Texture alignment in the Doom engine is affected by a number of factors. It can at times be more complicated than in modern true 3D games.
Ordinarily, most textures consist of a number of repeating shapes to create a pattern. Bricks for example almost always consist of a pattern of similarly shaped rectangles. The objective of aligning a texture appropriately is attempting to position the texture on the wall so that as many of the intended shapes included in the texture are displayed without being cutoff by a vertex in your wall. In the screenshot in the right, the rounded shapes in the texture are cut off by the differing height levels of the adjacent sectors that are decorated with the same wall texture. In many cases the texture can be aligned to repeat the texture so that none of the intended shapes are being cut off. Sometimes this can be very difficult to do, and can often be fixed by using a different texture designed to fit that particular length and height of the wall you are applying a texture to. In some cases, textures with no particular cracks (or too many to draw attentiont to) or bevels displayed in the image can be used in just about all lengths and heights of a wall, but using it excessively can make a map look very dull. The most rewarding part of mapping these days is how effectively a mapper can align the textures in a map.
Alignment of wall textures is a fairly complex subject. The way textures are aligned depends on the type of line (single or two sided line), the linedef flags controlling pegging, and the sidedef offsets.
- If a linedef is one sided (a solid wall), the texture is "pegged" to the top of the wall. That is to say, the top of the texture is at the ceiling. The texture continues downward to the floor.
- If the "lower unpegged" flag is set on the linedef, the texture is instead "pegged" to the bottom of the wall. That is, the bottom of the texture is located at the floor. The texture is drawn upwards from here. This is commonly used in the jambs in doors; because Doom's engine treats each door as a "rising ceiling", a doorjamb pegged to the top of the wall would "rise up" with the door.
- The alignment of the texture can be adjusted using the sidedef X and Y alignment controls. This is applied after the logic controlling pegging.
- If a linedef is two sided, acting as a "bridge" between two sectors - such as a window or a step - the upper texture is pegged to the lowest ceiling, and the lower texture is pegged to the highest floor. That is, the bottom of the upper texture will be at the lowest ceiling and drawn upwards, and the top of the lower texture will be at the highest floor and drawn downwards. This is appropriate for textures that "belong" to the surface behind them, such as lifts or platforms.
- If the "upper unpegged" flag is set on the linedef, the upper texture will begin at the higher ceiling and be drawn downwards.
- If the "lower unpegged" flag is set on the linedef, the lower texture will be drawn as if it began at the higher ceiling and continued downwards. So if the higher ceiling is at 96 and the higher floor is at 64, a lower unpegged texture will start 32 pixels from its top edge and draw downwards from the higher floor.
- Both forms of unpegging cause textures to be drawn the same way as if they were on a one-sided wall — relative to the ceiling — but with the middle section "cut out". Unpegging is thus appropriate for textures that "belong" to the surface in front of them, such as windows or recessed switches.
- The alignment of textures can be adjusted with the sidedef X and Y alignment control. This is applied after the logic controlling pegging and affects all textures.
- If a middle texture is set, it is clipped by the highest floor and the lowest ceiling and follows the same alignment logic as for a single sided linedef, but does not repeat vertically.
Level designers must ensure that textures tile in a consistent and aesthetically pleasing manner, such that the player is unaware that walls, floors and so forth are actually made of repeating textures rather than a single, continuous texture.
Floor and ceiling textures
All floor and ceiling textures are aligned on a 64-unit grid. Because of this, in some cases the only way to align a floor texture is to move the position of the sector which defines the floor itself.
Some ports such as Boom add the ability to change the alignment of floor and ceiling textures, even allowing effects such as moving conveyor belts. These are typically implemented using a separate linedef tagged to the sector.