Photo-finish

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photo-finish / line-scan mode

what is line-scan?

Line-scan is an image acquisition method that continuously samples (sensor) lines into a composite image. A Flatbed or hand held scanner is essentially also kind of a line-scanner. For some historic reasons (I guess due to the various application and realms they derived from) there a many terms: line scan, slit scan, strip photography, streak photography, scanography, photo-finish,.. Basically they all describe the same principle, a principle that is rather old and was already used in traditional analogue photography (for instance rotating panorama cameras), where you have a slit as an aperture, exposing the film as it moves by. In the digital realm one would cut out slices of existing image footage or – as in case of dedicated line-scan cameras – it is really just a one line sensor (to be correct: that’s true for monochrome cameras, color line-scan cameras normally have either 3 lines – in case of CCD sensor: one for each color – or 2, if the sensor is CMOS using Bayer pattern).

Applications of these line-scan method include industrial surface scanning, photo-finish, rotating panorama cameras, aerial imaginary, the arts (an interesting overview of art and research related slit-scan approaches can be found here) and certainly many more.

line-scan with Elphel

Since 2005 and the older 333 model, Elphel cameras have a “photo-finish” mode. With its 3Mpix Aptina CMOS sensor Elphel 333 was able so sample around 3072 lines per second. The photo-finsh mode was revived for the current Elphel 353 model with firmware release 8.0.8.7 by end of April 2010. The current 5Mpix Aptina CMOS sensor (width: 2536px) is a bit slower and allows a sample rate of approx. 2300 lines per second – or a line rate of 2.3 kHz (as it seems to be measured in dedicated line-scan cameras).

In photo-finish mode the camera then samples just lines and delivers composite images as video frames via RTSP network stream or directly to a hard-disc. Programming the camera into line-scan mode is as simple as setting PH_HEIGHT to the desired line height (in pixel):

  • set COMPRESSOR_RUN=0
  • set PH_HEIGHT=2
  • set COMPRESSOR_RUN=2
Quick link to access parameters that might be useful: http://192.168.0.9/parsedit.php?WB_EN&AUTOEXP_ON&EXPOS&COMPRESSOR_RUN&TRIG&TRIG_PERIOD&PF_HEIGHT&WOI_HEIGHT&WOI_TOP&WOI_LEFT&WOI_WIDTH
For more details see: accessing camera parameters

Due to the Bayer pattern of the sensor the minimal sample height is 2 lines.

Pf-height.png


Photo-finish mode per “default” then samples line-pairs as fast as (exposure) settings allow with a maximum rate of approx. 2300 line-pairs per second and a maximum width of the composite frame of 16384px (defined by WOI_HEIGHT). The sensor location that is used as source for these lines is defined by WOI_TOP, WOI_LEFT and WOI_WIDTH.

For variable control of the line sampling speed you can either:

  • change exposure settings.
  • slow down that process by defining a virtual size with VIRT_KEEP and VIRT_HEIGHT (which still samples as fast as it can for given virtual size and exposure settings)
  • use the camera’s internal trigger mode with TRIG=4 and TRIG_PERIOD settings to generate a regular trigger impulse. (but due to camera’s internal delays in that mode the line rate is a bit less )

(A common pitfall in trying out photo-finish mode is that one needs to take care of appropriate exposure setting before going into photo-finish mode. If you just start that mode in low light conditions you will end up waiting for frames endlessly – and get the impression the camera does not work anymore)

Examples


Free Software and Open Hardware. Elphel, Inc., 2005