While the macroblock edges in the 54Mbps frames are fairly well blended, more seamless results could potentially be produced by hacking the encoder's Deblocking Filter coefficients. However, the I and P-frames of both 24p and 30p videos are also contaminated with noticeable macroblock artifacts.įrame grabs from the 54Mbps 24p and 30p videos show significantly fewer macroblock artifacts and finer image details than the 24Mbps videos. This can be seen in the noticeably coarser macroblock artifacts in the 24p B-frame image recorded at the stock 24Mbps bitrate. They will typically encode highly compressed B-frames at lower image quality than the I and P-frames. This can produce not only smeared image details, but undesireable macroblock artifacts as well, which can be seen in the sporadic rectangular edges in detailed areas of the decoded frame grabs.Ĭonsumer-grade H.264 encoders are optimized to maximize data compression rather than image quality. With extremely detailed moving images, however, the encoder's image quality may deteriorate when it reaches the limits of its maximum bitrate. After the H.264 decoder reconstructs the encoded pixel data, it uses a built-in Deblocking Filter to blend the rectangular edges of the adjacent macroblocks together. Sophisticated compression techniques are used to compress the encoded data within each macroblock, reusing image data from adjacent macroblocks and video frames. H.264 encoders divide each video frame into horizontal and vertical rows of tiles called macroblocks. P-frame Comparison 1080p30 (IP 15-frame GOP): I-frame Comparison 1080p30 (IP 15-frame GOP): P-frame Comparison 1080p24 (IBP 12-frame GOP):ī-frame Comparison 1080p24 (IBP 12-frame GOP): I-frame Comparison 1080p24 (IPB 12-frame GOP): I then displayed and cropped them at 200% magnification in side-by-side 24Mbps versus 54Mpbs comparison shots, and saved screenshots of each comparison. After loading the video files into a 32-bit Adobe After Effects project, I selected I, P, and B-frames from the second GOP in each video sample. In 1080p30 mode, the D5100 records in an IP format using a 15-frame GOP. In 1080p24 mode, the D5100 records in H.264 IPB format using 12-frame GOP. This type of subject presents a myriad of high-contrast moving images that tax the motion tracking and bitrate allocation algorithms of the camera's H.264 video encoder. I selected a sunlit scene of a running stream of water with highlights on the water ripples. The camera was locked down on a tripod and used the Neutral Picture Profile. I took these shots with a Tamron 24-70mm f2.8 lens set at 70mm, f5.6, and ISO 100. I recently shot comparison tests of the D5100 in 1080pp30 video modes, using the stock 24Mbps bitrate, and hacked 36Mbps and 54Mbps bitrates. D5100 macroblock artifact comparison:stock 24Mbps versus hacked 54Mbps
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March 2023
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