# Fast Thumbnail Extraction for H.264/AVC, HEVC and VP9
## 3 Experimental Results and Discussion  
In order to evaluate the performance of the fast thumbnail extraction method proposed in this paper, the proposed algorithm for H.264/AVC, HEVC and VP9 was implemented on FFmpeg version 4.2.2 [16]. We also used ffmpegthumbnailer [17] to fairly evaluate the performance of thumbnail extraction with the FFmpeg decoder. The open software ffmpegthumbnailer drives the FFmpeg decoders and down-sampler for thumbnail extraction, as shown in Figure 8. Thus, the proposed algorithm also employs the open software for the same interface and fair evaluation. The experiment was conducted in a virtual Linux environment with a 3.40 GHz processor, 16.0 GB of memory and Windows 10 64-bit operating system, as shown in Table 3.  
Figure 8: A block diagram for performance evaluation. \begin{table}
\begin{tabular}{c c} \hline CPU & Intel® CoreTM 17-8700K Processor @3.70 GHz \\ RAM & 16.0 GB \\ Windows 10 64-bit Windows Subsystem \\ OS & for Linux Ubuntu 18.04 LTS \\ Software & FFmpeg 4.2.2 \\ \hline \end{tabular}
\end{table} 
Table 3: Experimental conditions. Figure 7: The proposed memory structure for a 3840 \(\times\) 2160 input bitstream. The experiment was conducted in a virtual Linux environment with a 3.70 GHz processor, 16.0 GB of memory and Windows 10 64-bit operating system, as shown in Table 3.  
For test sequences, six video sequences from class A1 and A2 with 4K resolution under the common test conditions of versatile video coding (VVC) were selected [18], and they were coded by H.264/AVC, HEVC and VP9 encoders. Table 4 shows the bit rates of the test bitstreams. For performance evaluation, the thumbnail of the first frame of each test sequence was extracted for H.264/AVC, HEVC and VP9 bitstreams, and the extraction times were compared. The time saving (TS) for measuring the thumbnail extraction speed comparison was calculated, as defined by:  
\[TS=\frac{\left(T_{org}-T_{proposal}\right)}{T_{org}}\times 100 \tag{2}\]  
Tables 5-7 show the comparison of the thumbnail extraction time of each codec compared to the conventional method. The acceleration rate of the proposed thumbnail extraction algorithm was 66% for H.264/AVC bitstreams, 52% for HEVC bitstreams and 48% for VP9 bitstreams. The acceleration ratios may differ slightly depending on the codec. The computing time to decode the inverse transform and intra prediction of H.264/AVC was higher than that in the others; in addition, the speed factors differed depending on image characteristics because the ratios of transform skip or zero residuals influenced the thumbnail extraction computing time. The proposed algorithm focuses on the removal of the inverse transformation and intra prediction portions. Table 8 shows the peak signal-to-noise ratio values of the thumbnails compared with the conventional full decoding method for each codec. The result shows that the PSNR value significantly differs depending on the sequences. This is because the proposed method stores only pixels necessary for the thumbnail in the thumbnail buffer, and the down-sampling process is removed. Therefore, in the case of sequences containing complex textures, they suffer aliasing; thus, PSNR can be lower than for others. Nevertheless, the thumbnails generated by the proposed method had a sufficient level of visual quality for commercial use in thumbnail applications. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sequence Name**} & \multicolumn{3}{c}{**Bitrate (Mb/s)**} \\ \cline{2-4}  & **H.264/AVC** & **HEVC** & **VP9** \\ \hline Campfire & 54.7 & 24.6 & 45.0 \\ FoodMarket4 & 19.4 & 6.4 & 6.5 \\ Tango2 & 23.6 & 10.1 & 16.4 \\ CatRobot & 20.7 & 9.8 & 20.3 \\ DaylightRoad2 & 25.1 & 12.0 & 28.7 \\ ParkRunning3 & 72.4 & 39.3 & 47.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Bitrates of the coded test bitstreams by H.264/AVC, HEVC and VP9 for test sequences.