Texture mapping

Texture maps may be acquired by scanning/digital photography, designed in image manipulation software such as GIMP, Photoshop, or painted onto 3D surfaces directly in a 3D paint tool such as Mudbox or zbrush.

This process is akin to applying patterned paper to a plain white box. Every vertex in a polygon is assigned a texture coordinate (which in the 2d case is also known as UV coordinates). This may be done through explicit assignment of vertex attributes, manually edited in a 3D modelling package through UV unwrapping tools. It is also possible to associate a procedural transformation from 3d space to texture space with the material. This might be accomplished via planar projection or, alternatively, cylindrical or spherical mapping. More complex mappings may consider the distance along a surface to minimize distortion. These coordinates are interpolated across the faces of polygons to sample the texture map during rendering. Textures may be repeated or mirrored to extend a finite rectangular bitmap over a larger area, or they may have a one-to-one unique "injective" mapping from every piece of a surface (which is important for render mapping and light mapping, also known as baking).

Texture mapping maps the model surface (or screen space during rasterization) into texture space; in this space, the texture map is visible in its undistorted form. UV unwrapping tools typically provide a view in texture space for manual editing of texture coordinates. Some rendering techniques such as subsurface scattering may be performed approximately by texture-space operations.

Multitexturing is the use of more than one texture at a time on a polygon.[8] For instance, a light map texture may be used to light a surface as an alternative to recalculating that lighting every time the surface is rendered. Microtextures or detail textures are used to add higher frequency details, and dirt maps may add weathering and variation; this can greatly reduce the apparent periodicity of repeating textures. Modern graphics may use more than 10 layers, which are combined using shaders, for greater fidelity. Another multitexture technique is bump mapping, which allows a texture to directly control the facing direction of a surface for the purposes of its lighting calculations; it can give a very good appearance of a complex surface (such as tree bark or rough concrete) that takes on lighting detail in addition to the usual detailed coloring. Bump mapping has become popular in recent video games, as graphics hardware has become powerful enough to accommodate it in real-time.[9]

The way that samples (e.g. when viewed as pixels on the screen) are calculated from the texels (texture pixels) is governed by texture filtering. The cheapest method is to use the nearest-neighbour interpolation, but bilinear interpolation or trilinear interpolation between mipmaps are two commonly used alternatives which reduce aliasing or jaggies. In the event of a texture coordinate being outside the texture, it is either clamped or wrapped. Anisotropic filtering better eliminates directional artefacts when viewing textures from oblique viewing angles.

Texture streaming is a means of using data streams for textures, where each texture is available in two or more different resolutions, as to determine which texture should be loaded into memory and used based on draw distance from the viewer and how much memory is available for textures. Texture streaming allows for rendering engine to use low resolution textures for objects far away from the viewer's camera, and resolve those into more detailed textures, read from a data source, as the point of view nears the objects.

As an optimization, it is possible to render detail from a complex, high-resolution model or expensive process (such as global illumination) into a surface texture (possibly on a low-resolution model). Baking is also known as render mapping. This technique is most commonly used for light maps, but may also be used to generate normal maps and displacement maps. Some computer games (e.g. Messiah) have used this technique. The original Quake software engine used on-the-fly baking to combine light maps and colour maps ("surface caching").

Baking can be used as a form of level of detail generation, where a complex scene with many different elements and materials may be approximated by a single element with a single texture, which is then algorithmically reduced for lower rendering cost and fewer drawcalls. It is also used to take high-detail models from 3D sculpting software and point cloud scanning and approximate them with meshes more suitable for realtime rendering.

Some hardware systems e.g. Sega Saturn and the NV1 traverse texture coordinates directly, interpolating the projected position in screen space through texture space and splatting the texels into a frame buffer. (in the case of the NV1, quadratic interpolation was used allowing curved rendering). Sega provided tools for baking suitable per-quad texture tiles from UV mapped models.

This has the advantage that texture maps are read in a simple linear fashion.

Forward texture mapping may also sometimes produce more natural looking results than affine texture mapping if the primitives are aligned with prominent texture directions (e.g. road markings or layers of bricks). This provides a limited form of perspective correction. However, perspective distortion is still visible for primitives near the camera (e.g. the Saturn port of Sega Rally exhibited texture-squashing artifacts as nearby polygons were near clipped without UV coordinates).

