One of the main selling points for people to upgrade from DVD to Blu-ray is that BDs offer high-definition video in a maximum resolution of 1080x1920 pixels at 24 frames per second (progressive) and 29.97 frames per second (interlaced), making it 4x sharper and cleaner than DVD, which has a resolution of 720 x 480 pixels (480i NTSC) and 720×576 pixels (576i PAL). 

The high-definition picture quality is truly exceptional. It provides a 24-bit YCrCB Rec. 709 color space that's capable of displaying an impressive 16,777,216 colors, with a bit depth of 8 bits dedicated to each color. 

Additionally, it features a 4:2:0 chroma subsampling for enhanced color accuracy. Finally, it offers a Standard Dynamic Range (SDR) for optimal contrast and brightness. Optionally, it supports xvYCC for expanded colors that are 1.8 times as large but requires an xvYCC-compatible television and player. 

Here's a fair-unaltered comparison between the video quality of the 2000 DVD (left) and the 2009 BD (right). It's clear as light and day between the 480i and 1080p resolutions. The differences in color and brightness are also noticeable between the two formats.

Is DVD a poor format in modern times? No, DVDs can look good (if appropriately mastered, which newer DVDs fail to do), but BD is better as it takes advantage of HDTVs.

 

Transport Stream & Codecs 

BD-ROMs use the MPEG-2 Transport Stream (M2TS) container file format under the .m2ts filename extension. Video, audio, graphics, and other streams are stored and multiplexed (muxed) into this container. Muxing is a method of combining multiple analog or digital signals into one signal over a shared medium. When a BD-ROM disc is inserted into a player, it reads and pre-loads the muxed data (Graghics/Audio/Text) into the memory before the Primary Video stream. The decoding and demuxing process operates on a clock of 27 Mhz.

There are three mandatory video codecs of the BD-ROM specification, AVC, MPEG-2, and VC-1. The choice of video codecs impacts the producer's licensing/royalty costs and the title's full run time due to differences in compression efficiency. A BD-ROM title must use one of the mandatory codecs for the Primary Video. Each codec is unique and has its own advantages and disadvantages.

BDMV Primary and Secondary Video Limitations   Green = Mandatory codecs

	MPEG-2	AVC	VC-1	MVC	MGVC
Standard	ISO/IEC 13818-2, ITU-T Rec. H.262	ISO/IEC 14496-10, ITU-T Rec. H.264	SMPTE 421M	ISO/IEC 14496-10, ITU-T H.264 (Annex H)	Proprietary
Profile and Level	MP@HL, MP@ML	HP@L4.1, MP@L4.1	AP@L3/AP@L2	HP@L4.1, MP@L4.	?
Chroma Format	4:2:0	4:2:0	4:2:0	4:2:0	4:2:0
Primary Video Stream Max. Data Rate (Mbps)	40.0	40.0	40.0	Base view: 40Mbps Dependent view: 40Mbps Base + Dependent: 60Mbps	Base view: 40Mbps Dependent view: 40Mbps Base + Dependent: 60Mbps
Secondary Video Stream Max. Data Rate (Mbps)	8.0 (SD), 40.0 (HD)	8.0 (SD), 40.0 (HD)	8.0 (SD), 40.0 (HD)	N/A	N/A

BDMV Primary Video Stream Limitations (2D)

Type	Frame Size	Frames / fields per second	Aspect Ratio
HD 1080 29.97i (60i)	1920×1080	29.97 frames interlaced / 59.94 fields	16:9
HD 1080 25i (50i)	1920×1080	25 frames interlaced / 50 fields	16:9
HD 1080 24p	1920×1080	24 frames progressive	16:9
HD 1080 23.976p (24p)	1920×1080	23.976 frames progressive	16:9
HD 1080 29.97i (60i)	1440×1080	29.97 frames interlaced / 59.94 fields	16:9
HD 1080 25i (50i)	1440×1080	25 frames interlaced / 50 fields	16:9
HD 1080 24p	1440×1080	24 frames progressive	16:9
HD 1080 23.976p (24p)	1440×1080	23.976 frames progressive	16:9
HD 720 59.94p (60p)	1280×720	59.94 frames progressive	16:9
HD 720 50p	1280×720	50 frames progressive	16:9
HD 720 24p	1280×720	24 frames progressive	16:9
HD 720 23.976p (24p)	1280×720	23.976 frames progressive	16:9
SD 480 29.97i (60i)	720×480	29.97 frames interlaced / 59.95 fields	4:3, 16:9
SD 576 25i (50i)	720×576	25 frames interlaced / 50 fields	4:3, 16:9

