1. A pixel processing system comprising: a register-collecting mechanism, modifying a first program which requires a plurality of first registers to a second program which requires a portion of the first registers of the first program for collecting the rest of the registers originally allocated to the first program; and a pixel shader coupled to the register-collecting mechanism, fetching the second program for executing the second program and reallocating the collected rest of the first registers of the first program to be available to other program, wherein the register-collecting mechanism collects the rest of the first registers in advance before the pixel shader executes the second program having the portion of the first registers therein.