The test setup used to test a graphics card for DVI Compliance.

Since the DVI standard specifies exactly how the receiver is supposed to process the transmitted data, the performance of a DVI transmitter (graphics card) can easily be tested. Silicon Image has developed a DVI Compliance Test Kit for this exact purpose. The card is connected to a test board that possesses a CRU (clock recovery unit), which is used to simulate an optimal receiver. The data is then evaluated using a Tektronix TDS7404 oscilloscope. Tektronix' TDSDVI software automatically records the eye diagram, including the appropriate limits.

The Tektronix software allows a very detailed analysis of the eye diagram.

This photo is from a TFT displaying a perfect image.

In this case, there's too much jitter in the signal. Don't blame your TFT if you see such a signal. In most cases the reason for this bad quality is the graphics card or a too long cable.