BDMV Primary Video Stream Limitations (3D)

Type	Frame Size	Frames / fields per second	Aspect Ratio
HD 1080 23.976p (24p)	1920×1080 (x2) (base + dependent)	23.976 frames progressive (x2)	16:9
HD 720 59.94p (60p)	1280×720 (x2)	59.94 frames progressive (x2)	16:9
HD 720 50p *	1280×720 (x2)	50 frames progressive (x2)	16:9

* Mandatory if 50 Hz system is used for both high and standard definition TV (Europe).

BDMV Secondary Video Stream (2D)

Type	Frame Size	Frames / Fields per second
HD 1080 29.97i (60i)	1920×1080	29.97 frames interlaced / 59.94 fields
HD 1080 24p	1920×1080	24 frames progressive
HD 1080 23.976p (24p)	1920×1080	23.976 frames progressive
HD 1080 29.97i (60i)	1440×1080	29.97 frames interlaced / 59.94 fields
HD 1080 24p	1440×1080	24 frames progressive
HD 1080 23.976p (24p)	1440×1080	23.976 frames progressive
HD 720 59.94p (60p)	1280×720	59.94 frames progressive
HD 720 50p	1280×720	50 frames progressive
HD 720 24p	1280×720	24 frames progressive
HD 720 23.976p (24p)	1280×720	23.976 frames progressive
SD 480 29.97p (30p)	720×480	29.97 frames progressive
SD 480 29.97i (60i)	720×480	29.97 frames interlaced / 59.95 fields
SD 480 25p	720×480	25 frames progressive
SD 480 24p	720×480	24 frames progressive
SD 480 23.976p (24p)	720×480	23.976 frames progressive
SD 576 25i (50i)	720×576	25 frames interlaced / 50 fields

BDMV Video Stream Allowable Codec Combinations (2D)

Primary Video Stream Codec	Secondary Video Stream MPEG-2	Secondary Video Stream AVC	Secondary Video Stream VC-1
MPEG-2	Yes	Yes	Yes
AVC	No	Yes	No
VC-1	No	No	Yes

 

Video Scanning

There are two techniques used in HD video: Progressive Scan (p) and Interlaced Scan (i). These techniques help in scanning the video frames. The refresh rate of a display, measured in Hertz (Hz), indicates how many times the screen can update in one second. A higher refresh rate results in smoother motion and less input lag because the image on the screen is updated more frequently. 

 

For Interlaced Scan, the lines of visual information are rotated as odds and evens. Only half of a frame’s visual information is transmitted at a time. The interlaced signal contains two fields of a video frame. A field is one of the many still images displayed sequentially to create the impression of motion on the screen. In 1080i resolution, it supports a refresh rate of 60Hz (59.94 fields per second) (American standard) and 50Hz (50 fields per second) (European standard). While it utilizes less bandwidth, it also presents certain drawbacks, including visual artifacts.

For Progressive Scan, all the lines that make up a single frame are drawn and transmitted at once. In 1080p resolution, it uses a 24Hz refresh rate becuase movies are almost always filmed 24 frames per second. The advantage of progressive scan is that it produces smoother and more realistic motion while maintaining high quality and absence of visual artifacts. One drawback is that it requires higher bandwidth to process an image. More frame rates are possible in 720p resolution that supports 59.94 frames per second.

Most Blu-ray releases opt for Progressive Scan despite higher bandwidth due to superior picture quality. Uncommonly, Blu-ray releases of some TV shows may use interlaced video (1080i) to preserve the original broadcast or because it was mastered that way.
 

