Nevertheless, there are source images where the loss due to 420 subsampling is not obvious to human perception and in such cases it can be advantageous to use 420 subsampling. Ideally, a codec should be able to support both subsampling formats. However, there are a few codecs that only support 420 subsampling — webp, discussed below, is one such popular codec.

Brief overview of image coding formats

The JPEG format was introduced in 1992 and is widely popular. It supports various color subsamplings including 420, 422 and 444. JPEG can ingest RGB data and transform it to a luma-chroma representation before performing lossy compression. The discrete cosine transform (DCT) is employed as the decorrelating transform on 8x8 blocks of samples. This is followed by quantization and entropy coding. However, JPEG is restricted to 8-bit imagery and lacks support for alpha channel. The more recent JPEG-XT standard extends JPEG to higher bit-depths, support for alpha channel, lossless compression and more in a backwards compatible way.

The JPEG 2000 format, based on the discrete wavelet transform (DWT), was introduced as a successor to JPEG in the year 2000. It brought a whole range of additional features such as spatial scalability, region of interest coding, range of supported bit-depths, flexible number of color planes, lossless coding, etc. With the motion extension, it was accepted as the video coding standard for digital cinema in 2004.

The webp format was introduced by Google around 2010. Google added decoding support on Android devices and Chrome browser and also released libraries that developers could add to their apps on other platforms, for example iOS. Webp is based on intra-frame coding from the VP8 video coding format. Webp does not have all the flexibilities of JPEG 2000. It does, however, support lossless coding and also a lossless alpha channel, making it a more efficient and faster alternative to PNG in certain situations.

High-Efficiency Video Coding (HEVC) is the successor of H.264, a.k.a. Advanced Video Coding (AVC) format. HEVC intra-frame coding can be encapsulated in the High-Efficiency Image File Format (HEIF). This format is most notably used by Apple devices to store recorded imagery.

Similarly, AV1 Image File Format (AVIF) allows encapsulating AV1 intra-frame coded content, thus taking advantage of excellent compression gains achieved by AV1 over predecessors. We touch upon some appealing technical features of AVIF in the next section.

The JPEG committee is pursuing a coding format called JPEG XL which includes features aimed at helping the transition from legacy JPEG format. Existing JPEG files can be losslessly transcoded to JPEG XL while achieving file size reduction. Also included is a lightweight conversion process back to JPEG format in order to serve clients that only support legacy JPEG.

AVIF technical features

Although modern video codecs were developed with primarily video in mind, the intraframe coding tools in a video codec are not significantly different from image compression tooling. Given the huge compression gains of modern video codecs, they are compelling as image coding formats. There is a potential benefit in reusing the hardware in place for video compression/decompression. Image decoding in hardware may not be a primary motivator, given the peculiarities of OS dependent UI composition, and architectural implications of moving uncompressed image pixels around.

In the area of image coding formats, the Moving Picture Experts Group (MPEG) has standardized a codec-agnostic and generic image container format: ISO/IEC 23000–12 standard (a.k.a. HEIF). HEIF has been used to store most notably HEVC-encoded images (in its HEIC variant) but is also capable of storing AVC-encoded images or even JPEG-encoded images. The Alliance for Open Media (AOM) has recently extended this format to specify the storage of AV1-encoded images in its AVIF format. The base HEIF format offers typical features expected from an image format such as: support for any image codec, ability to use a lossy or a lossless mode for compression, support for varied subsampling and bit-depths, etc. Furthermore, the format also allows the storage of a series of animated frames (offering an efficient and long-awaited alternative to animated GIFs), and the ability to specify an alpha channel (which sees tremendous use in UIs). Further, since the HEIF format borrows learnings from next-generation video compression, the format allows for preserving metadata such as color gamut and high dynamic range (HDR) information.

Image compression comparison framework

We have open sourced a Docker based framework for comparing various image codecs. Salient features include:

Encode orchestration (with parallelization) and insights generation using Python 3 Easy reproducibility of results and Easy control of target quality range(s).

Since the framework allows one to specify a target quality (using a certain metric) for target codec(s), and stores these results in a local database, one can easily utilize the Bjontegaard-Delta (BD) rate to compare across codecs since the target points can be restricted to a useful or meaningful quality range, instead of blindly sweeping across the encoder parameter range (such as a quality factor) with fixed parameter values and landing on arbitrary quality points.

An an example, below are the calls that would produce compressed images for the choice of codecs at the specified SSIM and VMAF values, with the desired tolerance in target quality:

main(metric='ssim', target_arr=[0.92, 0.95, 0.97, 0.99], target_tol=0.005, db_file_name='encoding_results_ssim.db') main(metric='vmaf', target_arr=[75, 80, 85, 90, 95], target_tol=0.5, db_file_name='encoding_results_vmaf.db')

For the various codecs and configurations involved in the ensuing comparison, the reader can view the actual command lines in the shared repository. We have attempted to get the best compression efficiency out of every codec / configuration compared here. The reader is free to experiment with changes to encoding commands within the framework. Furthermore, newer versions of respective software implementations might have been released compared to versions used at the time of gathering below results. For example, a newer software version of Kakadu demo apps is available compared to the one in the framework snapshot on github used at the time of gathering below results.

