In QTP, to develop a custom comparer, below are some importants to keep in mind:

1. To develop a custom comparer you must understand image processing and know how to develop COM objects.

2. You can implement a custom comparer using any language and development environment that supports creating COM objects.

3. Custom comparers run within the QuickTest context. You must therefore exercise care when developing your custom comparer, as its behavior and performance will affect the behavior and performance of QuickTest.

4. The custom comparer must be installed and registered on any computer that runs a test with a bitmap checkpoint using the custom comparer.

5. Before installing and registering a new version of a custom comparer, unregister the existing comparer.

6. More than one custom comparer can be installed and registered on the same QuickTest computer. In the Bitmap Checkpoint Properties dialog box, QuickTest displays all of the available custom comparers, and the QuickTest default comparer. The QuickTest user can select the appropriate comparer to use for each bitmap checkpoint.

7.The computer that runs the custom comparer must have installed the runtime environment associated with the configuration in which the custom comparer DLL was built.

8. You create the custom comparer DLL using a specific development environment version; the computer on which this DLL runs must have the corresponding runtime environment installed.