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Coding tree unit
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<h2 id="history">History</h2> <p>Macroblock encoding methods have been used in digital video coding standards since <a href="/facts/H.261/2XzQrNgo">H.261</a> which was first released in 1988. However, for <a href="/facts/Error_correction/o2rH8e3n">error correction</a> and <a href="/facts/Signal-to-noise_ratio/qohClhyG">signal-to-noise ratio</a> the standard 16x16 macroblock size is not capable of getting the kind of bit reductions that <a href="/facts/Information_theory/qNLH3U5P">information theory</a> and <a href="/facts/Coding_theory/SBqStoGL">coding theory</a> suggest are theoretically and practically possible.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> <h2 id="technical-details">Technical details</h2> <p>HEVC replaces <a href="/facts/Macroblock/L7XPoRNP">macroblocks</a>, which were used with previous video standards, with CTUs which can use larger block structures of up to 64×64 pixels and can better sub-partition the picture into variable sized structures.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>HEVC initially divides the picture into CTUs which are then divided for each luma/chroma component into coding tree blocks (CTBs).<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a><a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p><p>A CTB can be 64×64, 32×32, or 16×16 with a larger pixel block size usually increasing the coding efficiency.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> CTBs are then divided into one or more coding units (CUs), so that the CTU size is also the largest coding unit size.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p> <ul><li>The arrangement of CUs in a CTB is known as a <a href="/facts/Quadtree/6b23FQoD">quadtree</a> since a subdivision results in four smaller regions.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a></li> <li>CUs are then divided into prediction units (PUs) of either intra-picture or inter-picture prediction type which can vary in size from 64×64 to 4×4.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> To limit worst-case memory bandwidth when applying motion compensation in the decoding process, prediction units coded using inter-picture prediction are restricted to a minimum size of 8×4 or 4×8 if they are predicted from a single reference (uni-prediction) or 8×8 if they are predicted from two references (bi-prediction).<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a><a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a></li> <li>To code the prediction residual, a CU is divided into a quadtree of <a href="/facts/Discrete_cosine_transform/gPpDhPwP">DCT</a> transform units (TUs).<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a><a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> TUs contain coefficients for spatial block transform and quantization.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a><a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> A TU can be 32×32, 16×16, 8×8, or 4×4 pixel block sizes.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a></li></ul> <h2 id="standardization">Standardization</h2> <p>At the July 2012 HEVC meeting it was decided, based on proposal JCTVC-J0334, that HEVC level 5 and higher would be required to use CTB sizes of either 32×32 or 64×64.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a><a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> This was added to HEVC in the Draft International Standard as a level limit for the Log2MaxCtbSize variable.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> </p><p>Log2MaxCtbSize was renamed CtbSizeY in the October 2012 HEVC draft and then renamed CtbLog2SizeY in the January 2013 HEVC draft.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a><a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> </p> <h2 id="coding-efficiency">Coding efficiency</h2> <p>The design of most video coding standards is primarily aimed at having the highest coding efficiency.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> Coding efficiency is the ability to encode video at the lowest possible bit rate while maintaining a certain level of video quality.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> HEVC benefits from the use of larger CTB sizes.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> </p><p>This has been shown in <a href="/facts/Peak_signal-to-noise_ratio/FPlWpNkI">peak signal-to-noise ratio</a> (PSNR) tests with a HM-8.0 HEVC encoder where it was forced to use progressively smaller CTU sizes.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> For all test sequences when compared to a 64×64 CTU size it was shown that the HEVC bit rate increased by 2.2% when forced to use a 32×32 CTU size and increased by 11.0% when forced to use a 16×16 CTU size.<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a> </p><p>In the Class A test sequences, where the resolution of the video was 2560×1600, when compared to a 64×64 CTU size it was shown that the HEVC bit rate increased by 5.7% when forced to use a 32×32 CTU size and increased by 28.2% when forced to use a 16×16 CTU size.