History

The attributes and functions explained on this page can be found in the History category.

The Image Watch control offers the option of temporarily saving a defined number of received images. In order to use the history functions and to be able to choose and display these images, the ImageFreeze property must be activated as a prerequisite.

Property: QueueSize

The QueueSize property fixes how many of the most recently received images are to be saved and, accordingly, how large the history queue is. The image history function can be deactivated at 0. If the queue is full and another image is received, the oldest image is deleted according to the FIFO principle (First In First Out). To access the history, ImageFreeze must be activated. You can then use QueueIndex to determine which image should be displayed. The 0 entry always corresponds to the latest image that is at the top of the queue and was received last.

Property: ImageTimestamp (read only)

The timestamp of the currently selected history image can be read out in millisecond format via the ImageTimestamp property. To process this further, e.g. for display, the JavaScript Date class can be used, as the following example shows:

History 1:
new Date(/**TimestampSymbol**/).toLocaleString()
// output: "4/8/2025, 1:45:13 PM"

Function: clearHistory

The clearHistory function deletes the entire history queue.

Temporary data storage

The image data in the history queue is stored temporarily and deleted when the respective ImageWatch-Control instance is released. This happens, for example, when leaving a page or when changing content.

Performance

When configuring QueueSize, particular attention should be paid to the image size. This depends on the image resolution, the number of channels and the transmission format. The converted image size can be taken from the statistics in the server extension. With the BMP transfer format, the image data is uncompressed, which results in a larger amount of data. In connection with a larger QueueSize, this can lead to "OutOfMemory Exceptions" from the browser. We therefore recommend that you do not select QueueSize too large and that you use the compressed JPEG or PNG transfer formats.