Hello! My task is creation of real-time system which can draw simple graphics figure on monitor (VGA) when the system register input pulse. The main difficulty is to draw this image only once (for measuring man's reaction time: time between image drawing and the button pressing). I think it'll possible to use FPGA to register input pulse and to generate VGA signal with required parameters. But I don't undestand can the ONLY ONE image frame be drawn at input pulse interrupting or not? 

The main task is to minimize time delays and in this case I can't use the standart solutions. 

For example, using Raspberry Pi or a similar device is accompanied by the necessary of screen refreshing and image drawing binding. This adds a random time delay from 0ms (if the screen refresh time coincides with time of input external pulse which starts drawing figures) to 1/60Hz=16.7ms. 

I need your advices or suggestions. Thanks!

Hi it is possible, you might want to check out http://adiuvoengineering.com/?p=23 for how to create the vga pattern. The rest of it is just simple logic.

You will always have to re generate the entire VGA frame however