The Particle Location Paramters submenu controls how the two dimensional particle tracking subsystem within DigImage will locate particles within a frame. In addition, it sets down the criteria which will be used to determine whether a "blob" on the screen represents a single or multiple particles.
The basic principle is that each image is scanned for pixels satisfying a given intensity threshold. Once such a pixel is found, DigImage determines the size of the "blob", of which this pixel is a member, satisfying the threshold, along with a number of statistics describing the blob. Of particular interest are the location of the blob centroid, the size of the blob, the ellipticity of the blob, and the average intensity.
This entry sets a limit on the intensity of the blob in addition to the threshold criterion. If the difference between the average intensity of the blob and the threshold is less than the value specified by this entry, the blob will not be considered as a particle. In effect this requires the blob to satisfy the threshold "sufficiently well" in order to be considered as a particle.
This option provides access to a menu setting parameters which control the manner in which the local background intensity is calcualted either for background pricing or background intensity recording. If neither of these is enabled, or we are tracking more than 511 particles, then the settings in this menu will have no effect.
The particle positions are ultimately described in terms of the location of their centroids. This centroid may be calculated either as the centroid of the area the particle appears to occupy (ie. the area above or below a specified threshold) - type A - or the projection (onto the physical plane) of the centroid of the 'volume' occupied by the intensity profile (effectively the area centroid weighted by the intensity) - type V. This option enables either the Area or Volume centroid to be used to describe the particles location.
The apparent ellipticity of a particle is related to the correlation coefficient for that particle. To trap very elliptic particles, we may place a limit on the correlation coefficient. If the absolute value of the correlation coefficient exceeds this value, then the blob may either be considered carefully with the possibility that it may represent two particles very close together, or ignored. The way in which the nominal second particle of the pair is treated is determined in the [qP Pricing policy] menu.
This option specifies the maximum mismatch (in pixel coordinates) between the location of the area and volume centroids (see [C Area/Volume centroid]). If this mismatch is exceeded, then a warning message is generated and the corresponding 'particle' is ignored.
Particles are identified initially by their intensity. This field indicates if a particle has an intensity Greater than (G) or Less than (L) the background. If the particles are less than the background intensity, then the image is inverted on acquisition so that all processing is undertaken on images with light particles and a dark background.
This field specifies a lower size limit for a 'particle' (in pixels area). Particles smaller than this size may either be treated with caution, or ignored. Note that the particle must also satisfy the horizontal and vertical extents specified by the 'X' and 'Y' options. If it does not, then it will be ignored, even if it is sufficiently large according to this field.
This field specifies the upper size limit for a 'particle' (in pixels area). If a particle exceeds this area, then it may be treated as two particles, the second incurring a price penalty specified in the [qP Pricing policy] submenu, or ignored.
Two intensity thresholds must be defined for locating particles. This option allows the lower threshold to be defined. The threshold is defined relative to the background image as specified in [qB Background removal]. Note that the processing is always performed on light particles on a dark background, after the background image has been subtracted. The upper threshold may be defined by [U Upper threshold]. A particle which satisfies the upper threshold and the average intenstiy excess (see [A Minimum average intensity excess]), then it will be treated as a "normal" particle. On the other hand, if a particle satisfies only this lower threshold and the average intensity The reason for this penalty (specified in the [qP Pricing policy] not just noise (for example).
This option specifies the upper (more stringent) threshold for particles. A particle satisfying this threshold and the average intensity excess (see [A Minimum average intensity excess]) will be considered without any intensity penalty. See [T Lower threshold] for more details.
The threshold interval specified by [T Lower threshold] and [U Upper threshold] is broken into the number of steps specified by this field. Particles with an intensity above the midpoint of this field are treated as "normal" intensity, while those below are considered "faint". Note that the tracking process will not use threshold steps smaller than four intensity units, even if such steps are implied by the value for this field.
This option specifies how the thresholds defining the particles are to be determined. The thresholds may either be set in advance by the user, or determined automatically for each image processed based on the intensity histogram of that image. In most situations manually choosing the thresholds in advance will produce better results, however for inexperienced users or for flows where the overall brightness of the images change with time, then automatic thresholding may be preferable.
This option sets a limit on the horizontal size of a particle as defined by the left-most and right-most pixels. If a "blob" has a horizontal size of less than the amount specified by this field, it will be ignored. The justification for this is that if a particle is too small, then its position is not adequately resolved (eg. if the horizontal size is only one pixel, then subpixel resolution is not possible).
As with the [X Minimum horizontal size] option, this option works in conjunction with the [R Lower size limit] option. Particles which have a vertical extent (ie. between the top-most and bottom-most pixels) of less than the amount specified by this field will be ignored. The justification for this is that if a particle is too small, then its position is not adequately resolved. Typically a particle must extend at least two pixels vertically for adequate subpixel horizontal resolution if no interlace filters are employed, or four pixels vertically if an interlace filter is in use. Particles which have a vertical extent of less than this amount are limited to pixel resolution.
This option will count the number of particles falling within the tracking window in a buffer to which an image has been acquired previously. This offers a convenient way of checking the location parameters and/or counting particles. Note that the background is NOT subtracted from the image, nor are any filters applied (except that if single field sample spacing has been specified then the odd lines will be selected).
This option utilises the intensity histogram and particle statistics of the tracking window to suggest an appropriate upper and lower threshold for the particle tracking.