1. A video coding method performed by a video coding apparatus, comprising: utilizing a visual quality evaluation circuit for evaluating visual quality based on data involved in a coding loop; and referring to at least the evaluated visual quality for performing sample adaptive offset (SAO) filtering upon a reconstructed frame generated by the coding loop; wherein the evaluated visual quality comprises visual quality information evaluated for a plurality of candidate regions in the reconstructed frame, wherein visual quality information evaluated for each of the candidate regions is decided based on at least one visual quality metric that is calculated according to: raw data of a source frame, transformed coefficients, quantized coefficients, reconstructed pixel data fed into de-blocking filtering, reconstructed pixel data output by the de-blocking filtering, reconstructed pixel data fed into the SAO filtering, reconstructed pixel data output by the SAO filtering, reconstructed pixel data stored in a frame buffer, motion-compensated pixel data, or intra-predicted pixel data, and wherein, the SAO filtering performs first visual quality comparison using the visual quality information of the candidate regions, and decides one partitioning of the reconstructed frame according to a result of the first visual quality comparison; or wherein the reconstructed frame is partitioned into a plurality of regions, the evaluated visual quality comprises visual quality information evaluated for a plurality of candidate SAO types of each of the regions, visual quality information evaluated for each of the candidate SAO types is decided based on at least one visual quality metric that is calculated according to: raw data of the source frame, transformed coefficients, quantized coefficients, reconstructed pixel data fed into the de-blocking filtering, reconstructed pixel data output by the de-blocking filtering, reconstructed pixel data fed into the SAO filtering, reconstructed pixel data output by the SAO filtering, reconstructed pixel data stored in the frame buffer, motion-compensated pixel data, or intra-predicted pixel data, and wherein, the SAO filtering performs second visual quality comparison using the visual quality information of the candidate SAO types, and decides one SAO type of said each of the regions by a candidate SAO type with best visual quality that is selected from the candidate SAO types according to a result of the second visual quality comparison, by which an improved video compression efficiency and enhanced visual quality are obtained.