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Rate control plays a crucial role for video communication applications. It ensures that the generated compressed bit streams satisfy bandwidth and buffer constraints. Rate control algorithms recommended by H.264/AVC adopt rate-distortion (R-D) models for inter-frames to determine quantization parameters (QPs) but not for intra-frames. Instead, they directly compute QPs without any considerations of bitrates and coding complexities for intra-frames. In order to obtain more accurate target bit prediction for intra-frames, we first introduce the geometry gradient information as a new complexity measure to accurately represent the complexities for intra-frames. Then, we propose a novel R-D model which is an integration of a linear rate-complexity model and an exponential rate-quantization model. Finally, we develop an accurate and robust intra-only rate control algorithm for H.264/AVC. Experimental results demonstrate that, compared with JVT-W042, the proposed algorithm achieves higher precise bit estimation, provides more robust buffer control, and also improves coding quality.