<a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a> </p><p>The tests showed that large CTU sizes become even more important for coding efficiency with higher resolution video.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> The tests also showed that it took 60% longer to decode HEVC video encoded at 16×16 CTU size than at 64×64 CTU size.<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a> The tests showed that large CTU sizes increase coding efficiency while also reducing decoding time.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a> The tests were conducted with the Main profile of HEVC based on equal PSNR.<a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> </p> Increase in video bit rate when smaller CTU sizes were used<a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a><table><tbody><tr><th rowspan="2">Video test sequences</th><th colspan="3">Maximum CTU size used in video encodingin comparison to 64×64 CTUs</th></tr><tr><th>64×64 CTUs</th><th>32×32 CTUs</th><th>16×16 CTUs</th></tr><tr><th>Class A (2560×1600 pixels)</th><td>0%</td><td>5.7%</td><td>28.2%</td></tr><tr><th>Class B (1920×1080 pixels)</th><td>0%</td><td>3.7%</td><td>18.4%</td></tr><tr><th>Class C (832×480 pixels)</th><td>0%</td><td>1.8%</td><td>8.5%</td></tr><tr><th>Class D (416×240 pixels)</th><td>0%</td><td>0.8%</td><td>4.2%</td></tr><tr><th>Overall</th><td>0%</td><td>2.2%</td><td>11.0%</td></tr><tr><th>Encoding time</th><td>100%</td><td>82%</td><td>58%</td></tr><tr><th>Decoding time</th><td>100%</td><td>111%</td><td>160%</td></tr></tbody></table> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/High_Efficiency_Video_Coding/6gp7DC1a">High Efficiency Video Coding</a> (HEVC) - Video standard that supports 8K UHDTV and resolutions up to 8192 × 4320</li> <li><a href="/facts/H.264/MPEG-4_AVC/soCzKcmJ">H.264/MPEG-4 AVC</a> - The predecessor video standard of HEVC</li> <li><a href="/facts/VP9/dBP8ffoc">VP9</a> - A video codec with superblocks, which are similar to CTUs</li> <li><a href="/facts/Macroblock/L7XPoRNP">Macroblock</a> - The basic processing unit used in several previous video standards</li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf">HEVC overview</a></li> <li><a href="http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Performance.pdf">HEVC coding efficiency</a></li> <li><a href="http://www.itu.int/rec/T-REC-H.265">ITU-T Recommendation H.265 - High Efficiency Video Coding</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>G.J. Sullivan; J.-R. Ohm; W.-J. Han; T. Wiegand (2012-05-25). "Overview of the High Efficiency Video Coding (HEVC) Standard" (PDF). IEEE Transactions on Circuits and Systems for Video Technology new ownership of coding:GilbertLeeGrimaldoJr.(c)(r)(tm). Retrieved 2013-04-26. <a href="/wiki/Thomas_Wiegand" target="_blank">/wiki/Thomas_Wiegand</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>G.J. Sullivan; Heiko Schwarz; Thiow Keng Tan; Thomas Wiegand (2012-08-22). "Comparison of the Coding Efficiency of Video Coding Standards – Including High Efficiency Video Coding (HEVC)" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. Retrieved 2013-04-26. <a href="/wiki/Thomas_Wiegand" target="_blank">/wiki/Thomas_Wiegand</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Gary Sullivan; Jens-Rainer Ohm (2012-10-13). "Meeting report of the 10th meeting of the Joint Collaborative Team on Video Coding (JCT-VC), Stockholm, SE, 11-20 July 2012". JCT-VC. Retrieved 2013-04-28. <a href="http://phenix.it-sudparis.eu/jct/doc_end_user/current_document.php?id=6466" target="_blank">http://phenix.it-sudparis.eu/jct/doc_end_user/current_document.php?id=6466</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>G.J. Sullivan; J.-R. Ohm; W.-J. Han; T. Wiegand (2012-05-25). "Overview of the High Efficiency Video Coding (HEVC) Standard" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. Retrieved 2013-04-26. <a href="http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf" target="_blank">http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>G.J. Sullivan; Heiko Schwarz; Thiow Keng Tan; Thomas Wiegand (2012-08-22). "Comparison of the Coding Efficiency of Video Coding Standards – Including High Efficiency Video Coding (HEVC)" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. Retrieved 2013-04-26. <a href="/wiki/Thomas_Wiegand" target="_blank">/wiki/Thomas_Wiegand</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"ITU-T Home : Study groups : ITU-T Recommendations : ITU-T H.265 (04/2013)". ITU. 2013-04-13. Retrieved 2013-04-16. <a href="http://www.itu.int/ITU-T/recommendations/rec.aspx?rec=11885" target="_blank">http://www.itu.int/ITU-T/recommendations/rec.aspx?rec=11885</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"AAP Recommendation: H.265". ITU. 2013-04-13. Retrieved 2013-04-16. <a href="http://www.itu.int/ITU-T/aap/AAPRecDetails.aspx?AAPSeqNo=2741" target="_blank">http://www.itu.int/ITU-T/aap/AAPRecDetails.aspx?AAPSeqNo=2741</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"AAP Announcement No. 09". ITU. 2013-04-15. Retrieved 2013-04-16. <a href="http://www.itu.int/dms_pubaap/01/T0101000F09.htm" target="_blank">http://www.itu.int/dms_pubaap/01/T0101000F09.htm</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>"Diary of an x264 Developer » the problems with wavelets". Archived from the original on 2014-01-29. Retrieved 2014-02-06. <a href="https://web.archive.org/web/20140129171425/http://x264dev.multimedia.cx/archives/317" target="_blank">https://web.archive.org/web/20140129171425/http://x264dev.multimedia.cx/archives/317</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>G.J. Sullivan; J.