what's the difference between material and texture?

Question

In Computer graphics, what's the difference between material and texture?

Answer 1

In OpenGL, a material is a set of coefficients that define how the lighting model interacts with the surface. In particular, ambient, diffuse, and specular coefficients for each color component (R,G,B) are defined and applied to a surface and effectively multiplied by the amount of light of each kind/color that strikes the surface. A final emmisivity coefficient is then added to each color component that allows objects to appear luminous without actually interacting with other objects.

A texture, on the other hand, is a set of 1-, 2-, 3-, or 4- dimensional bitmap (image) data that is applied and interpolated on to a surface according to texture coordinates at the vertices. Texture data alters the color of the surface whether or not lighting is enabled (and depending on the texture mode, e.g. decal, modulate, etc.). Textures are used frequently to provide sub-polygon level detail to a surface, e.g. applying a repeating brick and mortar texture to a quad to simulate a brick wall, rather than modeling the geometry of each individual brick.

In the classical (fixed-pipeline) OpenGL model, textures and materials are somewhat orthogonal. In the new programmable shader world, the line has blurred quite a bit. Frequently textures are used to influence lighting in other ways. For example, bump maps are textures that are used to perturb surface normals to effect lighting, rather than modifying pixel color directly as a regular "image" texture would.

Answer 2

The question suggests a common misunderstanding of various computer graphics concepts. It is one born of pre-shader thinking and coding.

A texture is nothing more than a container for a series of one or more images, where an image is an array of some dimensionality (1D, 2D, etc) of values, where each value can be a vector of between 1 and 4 numbers. Textures also have some special techniques for accessing values from them that allow for interpolation and the minimizing of aliasing artifacts from sampling.

A texture can contain colors, but textures do not have to contain colors. Textures can be used to vary parameters across an objects surface, but that is not all textures can be used for.

Textures have no direct association with "materials"; you can use them for a wide variety of things (or nothing at all).

A material is a concept in lighting. Given a particular light and a point on the surface, the intensity (ie: color) of light reflected from that surface at that point is governed by a lighting equation. This equation is a function of many parameters. But those parameters are grouped into two categories.

One category of light equation parameters are the light parameters. These describe the characteristics of the light source. Point lights vs. directional lights vs. spot lights. The light intensity (again: color) is another parameter. Since the intensity itself may vary depending on the direction of the surface point relative to the light (think flashlights or spotlights), the intensity may be accessed from a texture. That's how many games project flashlights over a dark room.

The other category of light equation parameters describes the characteristics of the surface at that point. These are called material parameters. The material parameters, or material for short, describe important properties of the surface at the point in question. The normal at that point is an important one. There is also the diffuse reflectance (color), specular reflectance, specular shininess (exponent for Phong/Blinn-Phong) and various other parameters, depending on how comprehensive your lighting equation is.

Where do these values come from? Light parameters tend to be fixed in the world. Lights don't move per-object (though if you're doing lighting in object space, then each object would have its own light position). The light intensity may vary. But that's mostly it; anything else happens between frames, not within a single frame's rendering. So most light parameters are shader uniforms.

Material parameters can come from a variety of sources. Using a shader uniform effectively means that all points on the surface have that same value. So your could have the diffuse color come from a uniform, which would give the surface a uniform color (modified by lighting, of course). You can vary material parameters per-vertex, by passing them as vertex attributes or computing them from other attributes.

Or you can provide a parameter by mapping a texture to a surface. This mapping involves associating texture coordinates with vertex positions, so that the texture is directly attached to the surface. The texture is sampled at that location during rendering, and that value is used to perform the lighting.

The most common textures you're familiar with, "color textures", are simply varying the diffuse reflectance of the surface. The texture provides the diffuse color at each point along the surface.

This is not the only function of textures. You could just as easily vary the specular shininess over the surface. Or the light intensity. Or anything else.

Textures are tools. Materials are just a group of light equation parameters.