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Shading

shading, shading techniques
Shading refers to depicting depth perception in 3D models or illustrations by varying levels of darkness

Example of flat shading vs Phong shading interpolation Phong shading is a more realistic shading technique, developed by Bui Tuong Phong in 1973

Contents

  • 1 Drawing
  • 2 Computer graphics
    • 21 Angle to light source
    • 22 Lighting
      • 221 Ambient lighting
      • 222 Directional lighting
      • 223 Point lighting
      • 224 Spotlight lighting
      • 225 Area lighting
      • 226 Volumetric lighting
    • 23 Distance falloff
  • 3 Flat shading
  • 4 Smooth shading
    • 41 Gouraud shading
      • 411 Data structures
      • 412 Advantages
      • 413 Problems
    • 42 Phong shading
    • 43 Other Approaches
  • 5 Flat vs smooth shading
  • 6 See also
  • 7 References
  • 8 Further reading

Drawingedit

Example of shading

Shading is used in drawing for depicting levels of darkness on paper by applying media more densely or with a darker shade for darker areas, and less densely or with a lighter shade for lighter areas There are various techniques of shading including cross hatching where perpendicular lines of varying closeness are drawn in a grid pattern to shade an area The closer the lines are together, the darker the area appears Likewise, the farther apart the lines are, the lighter the area appears

Light patterns, such as objects having light and shaded areas, help when creating the illusion of depth on paper12

Powder shading is a sketching shading method In this style, the stumping powder and paper stumps are used to draw a picture This can be in color The stumping powder is smooth and doesn't have any shiny particles The paper to be used should have small grains on it so that the powder remains on the paper

Computer graphicsedit

In computer graphics, shading refers to the process of altering the color of an object/surface/polygon in the 3D scene, based on its angle to lights and its distance from lights to create a photorealistic effect Shading is performed during the rendering process by a program called a shader

Angle to light sourceedit

Shading alters the colors of faces in a 3D model based on the angle of the surface to a light source or light sources

The first image below has the faces of the box rendered, but all in the same color Edge lines have been rendered here as well which makes the image easier to see

The second image is the same model rendered without edge lines It is difficult to tell where one face of the box ends and the next begins

The third image has shading enabled, which makes the image more realistic and makes it easier to see which face is which

Rendered image of a box This image has no shading on its faces, but uses edge lines to separate the faces This is the same image with the edge lines removed This is the same image rendered with shading of the faces to alter the colors of the 3 faces based on their angle to the light sources

Lightingedit

Shading effects from floodlight

Shading is also dependent on the lighting used Usually, upon rendering a scene a number of different lighting techniques will be used to make the rendering look more realistic Different types of light sources are used to give different effects

Ambient lightingedit

An ambient light source represents an omni-directional, fixed-intensity and fixed-color light source that affects all objects in the scene equally Upon rendering, all objects in the scene are brightened with the specified intensity and color This type of light source is mainly used to provide the scene with a basic view of the different objects in it This is the simplest type of lighting to implement and models how light can be scattered or reflected many times producing a uniform effect

Ambient lighting can be combined with ambient occlusion to represent how exposed each point of the scene is, affecting the amount of ambient light it can reflect This produces diffuse, non-directional lighting throughout the scene, casting no clear shadows, but with enclosed and sheltered areas darkened The result is usually visually similar to an overcast day

Directional lightingedit

A directional light source illuminates all objects equally from a given direction, like an area light of infinite size and infinite distance from the scene; there is shading, but cannot be any distance falloff

Point lightingedit

Light originates from a single point, and spreads outward in all directions

Spotlight lightingedit

Models a Spotlight Light originates from a single point, and spreads outward in a cone

Area lightingedit

Light originates from a small area on a single plane A more realistic model than a point light source

Volumetric lightingedit

Light originating from a small volume, an enclosed space lighting objects within that space

Shading is interpolated based on how the angle of these light sources reach the objects within a scene Of course, these light sources can be and often are combined in a scene The renderer then interpolates how these lights must be combined, and produces a 2d image to be displayed on the screen accordingly

Distance falloffedit

Theoretically, two surfaces which are parallel are illuminated the same amount from a distant light source, such as the sun Even though one surface is further away, your eye sees more of it in the same space, so the illumination appears the same

Notice in the first image that the color on the front faces of the two boxes is exactly the same It appears that there is a slight difference where the two faces meet, but this is an optical illusion because of the vertical edge below where the two faces meet

Notice in the second image that the surfaces on the boxes are bright on the front box and darker on the back box Also the floor goes from light to dark as it gets farther away

This distance falloff effect produces images which appear more realistic without having to add additional lights to achieve the same effect

Two boxes rendered with an OpenGL renderer Note that the colors of the two front faces are the same even though one box is farther away The same model rendered using ARRIS CAD which implements "Distance Falloff" to make surfaces that are closer to the eye appear brighter

Distance falloff can be calculated in a number of ways:

