Hvad er tekstur?

Texture - What is Texture?

From Helen South's Drawing/Sketching Glossary

Definition: Texture describes the tactile quality of a form. Accurate rendering of an object's texture is the key to very realistic (particularly 'photo-realistic') drawing.

The textures of some objects can be particularly challenging due to movement (water), fine detail (skin surface and hair, grass, leaves) or their ethereal quality (cloud, glass).
The texture in a drawing is also a product of the support, such as paper or canvas. Some papers, particularly those for pastel and watercolor, have a textured surface due to the fibres or the mold used to make the paper. Different mediums will show up the inherent texture in the paper.
As well as creating the illusion of texture in drawing, artists often use the inherent qualities of a medium combined with various forms of mark-making, such as rough shading, smooth blending, hatching or scumbling, to create interest within an image.

Examples:

I search for the realness, the real feeling of a subject, all the texture around it... I always want to see the third dimension of something... I want to come alive with the object. - Andrew Wyeth


Definition: (noun) - Texture, another element of art, is used to describe either the way a three-dimensional work actually feels when touched, or the visual "feel" of a two-dimensional work.
Take rocks, for example. A real, 3-D rock might feel rough or smooth, and definitely feels hard when touched or picked up. A painter, depicting a rock, would create the illusions of these qualities through use of color, line, shape, etc.

What is texture?

Everyday texture terms - rough, silky, bumpy - refer to touch.

A texture that is rough to touch has:

• a large difference between high and low points, and
• a space between highs and lows approximately the same size as a finger.

Silky would have

• little difference between high and low points, and
• the differences would be spaced very close together relative to finger size.

Image texture works in the same way, except the highs and lows are brightness values (also called grey levels, GL,  or digital numbers, DN) instead of elevation changes. Instead of probing a finger over the surface, a "window" - a (usually square) box defining the size of the probe - is used.

Textures in images quantify:

• Grey level differences (contrast)
• Defined size of area where change occurs (window)
• Directionality or lack of it

tekstur

What is a Texture?

The natural world is rich in texture: the surface of any visible object is textured at certain scale. A wealth of textures are observed on both artificial and natural objects such as those on wood, plants, materials and skin. In a general sense, the word texture refers to surface characteristics and appearance of an object given by the size, shape, density, arrangement, proportion of its elementary parts [99]. A texture is usually described as smooth or rough, soft or hard, coarse of fine, matt or glossy, and etc.
Textures might be divided into two categories, namely, tactile and visual textures. Tactile textures refer to the immediate tangible feel of a surface. Visual textures refer to the visual impression that textures produce to human observer, which are related to local spatial variations of simple stimuli like colour, orientation and intensity in an image. This thesis focuses only on visual textures, so the term `texture' thereafter is exclusively referred to `visual texture' unless mentioned otherwise.
Textures are usually given by examples in digitised images. Figures 4.1 and 4.2 show a few natural and man-made textures, respectively, which could be met in daily life.
Although texture is an important research area in computer vision, there is no precise definition of the notion texture. The main reason is that natural textures often display different yet contradicting properties, such as regularity versus randomness, uniformity versus distortion, which can hardly be described in a unified manner. Many researchers have been trying to define textures from a certain perspective of their nature. Haralick considers a texture as an ``organised area phenomenon'' which can be decomposed into `primitives' having specific spatial distributions [43]. This definition, also known as structural approach, comes directly from human visual experience of textures. For instance, each texture in Figs 4.1 and 4.2 is composed of particular texture elements, e.g., objects (windows), shapes (jigsaw pieces), or simply colour patterns. Meanwhile, these primitives are organised in a particular spatial structure indicating certain underlying placement rules. Alternatively, as Cross and Jain suggested, a texture is ``a stochastic, possibly periodic, two-dimensional image field'' [20]. This definition describes a texture by a stochastic process that generates the texture, which is also known as stochastic approach. These different definitions usually lead to different computational approaches to texture analysis.

Figure 4.1: Examples of natural textures.

Figure 4.2: Examples of artificial regular textures.

Nevertheless, an apparent consensus that spatial homogeneity is the most important property of a texture has been reached. From the statistical point of view, homogeneity means statistical stationarity, i.e. that certain signal statistics of each texture region have the same values. This property relates directly to self-similarity: the patterns at different magnifications, although not identical, are represented by the same signal statistics [95].
Textures also exhibit local non-homogeneity, i.e. departures from strict homogeneity to some extent in a local image region. For example, in the image `leaves' in Fig 4.1, every single leaf is slightly different from another (local non-homogeneity), but as a whole they display approximate spatial uniformity and consistency (global homogeneity).

Figure 4.3: Examples of natural regular textures.

Due to the diversity and complexity of natural textures, it is useful to separate them into categories. For instance, textures can be classified into regular and stochastic ones by their degree of randomness. A regular texture is formed by regular tiling of easily identifiable small size elements organised into strong periodic patterns. A stochastic texture exhibits less noticeable elements and display rather random patterns. For examples, textures in Figs 4.1 and 4.4 are mostly stochastic, and those in Figs 4.2 and 4.3 are regular. Most of real world textures, however, are mixtures of the above-mentioned categories.

Figure 4.4: Examples of stochastic textures.

By spatial homogeneity, textures can be classified into homogeneous, weakly-homogeneous, and inhomogeneous patterns. Specifically, homogeneous texture contains ideal repetitive structures, and such uniformity produces idealised patterns. Weak homogeneity involves local spatial variation in texture elements or their spatial arrangement, which leads to more or less violates the precise repetitiveness (See Fig 4.5). An `inhomogeneous texture' mostly refers to an image where repetition and spatial self-similarity are absent. Since spatial homogeneity is considered below as an essential property of a texture, an inhomogeneous image is not treated in this thesis as a `texture'.

Figure 4.5: Examples of weakly-homogeneous textures.