# Fast Thumbnail Extraction for H.264/AVC, HEVC and VP9
## 1 Introduction  
With the recent development of video capture, displays and processing capability, there is a growing demand for high-definition, high-quality video services in the market. Along with the market requirements, the ISO/IEC moving picture expert group (MPEG) and video coding expert group (VCEG) have developed subsequent video codec standards such as MPEG-2, H.264/AVC and high-efficiency video coding (HEVC) [1, 2, 3]. In addition, private entity codecs such as VP8 and VP9 [4, 5], among others, have been developed. As the codecs have evolved, their coding performance has increased by a bit-saving factor of two for the same visual quality. These advances have been made possible by adding a number of new tools with high computational complexity. Many tools have been developed for and added to intra frames. However, to reduce the amount of spatial redundancy of the frame, coding tools are interdependent, making it difficult to accelerate the video codecs in real time with a minimal computational load. In today's world, we are flooded with video content from multiple media services through broadcasting, over-the-top media service, the internet, and so on. Easy selection of these contents is necessary, and thumbnail display is used as a part of user interfaces to allow users to visually select the content they wish to engage with. However, video resolution is also increasing rapidly up to 8K or higher, resulting in a significant increase in the hardware requirements for fully decoding video content. Several attempts have been made, such as parallel decoding and decoder implementation using single Instruction multiple data (SIMD) instructions, to improve the decoding speed [6, 7, 8, 9]. However, it is still almost impossible to fully decode multiple 4K or 8K videos simultaneously in limited hardwares to enable thumbnail display. In addition, we need to reduce the required amount of memory used for thumbnail processing on embedded systems due to their memory limitations. Some attempts have been made to extract thumbnail images in the frequencydomain [10; 11] based on [PERSON]'s transform domain intra prediction method [12]. However, these methods require additional look-up tables, which increases the amount of memory use and is very hard to apply to other codecs. As such, a partial decoding method has been proposed [13]. This method restores only the right and bottom boundary pixels in 4 \(\times\) 4 units, which is the minimum transform unit (TU) size of HEVC. This method can be easily applied to other codecs; however, it always operates in 4 \(\times\) 4 units regardless of the block size. Therefore, unnecessary pixels that are not used for the thumbnail output or reference pixels for intra prediction are restored. Figure 1 shows the pixels used for a reference pixel or thumbnail output (gray) and the unnecessarily reconstructed pixels (yellow) of the method [12]. The number of unnecessarily restored pixels for the N \(\times\) N TU block are as follows:  
\[\frac{3N^{2}-12N}{8} \tag{1}\]  
In this paper, we present a fast thumbnail decoding method using a small amount of memory and partial decoding according to the prediction block size for intra frames of H.264/AVC, HEVC and VP9 with minimal visual quality loss and without any error propagation. The proposed partial decoding method restores the pixels constituting the thumbnail and the reference pixels used in the intra prediction by replacing the full inverse transform and intra prediction with the partial inverse transform and partial intra prediction. The computational load and memory usage are greatly reduced by omitting both the reconstruction process and the storage process for the other pixels. HEVC employs large transformations with dimensions such as 32 \(\times\) 32; thus, we reconstructed several pixels inside the block to preserve the visual quality of the reconstructed thumbnails.