calculating the ratio between the sizes of the object projected on distance of an object with arbitrary texture surface . Since the mapping w = log(z) transforms. PDF | The undesired () texture component for the magnetic properties mainly exists in transformation, and analyze the opposite relationship between the .. (d–f) Orientation map shown in IPF-Z of (a)–(c) in sequence. ODF's, the experimental textures are transformed into their bcc equivalents according to the Bain, according to the Bain, K-S and N-W relationships to yield textures correspond- ing to the bcc a .. Z. Physik 64, Liicke, K.
Texture mapping - Wikipedia
Texture also determines local spectral or frequency content in an image; changes in local texture should cause changes in the local spatial frequency. Texture analysis is of interest in medical imaging because, as biological tissues become abnormal during a disease process, their underlying texture may also change.
Various mathematical techniques to quantify image texture, including statistical, Fourier, and wavelet-based methods, have been applied to radiological images of numerous pathologies, such as multiple sclerosis 45brain tumors 6liver diseases 7infarcted myocardial tissue 8and normal tissues of the knee 9and has even been used for automated detection and classification based on phase-contrast microscopy images, such as those used in cervical cancer diagnosis A texture feature is a value that quantifies some characteristic of local intensity variation within an image 1.
A variety of approaches exist to quantify texture.
A common technique used in medical imaging is based on co-occurrence matrices Statistical measures of texture are calculated based on the frequency of specific gray levels occurring between pairs of points within an image.
The co-occurrence technique has been used in many studies, such as to classify benign and malignant solitary pulmonary nodules on computed tomography CT images 12 and to quantify pathological changes during treatment of multiple sclerosis However, the technique is limited to very small neighborhoods due to its computation complexity. Therefore, only the highest frequency textures can be analyzed. Broad, large-scale changes are difficult to detect using co-occurrence statistics.Going from Laplace to Z Transform
Furthermore, the resulting statistical measures are difficult to interpret and compare across patients. A more recent method of texture analysis relies on discrete wavelet transform DWT see 14 for a review. Wavelets provide a multi-scale representation of an image, allowing analysis of varying degrees of detail within an image.
Efficient algorithms and a solid mathematical framework make wavelets appealing for numerous applications, including texture analysis. For example, wavelet-based texture analysis has been used for automatic diagnosis and grading of breast tumor histology images The S-transform 16 ST is closely related to the continuous wavelet transform using a complex Morlet mother wavelet 17 and directly measures the local spatial frequency content of each pixel in an image.
The ST has been successfully used to analyze signals in numerous applications, such as seismic recordings 18ground vibrations 19hydrology 20gravitational waves 21and power system analysis The one-dimensional ST has shown to be a useful tool for analysis of medical signals, such as EEG 23functional magnetic resonance imaging 24and laser Doppler flowmetry The ST is particularly well suited to texture analysis of medical images due to its optimal space-frequency resolution and close ties to the Fourier transform FT —the basis of medical image reconstruction.
The ST uses complex Fourier basis functions, modulated by frequency-dependent Gaussian windows. The ST preserves the phase information, uses a linear frequency scale, and can be easily inverted to recover the Fourier domain of an image. The redundant nature of the ST algorithm has been the main obstacle in wider application of ST-based texture analysis for 2D images. Extensive processing time and a large amount of memory are required to calculate and store the texture descriptions of large medical images.
While other researchers have developed techniques to distribute these calculations over networks of machines 26the results are still difficult to manage and interpret Therefore, previous work on 2D images has been limited to analysis of small regions of interest ROIs and typically collapsed to 1D spectra 28 — Bump mapping has become popular in recent video games, as graphics hardware has become powerful enough to accommodate it in real-time.
The cheapest method is to use the nearest-neighbour interpolationbut 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. Baking[ edit ] As an optimization, it is possible to render detail from a high resolution model or expensive process such as global illumination into a surface texture possibly on a low resolution model.
This is also known as render mapping. This technique is most commonly used for lightmapping but may also be used to generate normal maps and displacement maps. Some video games e. Messiah have used this technique. The original Quake software engine used on-the-fly baking to combine light maps and colour texture-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.
Rasterisation algorithms[ edit ] Various techniques have evolved in software and hardware implementations. Each offers different trade-offs in precision, versatility and performance. Forward texture mapping[ edit ] Some hardware systems e. 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.
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. This provides a limited form of perspective correction.
However, perspective distortion is still visible for primitives near the camera e.
This technique is not used in modern hardware because UV coordinates have proved more versatile for modelling and more consistent for clipping. Inverse texture mapping[ edit ] 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.