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Text File | 1994-06-15 | 20KB | 438 lines

MANAGING TEXTURES AND THE "UNPEGGED" ATTRIBUTE ---------------------------------------------- Scott Amspoker (scott@basis.com) There have been recurring questions regarding the use of "pegged" and "unpegged" textures on walls as well as the tile rendering of textures by the Doom engine. This is not just a beginner's problem. Even the more sophisticated and popular PWADs submitted by netters have the occasional texturing oversight. Here is an explanation of how textures are rendered by Doom. The bulk of the following text deals with wall textures in particular. As you have already discovered, every section of wall in Doom has three rectangular areas that must be filled in by the Doom engine - the upper texture, the normal texture, and the lower texture. Each area may have a different texture map assigned to it or no texture at all inidicated by a texture name of "-". Any given wall can have all three areas exposed to the player but most walls will have one of more of these areas out of view. When choosing texture maps, the PWAD creator must consider how Doom will use that texture map to fill in the rectangular wall areas and understand how Doom's default behaviour can be changed via the "unpegged" attributes. Doom fills in each wall area by tiling its texture map horizontally and vertically until the entire area is covered. The texture map may be truncated horizontally and/or vertically if the wall area dimensions are not an even multiple of the dimensions of the texture map. In all cases, texture maps are placed on the horizonal axis beginning with the left edge of the wall and working towards the right. The placement of textures on the vertical axis is more complicated and differs depending on various conditions described below. Some of the texture maps found in the standard DOOM.WAD file are have a homogeneous quality to them. The effects of tiling/truncating them are not visible and the PWAD designer need not worry about unusual wall dimensions. On the other hand, some texture maps have distinct markings that won't appear correct when tiled/truncated without some intervention by the PWAD designer. It is desirable that the texture maps of adjoining wall segments line up with each other to create a seamless composite wall. Sometimes, deliberate texture mis-alignment is used as a hint to the player of a secret door. NOTE: some texture maps that have a height other than 64 or 128 (such as a stair riser) do not appear to tile very well. For some reason you might end up with small pink cracks in the wall texture. While this can have a really cool effect, treat it as a bug. Most texture maps found in the standard WAD are 128 units high. For most of the following examples, I will use the following texture map: -------------------- |TTTTTTTTTTTTTTTTTTTT| | | | | | | | | | | | | | | | | |BBBBBBBBBBBBBBBBBBBB| -------------------- For reference, I've labeled the top of the map with T's and the bottom with B's. THE NORMAL TEXTURE ------------------ Single-sided linedefs only require that the normal texture be defined. In this case the normal texture is the entire area of wall you see from floor to ceiling. By default, Doom will fill the normal texture with the assigned texture map starting with the top and working down. If the wall is somewhat taller than the texture map, it will look like this: -------------------- |TTTTTTTTTTTTTTTTTTTT| | | | | | | | | | | | | | | | | |BBBBBBBBBBBBBBBBBBBB| |TTTTTTTTTTTTTTTTTTTT| | | | | -------------------- If the wall is shorter than the texture, the result would be: -------------------- |TTTTTTTTTTTTTTTTTTTT| | | | | | | | | -------------------- Notice that the top of the texture is always at the top of the wall. The texture is rendered working down the wall towards the floor and repeated if necessary. By setting the "lower unpegged" bit in the linedef, the rendering of the normal texture will be performed from the bottom up instead of the top down. (The "lower unpegged" bit is normally associated with the lower texture but also has an effect on the normal texture.) Our tall wall would now look like this: -------------------- | | | | |BBBBBBBBBBBBBBBBBBBB| |TTTTTTTTTTTTTTTTTTTT| | | | | | | | | | | | | | | | | |BBBBBBBBBBBBBBBBBBBB| -------------------- Our short wall would be: -------------------- | | | | | | | | |BBBBBBBBBBBBBBBBBBBB| -------------------- Notice that the bottom of the texture map begins at the bottom of the wall and works up. THE UPPER AND LOWER TEXTURES ---------------------------- A linedef may have sectors on both sides of the line. In such a case, both sides of the linedef have wall definitions (sidedefs). If the "2-sided" attribute is set then Doom considers each of the two sectors to be exposed to the other sector. When a 2-sided line is used, the PWAD designer must always consider what they player should see when facing the line from both sides. A 2-sided line is most often used as a see-through connection between two sectors (such doorways, windows, stairs, and platforms). Therefore, the normal texture on both sides would typically be invisible ("-"). However, since the two sectors could have different floor and ceiling heights, the dimensions of wall space on either side of the line could be different. The "upper texture" is used to handle the difference (if any) between