JeVois  1.5
JeVois Smart Embedded Machine Vision Toolkit
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Advanced topic: Image pixel formats

Pixel formats supported by JeVois

  • BAYER RGGB: The sensor in the camera of JeVois provides color output, but not every pixel has 3 separate red, green, and blue photo-sensitive elements. Indeed, because the human eye perceives colors with lower visual acuity than it perceives shapes (based on luminosity), many camera sensor manufacturers provide high-resolution luminance information but lower resolution color information. In the so-called Bayer RGGB sensor type (used in the JeVois sensor), the camera sensor has one photosensitive element per pixel, and alternating color filters are placed in front of alternating pixels.

    See this excellent Wikipedia entry for more information:

  • YUYV: This is also a popular format where luminance information is available at full resolution, while color information is only provided at a lower resolution, by having two adjacent pixels share the same color information. This mode is of particular interest in JeVois because the camera sensor chip can output it natively (i.e., the sensor chip contains hardware that converts from RGGB to YUYV on the fly) and it is quite easy to use (each pixel just has 2 bytes, one for luminance and one for color information). In JeVois, we wrote simple functions that allow one to draw text, lines, circles, etc directly into YUYV images, so that the demo displays shown by many of the demo vision modules directly are created in YUYV format, eliminating the need to convert to and from more standard RGB image representation (which takes a lot of CPU time). YUYV is also sometimes called YUV422 interleaved.

  • GREY: This is a greyscale format, which has no color information and uses one byte per pixel.

  • RGB565 / RGBP: This is a format where three R, G, and B values for each pixel are packed into only 2 bytes, by using a smaller number of bits for each value (5 bits for red, 6 bits for green, 5 bits for blue, total 16 bits). The JeVois hardware sensor can also natively capture in RGBP format, and the JeVois GPU can output (render) results of its processing into RGBP format, thus RGBP is also a format of interest.

    Unfortunately, guvcview does not seem to properly work in RGBP mode. It just fails to allocate the image buffer and crashes. This is maybe because JeVois is the first USB camera to actually be capable of outputting RGBP and that code has not been thoroughly tested in guvcview. We will work with the guvcview team on fixing this. In the meantime, ffplay or ffmpeg are alternative video capture software that work well with RGBP, although they do not provide a graphical interface as nice and complete as guvcview.

  • BGR24: This is a mode where 3 bytes are used for each pixel, to encode red, green, and blue. This is computed on the JeVois CPU through interpolation from either native RGGB, RGBP, or YUYV. Thus it is not often used as it requires significant CPU time to create. Note that JeVois uses BGR24 (as opposed to RGB24) mostly because BGR24 is the native color format used by OpenCV.

  • MJPG: This is a compressed mode popular on many USB cameras. Each video frame is basically compressed as a JPEG image. The compression is also done by the JeVois CPU and hence uses some computing resources. For this reason, it is also not as widely used in the bundled demos.

Note that guvcview shows some additional pixels format, which actually are not declared by the JeVois camera as being supported by the JeVois hardware. It seems that guvcview provides those by converting from the natively supported ones which are listed above. It varies by host computer, guvcview version, etc. On our hosts, we see additional pixel formats under "Camera Output" in guvcview for RGB3, YU12, YV12. And when we try to select them, sometimes guvcview crashes. Just do not use them, stick to the native modes of GREY, RGGB, MJPG, RGBP, and YUYV.

Video mapping concept

A vision processing mode in JeVois is determined by the combination of pixel format, image resolution, and framerate. You can play with all these parameters in your camera software to trigger different modes. For example, in the default JeVois disk image, 320x240@60fps in MJPEG format uses the PassThrough vision module (no processing, just output the raw captured images after compressing them to MJPEG) while 320x240@60fps in YUYV format uses the SaveVideo module (which can capture and save video to the microSD card).