Maximum Resolution for PAL VOB Subpictures

This article covers the maximum graphic resolution of a subpicture embedded in a PAL VOB file, detailing its technical limits, color specifications, and display mechanics within the DVD-Video standard. In DVD authoring, subpictures are overlay streams used for subtitles, menu highlights, and captions, and their dimensions are strictly bounded by the television broadcast format standards implemented on the disc.

The maximum graphic resolution of a subpicture stream in a standard PAL VOB (Video Object) file is 720 × 576 pixels.

This resolution matches the maximum display resolution of standard PAL DVD-Video, commonly known as Full D1. Because the subpicture stream is designed to overlay directly onto the active video frame, it cannot exceed the horizontal or vertical boundaries of the underlying video standard.

Display Characteristics and Aspect Ratio

Although the maximum raw pixel grid is 720 × 576, PAL video uses non-square pixels. As a result, the subpicture graphic is stretched upon playback to fit the target display aspect ratio:

• 4:3 Aspect Ratio: The 720 × 576 pixel canvas is rendered with a Pixel Aspect Ratio (PAR) of approximately 1.09:1, displaying as a standard fullscreen image.
• 16:9 Anamorphic Widescreen: The 720 × 576 pixel canvas is stretched horizontally using a PAR of approximately 1.46:1 to fill a 16:9 widescreen display. In widescreen setups, separate subpicture assets or coordinate mappings are often defined for letterbox and pan-and-scan modes to prevent graphic distortion.

Subpicture Technical Limitations

While the canvas resolution allows up to 720 × 576 pixels, subpicture streams in a VOB container are subject to strict technical constraints governed by the DVD-Video specification:

• Color Depth: Subpictures are 2-bit bitmaps, meaning any single subpicture screen can display a maximum of four distinct colors simultaneously.
• Color Palette: These four colors are mapped from a larger 16-color palette (part of the PGC, or Program Chain, settings). Each of the four colors is assigned a specific function: background, pattern/foreground, emphasis 1, and emphasis 2.
• Alpha Transparency: Each of the four selected colors carries an independent transparency value (alpha level) ranging from 0 (completely transparent) to 15 (fully opaque).
• Compression: Subpicture images are compressed using run-length encoding (RLE). Because of the strict limits on the maximum bit rate allocated to the subpicture stream inside the MPEG-2 transport architecture, graphic overlays must remain visually simple to ensure proper decoding without buffer underflow.

A subpicture does not need to cover the entire 720 × 576 area; it can be defined with smaller rectangular coordinate boundaries anywhere within that canvas to conserve processing bandwidth and storage space. However, it cannot exceed the maximum 720 × 576 perimeter.