In the context of global communication, the construction of national images in the media has evolved from passive reporting to active meaning modulation. Using China as a case study, this research introduces the Semiotic Intensity Approach (SIA) to quantify how news media integrate verbal, visual, and layout resources to either amplify or attenuate specific narratives. By conceptualizing media discourse as a communicative event, this study conducts a systematic scoping review of a corpus of 56 core studies to construct a unified taxonomy of bias. The synthesis identifies 25 distinct cross-modal mechanisms (14 amplifying and 11 attenuating) that systematically deviate from a standard journalistic factual register. The analysis reveals a quantitative chasm in existing scholarship: while high-arousal amplification strategies dominate academic attention (70.2%), the equally strategic quiet mechanisms of attenuation (29.8%), such as spatial relegation or intersemiotic decoupling, remain under-theorized. The framework demonstrates how intersemiotic synergy acts as a multiplier effect to lock in preferred readings, whereas modal dissonance is frequently deployed to structurally suppress cooperative or positive narratives. The SIA reconceptualizes abstract framing concepts into operationalized metrics, serving as a diagnostic tool for researchers and educators to decode the anatomy of bias. This study establishes a foundation for reproducible, cross-cultural media analysis and highlights the theoretical necessity of moving beyond bias diagnosis toward the active restoration of narrative complexity.