Audio/Video Capture Devices: The Second Component of Quality in .NET

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Figure 53 Interlaced video and fields
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Credit: Osprey Video Division for the ViewCast Corporation
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Typically, the notion of a video frame implies two fields of video Thus a full-resolution video frame is 720 480 pixels However, most computer monitors are progressive and not interlaced display devices Progressive displays do not split video into fields and draw them one after the other On a progressive display, when video is viewed at its full resolution, viewers see both the odd and even fields at the same time (Figure 54) There are various issues with combining temporally different fields into a single frame, and these will be discussed later in this chapter However, it is sufficient to say at this point that if a user desires a video image less than or equal to the field height, most capture cards can return only video from one of the two fields that make up a frame Thus a request for a video capture size of 240 lines or less typically results in the capture of a single field
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Lines from the Old Field Lines from the Even Field
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Figure 54 Combining both odd and even fields into a single frame in a progressive display
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A final point to cover here is that some video sources are not interlaced These are called progressive sources A frame of a progressive source is composed of a full-resolution image in which each pixel is sampled at the same instant in time Various cameras have progressive capture modes Film is also progressive in nature The 24 frames per second of film is composed of 24 progressive images
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The width-height ratio for the physical pixels on a TV does not have a 1:1 relationship TVs are said to have a CCIR-601 aspect ratio Computer monitors do have a 1:1 width-height ratio for pixels and are said to have a square aspect ratio Do not confuse this with the concept of the display resolution The latter is the ratio of the number of horizontal pixels compared with the number of vertical pixels This resolution is commonly 4:3 (640 480, for example) The reason that this concept is important to capture devices is that these devices often capture CCIR-601 video signals to be further processed and eventually displayed on a square aspect ratio display, such as a computer monitor The following example illustrates these issues: An image of circles is drawn on a piece of paper A video camera is then pointed at this paper, and a video capture device is used to capture the image and display the result on a computer monitor If the image is captured using a CCIR-601 aspect ratio by the capture device and then displayed on the square aspect ratio computer monitor, the image will appear stretched The correct procedure is to rescale the image if the video source provided a CCIR-601 digital signal In the case of analog video signals, sample the analog video using a square aspect ratio sampling procedure This results in a proper capture and display of the image
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As stated previously, a capture card is further defined by the video formats (video resolution and video color) it is able to provide to the host
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Audio/Video Capture Devices: The Second Component of Quality
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for further processing Pixels of uncompressed, or raw, video formats usually are based on a YUV color space, as discussed earlier in the digital video basics section If a capture can provide RGB (the basic color palette of video, composed of the colors red, green, and blue) video formats, then it is said to have the feature called color-space conversion Typical video formats are given in Table 53 Note that Four-Character Codes (FOURCC) are a set of codes that are four characters in length and were introduced by Microsoft to clearly identify video data stream formats
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NOTE are many variants of FOURCC codes for a given format, and Table 53 includes There
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only one variant Refer to wwwmicrosoftcom/hwdev/devdes/fourcchtml for a complete list
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Table 53
Capture Color Formats/FOURCC Codes FOURCC IYU2 YUY2 I420 Y41P YVU9 Y800 RGBA BI_RGB BI_RGB BI_RGB BI_RGB COMMENTS 4:4:4 sampling, 24 bits/pixel, packed format (U0Y0V0U1Y1V1) 4:2:2 sampling 16 bits/pixel, packed format (Y0UY1V) 4:2:0 sampling, 12 bits/pixel, planar format 4:1:1 sampling, 12 bits/pixel, planar format 9 bits/pixel, planar, subsample of CbCr by 4 vertically and horizontally 8-bit Y only 4:4:4 sampling, 32 bits/pixel, packed, RGB plus alpha channel 4:4:4 sampling, 24 bits/pixel, packed, RGB 4:4:4 sampling, 16 bits/pixel, 5 bits for R and B and 6 bits for G 4:4:4 sampling, 16 bits/pixel, 5 bits for R, G, and B, 1 unused bit 8 bits/pixel yielding 256 colors