A quiz on image representation and processing in software

Adrian F. Clark

1. How are the colours of an image normally represented in OpenCV?

   • blue, green, red

   • red, green, blue
   • blue, red, green
   • hue, saturation, value

   For reasons known only to its authors, colour images in OpenCV ignore the red, green, blue convention used everywhere else and normally represent images as blue, green, red. Crazy!

2. In OpenCV, if Y indexes lines, X columns and C channels, the right nesting >of loops is

   • X inside Y inside C
   • Y inside X inside C

   • C inside X inside Y

   • C inside Y inside X

   The C subscript should cycle fastest in all languages you will use OpenCV with, followed by the X subscript. The Y subscript should cycle slowest. This order mimics the way in which the pixels are stored in memory and thus minimises cache misses etc.

3. What is plotted along the abscissa (x-axis) of a histogram?

   • grey level

   • number of occurrences
   • mean value
   • standard deviation

   The x-axis of a histogram plots the grey level; the y-axis plots the number of occurrences of that grey level.

4. Why does EVE use float values for pixels?

   • it reduces the number of places that the effects of rounding have to be considered

   • that's the way images are stored in disk files
   • it supports a wider range of values
   • it should execute faster

   The use of float values in EVE is largely to reduce the effects of rounding: there are fewer places in a typical piece of code where the effects of rounding need to be considered. There are other advantages too, such as representing a widrer range of values but the rounding one is the most important.

5. The correlation coefficient between two images tells us

   • the amount of noise in the images
   • whether or not two images are identical
   • whether there are gain or offset changes between them

   • how similar they are, ignoring gain or offset changes

   Correlation is a measure which is independent of the means of signals and is normalized by the variances of the two signals, so it is also independent of their relative scales -- it measures only whether the variations in two images relative to their means are in step. The correlation coefficient lies in the range -1 -- +1 and the value describes the similarity of the two images.

6. Why are digital images usually rectangularly-sampled?

   • because of the way images are formed on photographic film
   • because that is a better match for the human eye's rods and cones
   • because it is impossible to make hexagonally-spaced CCDs

   • because they are easier to process with a computer

   Digital images are most easily processed by computer if they are rectangularly-sampled, as ordinary 2D arrays can be used. The rods and cones in the human eye are tightly-packed, which makes them approximately hexagonally-sampled. Hexagonally-sampled CCDs are entirely feasible and are used, for example, on some Fujifilm digital cameras. Finally, photographic film forms images in a purely analogue way, so there is no sampling involved at all.

7. How does histogram equalization work?

   • histogram bins with low occupancy are doubled

   • histogram bins with low occupancy are spread out

   • histogram bins with low occupancy are merged
   • histogram bins with low occupancy are deleted

   Histogram equalization spreads out low-occupancy parts of a histogram. It is inhereently non-linear in nature and generally a very silly thing to do for computer vision. Your lecture can count on the fingers of one foot the number of times it has helped him develop vision algorithms.

8. Why is imagery with 8 bits per pixel commonly used in vision software?

   • 8-bit values fit into a byte
   • computers don't have enough memory to use more than 8 bits/pixel

   • the human eye sees less than 8 bits worh of grey levels

   • 8-bit values are faster to process than integers

   The underlying reason is that the eye sees less than 256 shades of grey, though they are not evenly spaced in intensity. It is a convenient coincidence that this matches one byte per pixel. Depending on the hardware involved, 8-bit values are processed at the same speed as integers. Unless it is extremely large (and some of those your lecturer processes are), images are small compared to the size of memory these days.

9. Which of the following popular image processing packages provides a menu-based interface?

   • OpenCV

   • ImageJ

   • EVE
   • Matlab

   OpenCV and EVE are libraries to be used by a programmer, while Matlab has a GUI into which one typically types `commands'. However, ImageJ provides menus from which the image processing and analysis operations are chosen.

10. Why is the order of subscripts into an image important?

    • it affects the amount of memory used
    • it makes the code easier to read

    • it affects the run-time of an algorithm

    • it makes mapping code onto a GPU easier

    When the subscripts are indexed in the right order, adjacent pixels in the image are adjacent in memory. This reduces the number of cache hits, allows auto-increment instructions to be used, and so on.