Callbacks

A "Callback" function transforms the image's pixel data just prior to display or capture.  Typically, this function is intended to process an incoming image, or to add a graphic "overlay" to the image. The Callbacks tab allow the user to select a pre-defined callback on real-time or captured images.




Pixelink Capture OEM includes five sample callback functions (four image filters and a histogram stretch) and the option of using a custom bitmap overlay graphic (see table below). Custom filters or other callback functions can be incorporated into or called by applications created using the Pixelink Camera API.

Different callback functions may be applied to the preview window, captured images, and captured video clips.

Functions:


 Callback Effect
Median Filter (3X3)
Reduces random noise, particularly "shot noise" (extreme values of individual pixels), based on median brightness level of local pixel values.
Low Pass filter (3x3)
Smoothes image and reduces random noise, based weighting of local pixel values.
High pass filter (3x3)
Sharpens image and reduces gradual variations based on weighting of local pixel values.
Sobel Filter (3x3)
Detects edges based on gradients (differences) in local pixel values.
Histogram Equalization
Improves the contrast of "muddy" images by detecting the minimum and maximum pixel values in the input range calculating a linear conversion that maps the minimum intensity value to full black and the maximum intensity value to full white creating an enhanced image by converting all of the pixels of the input image accordingly. 
Bitmap Overlay
Adds a bitmap, such as company logo or a time stamp, on the image. Enter the name and location of the bitmap in the "File Name Box", or use the "Browse" button to locate the file.


Notes:


The bitmap must be a .bmp file in 24-bit RGB format.

The overlay image is placed in the upper left corner of the frame.

Any white areas (RGB 255, 255, 255) in the overlay image are treated as transparent i.e the video will show through them.

An overlay image will not be resized if the size of the ROI is changed.

NegativeTo invert the colors of an image. This filter is useful for turning a positive image into a negative, or to make a positive from a scanned negative. 
Grayscale
The following filter converts an image to grayscale. The red, green and blue colors in each pixels are averaged. Then the resulting color is set to grey scale with the average intensity.
Saturated and black
Displaying areas of saturated pixels and pure black pixels, thereby highlighting areas that have lost detail because it is too dark or too bright.
Threshold (50%)
The filter does image binarization using specified threshold value. All pixels with intensities equal or higher than threshold value are converted to white pixels. All other pixels with intensities below threshold value are converted to black pixels. 
Temporal Noise Filter
A very simple temporal filter that attempts to remove a bit of noise from images by comparing the current image to the previous image.
Motion Detector
Given a stream of images, pixel values that are consistent from one image to the next, are shown as black.  Similarly, pixels that change from one image to the next, are shown as saturated.
ASCII
The following filter converts an image to ASCII values.


Callbacks on the Preview Window
  • To apply a Callback function to the Preview Window, select desired callback, click on drop down menu and select the function you want.
  • To apply a bitmap overlay to the Preview Window, click on "Browse" and select desired bitmap from your files.

Callbacks on Image or Video Capture


Repeat for "Image Capture" and "Video Capture" functions if required