adjacent ceilings. The "lower texture" is similarly used to handle the different floor elevations (if any). In a nutshell, the normal texture is the area of wall that both sectors have in common. This would be the portion of the wall above the higher of the two floors and below the lower of the two ceilings. (Both sides of the line will normal textures.) The upper texture is the area of the wall between the two ceiling heights. This area is visible only on the side of the line with the higher ceiling. The lower texture is the wall space between the two floor heights and is only visible on the side with the lower floor. NOTE: If the ceiling texture of both sectors is F_SKY1 then the upper texture between them will also be invisible regardless of its assigned texture map. See the courtyard in E1M1 for an example of this. By default, the upper texture is tiled from the bottom up and the lower texture is tiled from the top down. Consider a window. The area above the window opening is the upper texture. The window opening itself is the normal texture. The area below the window opening is the lower texture. Let's use the following wall segment with our example texture map assigned to upper and lower areas: -------------------- | | | | <- upper texture |BBBBBBBBBBBBBBBBBBBB| |--------------------| | | <- normal texture ("-") | | |--------------------| |TTTTTTTTTTTTTTTTTTTT| | | <- lower texture | | -------------------- Notice how the bottom of the texture map rests on the bottom of the upper area whereas the lower area is just the opposite. If this wall section were placed adjacent to a regular wall panel, the texture pattern would not line up: --------------- -------------------- |TTTTTTTTTTTTTTT| | | | | | |BBBBBBBBBBBBBBBBBBBB| | |--------------------| | | | | | | | |--------------------| | |TTTTTTTTTTTTTTTTTTTT| | | | |BBBBBBBBBBBBBBB| | --------------- -------------------- By setting the "upper unpegged" attribute, the upper texture is rendered from the top down similar to a normal texture: --------------- -------------------- |TTTTTTTTTTTTTTT|TTTTTTTTTTTTTTTTTTTT| | | | | | | | |--------------------| | | | | | | | |--------------------| | |TTTTTTTTTTTTTTTTTTTT| | | | |BBBBBBBBBBBBBBB| | --------------- -------------------- Likewise, by setting the "lower unpegged" attribute, the lower texture will appear rendered from the bottom up: --------------- -------------------- |TTTTTTTTTTTTTTT|TTTTTTTTTTTTTTTTTTTT| | | | | | | | |--------------------| | | | | | | | |--------------------| | | | | | | |BBBBBBBBBBBBBBB|BBBBBBBBBBBBBBBBBBBB| --------------- -------------------- Actually, an unpegged lower texture is a tad more complicated than this. The bottom starting point on the texture map is not necessarily the bottom of the map. Instead, Doom choses the starting point based on the distance between the ceiling and the floor. The end result is that the bottom of the lower texture will line up with the bottom of the adjacent wall. If the above example were shorter, both wall sections would still align: --------------- -------------------- |TTTTTTTTTTTTTTT|TTTTTTTTTTTTTTTTTTTT| | | | | | | | |--------------------| | | | | | | | |--------------------| | | | <- bottom row cut off both walls --------------- -------------------- As you can see, unpegged upper and lower textures are used for seamless windows. NOTE: As mentioned earlier, the effect of "lower unpegged" on normal textures really does cause a bottom-up rendering of the texture. If the "lower unpegged" attribute were set for the adjacent wall in the above example, it would look like this: --------------- -------------------- | |TTTTTTTTTTTTTTTTTTTT| | | | | | | | |--------------------| | | | | | | | |--------------------| |BBBBBBBBBBBBBBB| | <- bottom rows don't match --------------- -------------------- APPLICATIONS OF UNPEGGED ATTRIBUTES ----------------------------------- The following are common situations where an understanding of pegged vs. unpegged textures is important: Windows and fixed doorways: As shown in the above example, use unpegged upper and lower textures to ensure alignment with surrounding wall sections. Doors: Doors are usually implemented as a sector where the ceiling height is the same as the floor height. The player will normally see only the upper texture which will contain the image of the door. When the door "opens", the ceiling of the door sector rises. The default pegged mode is used to let the door image rise with the ceiling since the upper texture is based at the bottom edge of the rising ceiling. The walls making up the inside tracks of the door are defined as normal textures since they are going to be on single-sided lines. Since normal textures are rendered from the ceiling down, the inside tracks will rise as the door (ceiling) rises. This is usually not the desired behaviour. By setting the "lower unpegged" bit for the inside tracks, they will remain stationary while the door opens since they will be based on the non-changing floor. The rising hallway on the other side of the acid pit in E1M3 is a classic example of the side walls remaining pegged so that they rise with the ceiling. Secret doors: Secret doors behave the same way as regular doors but add a little more complexity to texture decisions if the door is to blend in with the surrounding wall. Normally the door is closed so that the player only sees the upper texture from