Bitrate Compression Methods 

Primary video is loaded to the Primary memory buffer and the maximum video bitrate is 40 Mbit/s (5 MB memory), and all the video codecs support two compressing methods, CBR and VBR.

Constant bitrate (CBR)  maintains a consistent bitrate throughout the entire playback of the video. This makes videos easy to play and faster to load since everything is constant from start to finish, especially for low-end players. Developers seldom use CBR for movies due to its increased file size because it adds "bit stuffing" (useless data) in the stream. However, it's leveraged for fast loading of interactive content like FMV sequences in menus or games.

Variable bitrate (VBR)  allows a higher bitrate for more complex images and a lower bitrate for less detailed images. During playback, the bit rate adjusts to the level of detail needed, with fewer bits used in slow scenes and more in action-packed scenes. While VBR is generally hard to play and slower to load for players (both software and hardware), since the bitrate frequently changes, it provides much higher video quality without massive file sizes. Developers often use VBR encoding to achieve high picture quality while conserving disk space for movie.

For highly detailed live-action video, it's advisable to use a higher bitrate to avoid compression artifacts. For 2D animated video, a lower bitrate can be used as it's less detailed and more simplified. Bitrate can affect everything from file size to playback ability, which is why developers need to choose the right one.

Aspect Ratios

There are only two native aspect ratios, 16:9 and 4:3, and other aspect ratios are letterboxed in the encoded 16:9 picture, which means ultra widescreen movies will be shown with black bars on top and bottom. This was one of the criticisms of the Blu-ray format not natively supporting the ultra-widescreen 21:9 ratio.

Even though, as BD supports native 4:3 aspect ratio (1440 x 1080), most BD releases use pillar-boxing. Pillar-boxing is the display of an image within a wider image frame by adding lateral mattes (vertical bars at the sides). Example, a 4:3 image has lateral mattes when displayed on a 16:9 aspect ratio television screen. This has been criticized because it adds more size to the video file. (On newer DVDs, pillar-boxing is more common, consequently lowering the resolution of the picture.) Another criticism is there is no support for anamorphic video like there was on DVD.

 

Picture-in-Picture^[1]

Blu-ray also offers a Picture-in-Picture component that can play 2 video streams simultaneously, Primary Video and Secondary Video. The Secondary Video (typically displaying extra supplemental material) can be scaled, positioned, and overlayed on the Primary Video (showing the main feature). Secondary Video has a maximum resolution of 1080x1920 pixels at 24 frames per second (progressive) and 29.97 frames per second (interlaced). Secondary video is loaded to the Primary memory buffer and has a maximum bitrate of 40 Mbps (5 MB memory). You can combine two different codecs for both streams, but some of them may not be compatible with each other for playback due to technical constraints. So, it's recommended to use the same codec.

Here's an example of Beowulf using the Picture-in-Picture feature, showing viewers the motion capture performance in a small box overlaying the finished movie. (Image: Blu-ray.com)

 

Multi-angle

Like DVD, Blu-ray allows multi-angle viewing, giving viewers the ability to observe different perspectives of the same scene using the Angle button on the remote. This feature is made possible through the use of multiple viewpoints in one stream, enabling users to easily switch between them and explore the scene from various angles. Users can also use the angle buttons to see different versions of the same scene like storyboards or previsualizations. It supports a maximum of up to 9 camera angles.
Despite being a cool additional feature, it's not used often anymore in mainstream releases. Today, it's a niche in markets such as yoga, martial arts, erotica, animation (e.g. for storyboards), and live performances.

Alita: Battle Angel offers multi-angle using colored buttons to view the stages of production of the same scene.

Stereographic 3D

Optionally, Blu-ray is capable of playing 3D content for Blu-ray 3D players and TVs. 3D Video uses a modified version of AVC called MVC. 

    See: Stereographic 3D

Footnotes

[1] Ultra HD Blu-ray unfortunately does not support Secondary Video/Picture-in-Picture mode.

Author(s) : Æ Firestone