This technique is not used in modern hardware because UV coordinates have proved more versatile for modelling and more consistent for clipping.

Most approaches use inverse texture mapping, which traverses the rendering primitives in screen space whilst interpolating texture coordinates for sampling. This interpolation may be affine or perspective correct. One advantage is that each output pixel is guaranteed to only be traversed once; generally the source texture map data is stored in some lower bit-depth or compressed form whilst the frame buffer uses a higher bit-depth. Another is greater versatility for UV mapping. A texture cache becomes important for buffering reads, since the memory access pattern in texture space is more complex.

Because affine texture mapping does not take into account the depth information about a polygon's vertices, where the polygon is not perpendicular to the viewer, it produces a noticeable defect.

Affine texture mapping linearly interpolates texture coordinates across a surface, and so is the fastest form of texture mapping. Some software and hardware (such as the original PlayStation) project vertices in 3D space onto the screen during rendering and linearly interpolate the texture coordinates in screen space between them ("inverse texture mapping"). This may be done by incrementing fixed point UV coordinates, or by an incremental error algorithm akin to Bresenham's line algorithm.

In contrast to perpendicular polygons, this leads to noticeable distortion with perspective transformations (see figure: the checker box texture appears bent), especially as primitives near the camera. Such distortion may be reduced with the subdivision of the polygon into smaller ones.

Doom engine renders vertical and horizontal spans with affine texture mapping, and is therefore unable to draw ramped floors or slanted walls.

Perspective correct texturing accounts for the vertices' positions in 3D space, rather than simply interpolating coordinates in 2D screen space.[10] This achieves the correct visual effect but it is more expensive to calculate.[10]

To perform perspective correction of the texture coordinates ${\displaystyle u}$ and ${\displaystyle v}$, with ${\displaystyle z}$ being the depth component from the viewer's point of view, we can take advantage of the fact that the values ${\displaystyle {\frac {1}{z}}}$, ${\displaystyle {\frac {u}{z}}}$, and ${\displaystyle {\frac {v}{z}}}$ are linear in screen space across the surface being textured. In contrast, the original ${\displaystyle z}$, ${\displaystyle u}$ and ${\displaystyle v}$, before the division, are not linear across the surface in screen space. We can therefore linearly interpolate these reciprocals across the surface, computing corrected values at each pixel, to result in a perspective correct texture mapping.

To do this, we first calculate the reciprocals at each vertex of our geometry (3 points for a triangle). For vertex ${\displaystyle n}$ we have ${\displaystyle {\frac {u_{n}}{z_{n}}},{\frac {v_{n}}{z_{n}}},{\frac {1}{z_{n}}}}$. Then, we linearly interpolate these reciprocals between the ${\displaystyle n}$ vertices (e.g., using Barycentric Coordinates), resulting in interpolated values across the surface. At a given point, this yields the interpolated ${\displaystyle u_{i},v_{i}}$, and ${\displaystyle zReciprocal_{i}={\frac {1}{z_{i}}}}$. Note that this ${\displaystyle u_{i},v_{i}}$ cannot be yet used as our texture coordinates as our division by ${\displaystyle z}$ altered their coordinate system.

To correct back to the ${\displaystyle u,v}$ space we first calculate the corrected ${\displaystyle z}$ by again taking the reciprocal ${\displaystyle z_{correct}={\frac {1}{zReciprocal_{i}}}={\frac {1}{\frac {1}{z_{i}}}}}$. Then we use this to correct our ${\displaystyle u_{i},v_{i}}$: ${\displaystyle u_{correct}=u_{i}\cdot z_{i}}$ and ${\displaystyle v_{correct}=v\cdot z_{i}}$.[11]

This correction makes it so that in parts of the polygon that are closer to the viewer the difference from pixel to pixel between texture coordinates is smaller (stretching the texture wider) and in parts that are farther away this difference is larger (compressing the texture).

Affine texture mapping directly interpolates a texture coordinate ${\displaystyle u_{\alpha }^{}}$ between two endpoints ${\displaystyle u_{0}^{}}$ and ${\displaystyle u_{1}^{}}$:

${\displaystyle u_{\alpha }^{}=(1-\alpha )u_{0}+\alpha u_{1}}$ where ${\displaystyle 0\leq \alpha \leq 1}$

Perspective correct mapping interpolates after dividing by depth ${\displaystyle z_{}^{}}$, then uses its interpolated reciprocal to recover the correct coordinate:

${\displaystyle u_{\alpha }^{}={\frac {(1-\alpha ){\frac {u_{0}}{z_{0}}}+\alpha {\frac {u_{1}}{z_{1}}}}{(1-\alpha ){\frac {1}{z_{0}}}+\alpha {\frac {1}{z_{1}}}}}}$

3D graphics hardware typically supports perspective correct texturing.

