In computer-generated imagery and real-time 3D computer graphics, portal rendering is an algorithm for visibility determination.

A game level might contain many graphical polygons; only a few of which may be visible on screen at a given time. Visibility determination allows the renderer to decide which of those fall into that category and thus reduce rendering time.

For example, in a computer game, the game area might be divided to several sectors. These sectors would be then connected to each other by small openings such as doors or windows. These openings are referred as portals. When the sector behind a portal needs to be drawn, the only parts that are visible are the parts that can be seen through the portal. Therefore, the sector can be clipped against the portal boundaries to remove overdraw.

The use of portals simplifies the game engine's task of determining visible areas and objects from any given point of view of the level, and simplifies rendering by allowing it to use each portal as a viewing frustum for the area it leads to.

Wyndham Raymond Portal, 1st Viscount Portal PC GCMG DSO MVO (9 April 1885 – 6 May 1949) was a British politician. The eldest son of Sir William Wyndam Portal, 2nd Baronet, and [Florence Elizabeth Mary Glyn CBE, daughter of Hon. St Leger Glyn, 2nd son of George Glyn, 1st Baron Wolverton, he was educated at Eton and Christ Church, Oxford. In 1909 he married Lady Louise Rosemary Kathleen Virginia Cairns, MBE, only child of Arthur Cairns, 2nd Earl Cairns.

Portal was commissioned into the Hampshire Yeomanry in 1903, was promoted Lieutenant in 1905, and transferred to the 9th Lancers later the same year. He transferred to the 1st Life Guards as a Second Lieutenant in 1908 and was promoted Lieutenant again later the same year, but left the Army in 1911. He rejoined the Hampshire Yeomanry in 1914 and served in World War I. He was promoted Captain in 1914 while serving as adjutant of the Royal 1st Devon Yeomanry.

Transferring back to the Life Guards (Special Reserve) in 1915, he was promoted Lieutenant-Colonel in 1916 when he took command of the Household Battalion. In 1917 he was awarded the Distinguished Service Order and appointed a Member of the Royal Victorian Order (MVO). He relinquished command of the battalion in 1918 and reverted to the rank of Captain, but was soon promoted Major and attached to the Machine Gun Corps as a battalion commander, again with the rank of Lieutenant-Colonel. He resigned his commission in 1919. He was succeeded to the Baronetcy only by his uncle, Sir Spencer John Portal.

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• Doors — panels or barriers used to cover an opening
• Gates — entry points to an enclosed space
• Portals — focus pages which highlight a particular subject
• Portals on Wikimedia Commons - images of portals