Differences between HDR and SDR

Regardless of all its drawbacks and outdated technological process, video content in the SDR (Standard Dynamic Range) format still holds the leading position in the media market today. HDR (High Dynamic Range) format is only just beginning its expansion. In this article, we will consider the difference between SDR and HDR.

Colorimetric parameters

SDR format is based on the colorimetric parameters described in Rec. ITU-R BT.709. They cover only 35.9% of the spectrum visible to the human eye of the CEI 1931 system (Figure 1). HDR by contrast uses color parameters Rec. ITU-R BT.2020, covering 75.8% of the spectrum.

Figure 1. Color space of the CEI 1931 system.

Color depth

SDR has a color depth of 8 bits. SDR does not prohibit the use of 10 bits, but in practice the vast majority of video content is compressed at а color depth of 8 bits. This means that each of the primary colors — red, green and blue — can have 28 values, equal to 256, or a total number of 256x256x256 = 16,777,216 colors. This is a large number, but the human eye sees many more, therefore, in practice, it distinguishes aliased transitions in SDR videos (Figure 2). This is especially noticeable on gradient background scenes, such as the sky.

Figure 2. Color depth of 8 and 10 bits.

The minimum color depth in HDR is 10 bits: 1024 possible values for each primary color, or 1 073 741 824 colors in total, which is 64 times more than SDR. Such images are much closer to reality, however, under certain circumstances, the human eye is still able to notice aliasing in color transitions.

Luminance

The human eye is structured in such a way that, in addition to the color components, it also distinguishes the luminance or luma component, which is perceived much more intensively. SDR is limited to 100 cd/m2 luminance, while HDR standards theoretically achieve 10,000 cd/m2.

In practice, HDR-supporting displays in the mid-price segment claim a luminance of 1,000 cd/m2, while the premium segment offers a luminance of up to 4,000 cd/m2, but only for certain scenes and for a short time.

Read our article about the main HDR standards, their identification and validation for H.264/AVC, H.265/HEVC, VP9 and AV1.