  • None - The light intensity received is the same regardless of the distance between the point and the light source
  • Linear - For a given point at a distance x from the light source, the light intensity received is proportional to 1/x
  • Quadratic - This is how light intensity decreases in reality if the light has a free path ie no fog or any other thing in the air that can absorb or scatter the light For a given point at a distance x from the light source, the light intensity received is proportional to 1/x2
  • Factor of n - For a given point at a distance x from the light source, the light intensity received is proportional to 1/xn
  • Any number of other mathematical functions may also be used

Flat shadingedit

Flat shading is a lighting technique used in 3D computer graphics to shade each polygon of an object based on the angle between the polygon's surface normal and the direction of the light source, their respective colors and the intensity of the light source It is usually used for high speed rendering where more advanced shading techniques are too computationally expensive As a result of flat shading all of the polygon's vertices are colored with one color, allowing differentiation between adjacent polygons Specular highlights are rendered poorly with flat shading: If there happens to be a large specular component at the representative vertex, that brightness is drawn uniformly over the entire face If a specular highlight doesn’t fall on the representative point, it is missed entirely Consequently, the specular reflection component is usually not included in flat shading computation

Smooth shadingedit

In contrast to flat shading with smooth shading the color changes from pixel to pixel It assumes that the surfaces are curved and uses interpolation techniques to calculate the values of pixels between the vertices of the polygons

Types of smooth shading include:

  • Gouraud shading 3
  • Phong shading 4

Gouraud shadingedit

  1. Determine the normal at each polygon vertex
  2. Apply an illumination model to each vertex to calculate the vertex intensity
  3. Interpolate the vertex intensities using bilinear interpolation over the surface polygon

Data structuresedit

  • Sometimes vertex normals can be computed directly eg height field with uniform mesh
  • More generally, need data structure for mesh
  • Key: which polygons meet at each vertex

Advantagesedit

  • Polygons, more complex than triangles, can also have different colors specified for each vertex In these instances, the underlying logic for shading can become more intricate

Problemsedit

  • Even the smoothness introduced by Gouraud shading may not prevent the appearance of the shading differences between adjacent polygons
  • Gouraud shading is more CPU intensive and can become a problem when rendering real time environments with many polygons
  • T-Junctions with adjoining polygons can sometimes result in visual anomalies In general, T-Junctions should be avoided

Phong shadingedit

Phong shading is similar to Gouraud shading, except that the Normals are interpolated Thus, the specular highlights are computed much more precisely than in the Gouraud shading model:

  1. Compute a normal N for each vertex of the polygon
  2. From bilinear interpolation compute a normal, Ni for each pixel This must be renormalized each time
  3. From Ni compute an intensity Ii for each pixel of the polygon
  4. Paint pixel to shade corresponding to Ii

Other Approachesedit

Both Gouraud shading and Phong shading can be implemented using bilinear interpolation Bishop and Weimer 5 proposed to use a Taylor series expansion of the resulting expression from applying an illumination model and bilinear interpolation of the normals Hence, second degree polynomial interpolation was used This type of biquadratic interpolation was further elaborated by Barrera et al,6 where one second order polynomial was used to interpolate the diffuse light of the Phong reflection model and another second order polynomial was used for the specular light

Spherical Linear Interpolation Slerp was used by Kuij and Blake 7 for computing both the normal over the polygon as well as the vector in the direction to the light source A similar approach was proposed by Hast,8 which uses Quaternion interpolation of the normals with the advantage that the normal will always have unit length and the computationally heavy normalization is avoided

Flat vs smooth shadingedit

Flat Smooth
Uses the same color for every pixel in a face - usually the color of the first vertex Smooth shading uses linear interpolation of colors between vertices
Edges appear more pronounced than they would on a real object because of a phenomenon in the eye known as lateral inhibition The edges disappear with this technique
Same color for any point of the face Each point of the face has its own color
Individual faces are visualized Visualize underlying surface
Not suitable for smooth objects Suitable for any objects
Less computationally expensive More computationally expensive

See alsoedit

  • Computer graphics portal
  • 3D computer graphics
  • Shader
  • List of common shading algorithms
  • Zebra striping computer graphics

Referencesedit

  1. ^ "Drawing Techniques" Drawing With Confidence Retrieved 19 September 2012 
  2. ^ "Shading Tutorial, How to Shade in Drawing" Dueysdrawingscom 2007-06-21 Retrieved 2012-02-11 
  3. ^ Gouraud, Henri 1971 "Continuous shading of curved surfaces" IEEE Transactions on Computers C–20 6: 623–629 doi:101109/T-C1971223313 
  4. ^ B T Phong, Illumination for computer generated pictures, Communications of ACM 18 1975, no 6, 311–317
  5. ^ Gary Bishop and David M Weimer 1986 Fast Phong shading SIGGRAPH Comput Graph 20, 4 August 1986, 103-106
  6. ^ T Barrera, A Hast, E Bengtsson Fast Near Phong-Quality Software Shading WSCG'06, pp 109-116 2006
  7. ^ Kuijk, A A M and E H Blake, Faster Phong shading via angular interpolation Computer Graphics Forum 84:315-324 1989
  8. ^ A Hast Shading by Quaternion Interpolation WSCG'05 pp 53-56 2005

Further readingedit

  • Introduction to Shading

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    Shading beatiful post thanks!

    29.10.2014


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