Figure 1.

Up: image captures by EyeSys topographer for a spherical surface of 7.1 mm radius after software modification. It presents the reflection image of the placido rings on the ball. Once the videokeratograph was captured, we used an option in the topographer software to edit and modify two adjacent rings in the image. Down: explanation of the manipulation process of the computer algorithm in the EyeSys topographer. The schematic box shows the modified image due to software manipulation simulating ring tracing errors. A section of each of the two rings was deleted and a line was drawn connecting a free end of one of the rings with the other opposite free adjacent ring.