameras are often overlooked by photogra-
phers. We take photographs, not video clips, right? Well, with the CHDK
activated you may change your mind.
Video sequences are quite memory-hungry. That is the reason why the
camera compresses all video frames. Doing so saves a large amount of
memory but also reduces quality. To support different requirements, the
camera offers different video modes:
640 (640x480 pixels), 640 LP, and 320
(320x240 pixels). Compared to 640, the 640 LP mode generates only half
the amount of data, so that the maximum length of a video sequence is
doubled—albeit at a lower quality. The
320 mode produces only a third of
the data, resulting in a tripled maximum length. Some cameras also offer a
Compact mode (160x120 pixels for sending videos by email) that reduces
the amount of data by around a factor of nine. Here, the clip length is re-
stricted to three minutes.
Generally, the clip length is restricted to 4 GB or one hour, whichever is
shorter. Some older cameras only allow for 1 GB, but the CHDK removes
that limit. In addition, the CHDK allows you extended and fine-grained
control over video quality and compression. You have the option (in menu
entry
Video Mode ) to specify the quality, either in terms of compression
(CBR mode, Constant Bitrate) or quality (VBR mode, Variable Bitrate) . When
the quality mode is used, the quality remains constant and the camera
automatically adapts the bitrate to the quality setting and subject matter.
A higher quality setting leads, of course, to a higher bit rate—and a subject
with finer details as well.
f
The Video Bitrate can be set to the values 0.25, 0.5, 0.75, 1, 1.25, 1.5,
1.75, 2, 2.5 and 3.
f
The Video Quality can be set to a value between 1 and 99, with 99 rep-
resenting the best quality with the least compression. The camera’s
native
640 mode is equivalent to a setting of 70–75.
So, you can in fact get a better quality than what the native video mode
provides. In particular, close-ups and out-of-focus objects will look more
natural. However, it may well be that the camera or the memory card is
unable to handle such a large amount of data. In this case, the CHDK shows
a warning sign (
! ) on the display. When the camera’s internal buffers over-
flow, the video sequence is aborted in a controlled manner.
Figure 4-42
The submenu for Video Parameters