iphone ios 视频采集AVCaptureSessionPresetHigh/Medium/Low分辨率等参数

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做视频capture的时候老是要用到这些参数，记录一下，以下均来自官网
http://developer.apple.com/library/mac/#documentation/AudioVideo/Conceptual/AVFoundationPG/Articles/04_MediaCapture.html#//apple_ref/doc/uid/TP40010188-CH5-SW3

Symbol	Resolution	Comments
AVCaptureSessionPresetHigh	High	Highest recording quality. This varies per device.
AVCaptureSessionPresetMedium	Medium	Suitable for WiFi sharing. The actual values may change.
AVCaptureSessionPresetLow	Low	Suitable for 3G sharing. The actual values may change.
AVCaptureSessionPreset640x480	640x480	VGA.
AVCaptureSessionPreset1280x720	1280x720	720p HD.
AVCaptureSessionPresetPhoto	Photo	Full photo resolution. This is not supported for video output

Preset	iPhone 3G	iPhone 3GS	iPhone 4 (Back)	iPhone 4 (Front)
High	No video Apple Lossless	640x480 3.5 mbps	1280x720 10.5 mbps	640x480 3.5 mbps
Medium	No video Apple Lossless	480x360 700 kbps	480x360 700 kbps	480x360 700 kbps
Low	No video Apple Lossless	192x144 128 kbps	192x144 128 kbps	192x144 128 kbps
640x480	No video Apple Lossless	640x480 3.5 mbps	640x480 3.5 mbps	640x480 3.5 mbps
1280x720	No video Apple Lossless	No video 64 kbps AAC	No video 64 kbps AAC	No video 64 kbps AAC
Photo	Not supported for video output	Not supported for video output	Not supported for video output	Not supported for video output

Feature	iPhone 3G	iPhone 3GS	iPhone 4 (Back)	iPhone 4 (Front)
Focus mode	NO	YES	YES	NO
Focus point of interest	NO	YES	YES	NO
Exposure mode	YES	YES	YES	YES
Exposure point of interest	NO	YES	YES	YES
White balance mode	YES	YES	YES	YES
Flash mode	NO	NO	YES	NO
Torch mode	NO	NO	YES	NO

Capturing Still Images

You use an AVCaptureStillImageOutput output if you want to capture still images with accompanying metadata. The resolution of the image depends on the preset for the session, as illustrated in this table:

Preset	iPhone 3G	iPhone 3GS	iPhone 4 (Back)	iPhone 4 (Front)
High	400x304	640x480	1280x720	640x480
Medium	400x304	480x360	480x360	480x360
Low	400x304	192x144	192x144	192x144
640x480	N/A	640x480	640x480	640x480
1280x720	N/A	N/A	1280x720	N/A
Photo	1600x1200	2048x1536	2592x1936	640x480

Pixel and Encoding Formats

Different devices support different image formats:

iPhone 3G	iPhone 3GS	iPhone 4
yuvs, 2vuy, BGRA, jpeg	420f, 420v, BGRA, jpeg	420f, 420v, BGRA, jpeg