floor to ceiling. Since the upper texture is rendered from the bottom up, the bottom of the door (where it meets the floor) will show the bottom of the texture map. If the room has a non-standard ceiling clearance, the adjacent wall panel won't meet the floor the same way as the closed door. Rather than make the door match the adjacent walls, you can make the adjacent walls match the door by unpegging their lower texures (which re-aligns the normal textures) so that everybody builds from the floor up. The "secret" attribute may be set so that the door doesn't show up on the map. However, this will only work if the floor/ceiling height of the closed door sector is less than or equal to the floor height of the room sector. In other words, if the doorway is a step up from the room, the map will give away the secret by showing the lines immediately beyond the door even before the door is first opened. Switches and other wall decorations: The standard textures containing wall switches are 128 units high. (This also includes decorations such as "POISON" signs and gargoyles). When using these textures on a wall that is shorter, the switch will move down towards the floor since the normal wall texture is rendered from the ceiling down. To keep the switch at a reasonable height on the wall, set the "lower unpegged" bit. The wall will build up from the floor and the switch will always be at a proper height regardless of where the ceiling is. MANUAL ADJUSTMENT OF TEXTURE OFFSETS ------------------------------------ There are situations where manipulation of the "unpegged" bits will fail to create the desired alignment of wall sections. Since the "unpegged" bits only affect the vertical rendering of texture maps, they will not help when a horizontal adjustment is needed. Horizontal adjustments might be necessary when wall sections with non-standard widths are combined to create a single wall. If the texture map style creates a noticable seam, then you may adjust the horizontal and vertical offsets for each wall segment. The "X offset" parameter tells Doom how far horizontally into the texture map to begin rendering. A positive value would appear to shift the texture to the left. Likewise, the "Y offset" specifies how far vertically (from the top) to begin rendering. A positive value would appear to shift the texture upward. All three textures (upper, lower, and normal) are affected by these values. At first glance it might appear that these offets are troublesome to determine. In actual practice, it is usually easy to determine offsets for any given situation. Y offsets are the most common since they are necessary to make things appear aligned horizontally (by shifting a texture up and down, you affect the alignment of horizontal features). Generally, there are 3 values to consider when setting the Y offset: 1) The ceiling height of the current sector. 2) The ceiling height of a relevant adjoining sector. 3) The height of the texture map. Normally, the desired Y offset will turn out to be the difference between any 2 of these values. It is also useful to remember that an offset can be negative. For example instead of an offset of 100 into a 128-unit texture, an offset of -28 will do the same thing. (There's also a good chance that the "28" was easier to determine from the circumstances.) The X offset is less common and is most often used to center textures on wall segments that are not the same width as the texture. This happens when creating narrow doorways or placing switches and other decorations on short wall segments. In such cases, simply subtract the wall length from the texture length and set the X offset to half that value. A less common situation is when textures for several contiguous wall segments must appear as one wall. Most Doom textures would still appear just fine in such situations if the X offset were ignored. However, a few textures (such as computer panels) would look sloppy without carefully setting the X offsets. In such cases, the X offset for a wall would be the sum of the X offset and length of the preceeding wall (to the left). If the offset gets too large then wrap it around to the negative and continue. FLOORS AND CEILINGS ------------------- The textures used for floors and ceilings are all 64 x 64 units. Unlike wall textures, floor and ceiling textures are aligned and oriented with respect to the underlying map coordinate grid. Since these textures are tiled globally, they will always create seamless transitions from one sector to the next (assuming the adjoining sector has similar floor/ceiling textures). Some floor/ceiling textures have specific markings on them that are important. For example, some ceiling textures have a grid of lights. To ensure that the ceiling lights in a sector tile correctly, not only must the sector size be a proper multiple of a single light, but the sector itself must be placed on the map at an appropriate grid line. A transporter pad has a special design. In order for the image on the transporter pad to appear correct, the pad must be aligned on a 64 x 64 map grid. There are some special stair step textures (STEP1 and STEP2). These could be used for the top surface of each stair. If you wish to use these textures for your stairs, make sure you pick the texure with the proper north/south or east/west orientation and snap your stairs to a 64-unit width and a 32-unit depth.