Visual examples

This is the section where we get to admire the work of the compression community over the last 3 decades by looking at visual examples comparing JPEG and the state-of-the-art.

The encoded images shown below are illustrative and meant to compare visual quality at various target bitrates. Please note that the quality of the illustrative encodes is not representative of the high quality bar that Netflix employs for streaming image assets on the actual service, and is meant to be purely educative in nature.

Shown below is one original source image from the Kodak dataset and the corresponding result with JPEG 444 @ 20,429 bytes and with AVIF 444 @ 19,788 bytes. The JPEG encode shows very obvious blocking artifacts in the sky, in the pond as well as on the roof. The AVIF encode is much better, with less blocking artifacts, although there is some blurriness and loss of texture on the roof. It is still a remarkable result, given the compression factor of around 59x (original image has dimensions 768x512, thus requiring 768x512x3 bytes compared to the 20k bytes of the compressed image).

An original image from the Kodak dataset

JPEG 444 @ 20,429 bytes

AVIF 444 @ 19,788 bytes

For the same source, shown below is the comparison of JPEG 444 @ 40,276 bytes and AVIF 444 @ 39,819 bytes. The JPEG encode still has visible blocking artifacts in the sky, along with ringing around the roof edges and chroma bleeding in several locations. The AVIF image however, is now comparable to the original, with a compression factor of 29x.

JPEG 444 @ 40,276 bytes

AVIF 444 @ 39,819 bytes

Shown below is another original source image from the Kodak dataset and the corresponding result with JPEG 444 @ 13,939 bytes and with AVIF 444 @ 4,176 bytes. The JPEG encode shows blocking artifacts around most edges, particularly around the slanting edge as well as color distortions. The AVIF encode looks “cleaner” even though it is one-third the size of the JPEG encode. It is not a perfect rendition of the original, but with a compression factor of 282x, this is commendable.

Another original source image from the Kodak dataset

JPEG 444 @ 13,939 bytes

AVIF 444 @ 4,176 bytes

Shown below are results for the same image with slightly higher bit-budget; JPEG 444 @ 19,787 bytes versus AVIF 444 @ 20,120 bytes. The JPEG encode still shows blocking artifacts around the slanting edge whereas the AVIF encode looks nearly identical to the source.

JPEG 444 @ 19,787 bytes

AVIF 444 @ 20,120 bytes

Shown below is an original image from the Netflix (internal) 1142x1600 resolution “boxshots-1” dataset. Followed by JPEG 444 @ 69,445 bytes and AVIF 444 @ 40,811 bytes. Severe banding and blocking artifacts along with color distortions are visible in the JPEG encode. Less so in the AVIF encode which is actually 29kB smaller.

An original source image from the Netflix (internal) boxshots-1 dataset

JPEG 444 @ 69,445 bytes

AVIF 444 @ 40,811 bytes

Shown below are results for the same image with slightly increased bit-budget. JPEG 444 @ 80,101 bytes versus AVIF 444 @ 85,162 bytes. The banding and blocking is still visible in the JPEG encode whereas the AVIF encode looks very close to the original.

JPEG 444 @ 80,101 bytes

AVIF 444 @ 85,162 bytes

Shown below is another source image from the same boxshots-1 dataset along with JPEG 444 @ 81,745 bytes versus AVIF 444 @ 76,087 bytes. Blocking artifacts overall and mosquito artifacts around text can be seen in the JPEG encode.

Another original source image from the Netflix (internal) boxshots-1 dataset

JPEG 444 @ 81,745 bytes

AVIF 444 @ 76,087 bytes

Shown below is another source image from the boxshots-1 dataset along with JPEG 444 @ 80,562 bytes versus AVIF 444 @ 80,432 bytes. There is visible banding, blocking and mosquito artifacts in the JPEG encode whereas the AVIF encode looks very close to the original source.

Another original source image from the Netflix (internal) boxshots-1 dataset

JPEG 444 @ 80,562 bytes

AVIF 444 @ 80,432 bytes

Overall results

Shown below are results over public datasets as well as Netflix-internal datasets. The reference codec used is JPEG from the JPEG-XT reference software, using the standard quantization matrix defined in Annex K of the JPEG standard. Following are the codecs and/or configurations tested and reported against the baseline in the form of BD rate.

The encoding resolution in these experiments is the same as the source resolution. For 420 subsampling encodes, the quality metrics were computed in 420 subsampling domain. Likewise, for 444 subsampling encodes, the quality metrics were computed in 444 subsampling domain. Along with BD rates associated with various quality metrics, such as SSIM, MS-SSIM, VIF and PSNR, we also show rate-quality plots using SSIM as the metric.

Kodak dataset; 24 images; 768x512 resolution

We have uploaded the source images in PNG format here for easy reference. We give the necessary attribution to Kodak as the source of this dataset.

Given a quality metric, for each image, we consider two separate rate-quality curves. One curve associated with the baseline (JPEG) and one curve associated with the target codec. We compare the two and compute the BD-rate which can be interpreted as the average percentage rate reduction for the same quality over the quality region being considered. A negative value implies rate reduction and hence is better compared to the baseline. As a last step, we report the arithmetic mean of BD rates over all images in the dataset. We also highlight the best performer in the tables below.