-R. Ohm; W.-J. Han; T. Wiegand (2012-05-25). "Overview of the High Efficiency Video Coding (HEVC) Standard" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. Retrieved 2013-04-26. <a href="http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf" target="_blank">http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>"Description of High Efficiency Video Coding (HEVC)". JCT-VC. 2011-01-01. Retrieved 2012-09-15. <a href="http://mpeg.chiariglione.org/technologies/mpeg-h/HEVC.htm" target="_blank">http://mpeg.chiariglione.org/technologies/mpeg-h/HEVC.htm</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>G.J. Sullivan; J.-R. Ohm; W.-J. Han; T. Wiegand (2012-05-25). "Overview of the High Efficiency Video Coding (HEVC) Standard" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. Retrieved 2013-04-26. <a href="http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf" target="_blank">http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>"Description of High Efficiency Video Coding (HEVC)". JCT-VC. 2011-01-01. Retrieved 2012-09-15. <a href="http://mpeg.chiariglione.org/technologies/mpeg-h/HEVC.htm" target="_blank">http://mpeg.chiariglione.org/technologies/mpeg-h/HEVC.htm</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>G.J. Sullivan; J.-R. Ohm; W.-J. Han; T. Wiegand (2012-05-25). "Overview of the High Efficiency Video Coding (HEVC) Standard" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. Retrieved 2013-04-26. <a href="http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf" target="_blank">http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>G.J. Sullivan; J.-R. Ohm; W.-J. Han; T. Wiegand (2012-05-25). "Overview of the High Efficiency Video Coding (HEVC) Standard" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. Retrieved 2013-04-26. <a href="http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf" target="_blank">http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>G.J. Sullivan; J.-R. Ohm; W.-J. Han; T. Wiegand (2012-05-25). "Overview of the High Efficiency Video Coding (HEVC) Standard" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. Retrieved 2013-04-26. <a href="http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf" target="_blank">http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>G.J. Sullivan; J.-R. Ohm; W.-J. Han; T. Wiegand (2012-05-25). "Overview of the High Efficiency Video Coding (HEVC) Standard" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. Retrieved 2013-04-26. <a href="http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf" target="_blank">http://iphome.hhi.de/wiegand/assets/pdfs/2012_12_IEEE-HEVC-Overview.pdf</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>"Description of High Efficiency Video Coding (HEVC)". JCT-VC. 2011-01-01. 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Retrieved 2013-04-28. <a href="http://phenix.it-sudparis.eu/jct/doc_end_user/current_document.php?id=6466" target="_blank">http://phenix.it-sudparis.eu/jct/doc_end_user/current_document.php?id=6466</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></p></li> <li id="fn:27"><p>Wade Wan; Tim Hellman (2012-07-03). "Adding a Level Restriction on Coding Tree Block Size". JCT-VC. Retrieved 2012-09-22. <a href="http://phenix.it-sudparis.eu/jct/doc_end_user/current_document.php?id=6197" target="_blank">http://phenix.it-sudparis.eu/jct/doc_end_user/current_document.php?id=6197</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></p></li> <li id="fn:28"><p>"High Efficiency Video Coding (HEVC) text specification draft 8". JCT-VC. 2012-07-28. 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Retrieved 2012-10-23. <a href="http://phenix.it-sudparis.eu/jct/doc_end_user/current_document.php?id=6803" target="_blank">http://phenix.it-sudparis.eu/jct/doc_end_user/current_document.php?id=6803</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></p></li> <li id="fn:31"><p>G.J. Sullivan; Heiko Schwarz; Thiow Keng Tan; Thomas Wiegand (2012-08-22). "Comparison of the Coding Efficiency of Video Coding Standards – Including High Efficiency Video Coding (HEVC)" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. Retrieved 2013-04-26. <a href="/wiki/Thomas_Wiegand" target="_blank">/wiki/Thomas_Wiegand</a> <a href="#fnref:31" class="footnote-back-ref">↩</a></p></li> <li id="fn:32"><p>G.J. Sullivan; Heiko Schwarz; Thiow Keng Tan; Thomas Wiegand (2012-08-22). "Comparison of the Coding Efficiency of Video Coding Standards – Including High Efficiency Video Coding (HEVC)" (PDF). IEEE Transactions on Circuits and Systems for Video Technology. 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