Various techniques have evolved for rendering texture mapped geometry into images with different quality/precision tradeoffs, which can be applied to both software and hardware.

Classic software texture mappers generally did only simple mapping with at most one lighting effect (typically applied through a lookup table), and the perspective correctness was about 16 times more expensive.

The Doom engine restricted the world to vertical walls and horizontal floors/ceilings, with a camera that could only rotate about the vertical axis. This meant the walls would be a constant depth coordinate along a vertical line and the floors/ceilings would have a constant depth along a horizontal line. A fast affine mapping could be used along those lines because it would be correct. Some later renderers of this era simulated a small amount of camera pitch with shearing which allowed the appearance of greater freedom whilst using the same rendering technique.

Some engines were able to render texture mapped Heightmaps (e.g. Nova Logic's Voxel Space, and the engine for Outcast) via Bresenham-like incremental algorithms, producing the appearance of a texture mapped landscape without the use of traditional geometric primitives.[12]

Every triangle can be further subdivided into groups of about 16 pixels in order to achieve two goals. First, keeping the arithmetic mill busy at all times. Second, producing faster arithmetic results.

For perspective texture mapping without hardware support, a triangle is broken down into smaller triangles for rendering and affine mapping is used on them. The reason this technique works is that the distortion of affine mapping becomes much less noticeable on smaller polygons. The Sony PlayStation made extensive use of this because it only supported affine mapping in hardware but had a relatively high triangle throughput compared to its peers.

Software renderers generally preferred screen subdivision because it has less overhead. Additionally, they try to do linear interpolation along a line of pixels to simplify the set-up (compared to 2d affine interpolation) and thus again the overhead (also affine texture-mapping does not fit into the low number of registers of the x86 CPU; the 68000 or any RISC is much more suited).

A different approach was taken for Quake, which would calculate perspective correct coordinates only once every 16 pixels of a scanline and linearly interpolate between them, effectively running at the speed of linear interpolation because the perspective correct calculation runs in parallel on the co-processor.[13] The polygons are rendered independently, hence it may be possible to switch between spans and columns or diagonal directions depending on the orientation of the polygon normal to achieve a more constant z but the effort seems not to be worth it.

Screen space sub division techniques. Top left: Quake-like, top right: bilinear, bottom left: const-z

Another technique was approximating the perspective with a faster calculation, such as a polynomial. Still another technique uses 1/z value of the last two drawn pixels to linearly extrapolate the next value. The division is then done starting from those values so that only a small remainder has to be divided[14] but the amount of bookkeeping makes this method too slow on most systems.

Finally, the Build engine extended the constant distance trick used for Doom by finding the line of constant distance for arbitrary polygons and rendering along it.

Texture mapping hardware was originally developed for simulation (e.g. as implemented in the Evans and Sutherland ESIG image generators), and professional graphics workstations such as Silicon Graphics, broadcast digital video effects machines such as the Ampex ADO and later appeared in Arcade cabinets, consumer video game consoles, and PC video cards in the mid 1990s. In flight simulation, texture mapping provided important motion cues.

Modern graphics processing units (GPUs) provide specialised fixed function units called texture samplers, or texture mapping units, to perform texture mapping, usually with trilinear filtering or better multi-tap anisotropic filtering and hardware for decoding specific formats such as DXTn. As of 2016, texture mapping hardware is ubiquitous as most SOCs contain a suitable GPU.

Some hardware combines texture mapping with hidden-surface determination in tile based deferred rendering or scanline rendering; such systems only fetch the visible texels at the expense of using greater workspace for transformed vertices. Most systems have settled on the Z-buffering approach, which can still reduce the texture mapping workload with front-to-back sorting.

It is possible to use texture mapping hardware to accelerate both the reconstruction of voxel data sets from tomographic scans, and to visualize the results[15]

Many user interfaces use texture mapping to accelerate animated transitions of screen elements, e.g. Exposé in Mac OS X.