Lect 2 Projections and Shading Flashcards
- What is the window and how is it mapped onto a screen?
- It’s rarely necessary to display the whole of a scene, so clipping takes place.
- The actual part of the scene in world coordinates that is to be displayed is called a window.
- On a screen, the picture inside this window is mapped onto the viewport – the available display area.
- Describe the steps from world coordinates to **device coordinates. **
- Start with World Coordinates.
- Clip to Viewing Window
- Map to Viewport
- Convert to Device Coordinates.
Easy 2D to 2D.. 3D to 2D much harder!
3.
Display device (a screen) is 2D
How do we map 3D objects to 2D space?
• 2D to 2D is straight forward…
– 2D window to world.. and a viewport on the 2D surface. – Clip what won’t be shown in the 2D window, and map the remainder to the viewport.
• 3D to 2D is more complicated…
– Solution : Transform 3D objects on to a 2D plane using projections
- What are the main Types of Viewing?
-
Classical (many)
Orthographic One-, two-, and 3-point perspectives -
Computer
Orthographic Perspective
- What are (COP) and (DOP)?
• Center of Projection (COP)
– Point where all projectors meet
– Center of camera or eye lens
– Origin of synthetic camera frame
• Direction of Projection (DOP)
– Direction of projectors when COP is moved to infinity
- Describe how perspective defines a scene.
- Characterized by diminution of size of more distant objects
- Classically, viewer is symmetrical with respect to the projection plane
- One-, two-, and three- point perspectives depending on number of vanishing points
- How is a 3D scene projected?
In 2D we have a window in world coordinates.
In 3D we have a view volume!
– View volume in the world (a view volume is the 3D volume that contains everything visible from the point of view or direction)
– Projection onto the 2D projection plane
– A viewport to the view surface
- Model the 3D viewing Process. Highlight each stage.
• Process…
– 1… clip against the view volume,
– 2… project to 2D plane, or window,
– 3… map to viewport.
- Name the key terms used for **projections. **
• Projections: key terms…
– Projection from 3D to 2D is defined by straight projection rays (projectors) emanating from the ‘center of projection’ (COP), passing through each point of the object, and intersecting the ‘projection plane’ to form a projection.
- What are the two types of projections in computing?
The two types of projections are:
- Perspective
- Parallel
The key factor is (COP)
if distance to the center of projection (COP) is finite then perspective
if distance is infinite: Parallel
- Perspective v Parallel
**• Perspective: **- visual effect is similar to human visual system…
– has ‘perspective foreshortening’ (size of object varies inversely with distance from the center of projection).
- angles only remain intact for faces parallel to projection plane.
• Parallel:– less realistic view because of no foreshortening – however, parallel lines remain parallel.
– angles only remain intact for faces parallel to projection plane.
- Describe Two-point projection.
•** Two-point** perspective projection:
– This is often used in architectural, engineering and industrial design drawings.
– Three-point is used less frequently as it adds little extra realism to that offered by two-point perspective projection.
- Describe Parallel • Orthographic Projections.
2 principle types of Parallel projection
• Orthographic :
– direction of projection = normal to the projection plane.
– Useful because angle and distance measurements can be made…
– However, as only one face of an object is shown, it can be hard to create a mental image of the object, even with several views.
- Describe Oblique Parallel Projections.
• Oblique :
– direction of projection = normal to the projection plane.
– really a 3D system but a 2D view of an object with ‘forced depth’.
– E.g. instead of drawing the sides full size they are only drawn with half the depth creating ‘forced depth’ – adding an element of realism to the object
- Describe how light helps us see.
- The light that can be seen by the human eye is a mixture of all kinds of different lights scattered and reflected against the surroundings with different material properties.
- Each different color is simply energy which can be represented by a wavelength. Colour is a wavelength visible to the eye.
- Put simply, the colour that a material reflects is observed as that material’s colour. Also, the more light the material reflects the shinier it will appear to the viewer.
- What wavelengths of light does the human eye respond to?
• The visible spectrum is the portion of the electromagnetic spectrum that can be detected by the human eye. A typical human eye will respond to wavelengths from about 380nm to 750 nm.
- How does light allow us to perceive colour?
- Any colour that is reflected from an object is the colour we perceive.
• If an object absorbed all the light except for red then the object would appear to be red in a white light. So the reflectance spectrum of a “red” object would look like this:
- Why is shading needed?
- When a 3D model is rendered with colour but no shading it is difficult or impossible to make out its depth and form.
- In the real-world, the interaction of light on surfaces gives shading, which humans use as an important depth cue.
- Light-material interactions cause each point to have a different color or shade.
- What should be considered when shading?
Need to consider:
- Light sources
- Material properites (how shiny? How dull is it?)
- Location of viewer (positioning very important)
- Surface orientation (what way is the object facing?)
- What is a point light?
- The point light source emits rays in radial directions from its source.
A point light source is a fair approximation to a local light source such as a light bulb.
For many scenes a point light gives the best approximation to lighting conditions.
- What is a directional light?
All of the rays from a directional light source have a common direction and no point of origin. It is as if the light source was infinitely far away from the surface that it is illuminating.
For outdoor scenes, the sun is so far away that its illumination is simulated as a directional light source with all rays arriving at the scene in a parallel direction.
- What is a spotlight?
A spotlight is a point source whose intensity falls off away from a given direction.
The beam of the spotlight is normally assumed to have a graduated edge so that the illumination is at its maximum inside a cone, falling to zero intensity outside a cone.
- What is ambient light?
Even though an object in a scene is not directly lit it will still be visible. This is because light is reflected indirectly from nearby objects.
Ambient light does not model a ‘true’ light source – it’s a hack consisting of a constant value that mimics indirect lighting.
- How is a pixel colour calculated? Give the equation.
Take the r,g,b.
- Discuss modelling a smooth sphere.
- To model a sphere so that it looks smooth by increasing or decreasing the number of facets is very impractical.
- Although the outlines of these spheres don’t look particularly circular, they differ in appearance, yet both have the same number of facets.
- We need to fool the eye by showing continuity in shading.
- How is Gouraud shading implemented?
- Calculate the intensity of light at each vertex.
- Interpolate the RGB values between the vertical vertices along each edge.
- This gives us the RGB components for the left and right edges of each scan line (pixel row).
- We then display each row of pixels by horizontally interpolating the RGB values between that row’s left and right edges.
- How is Phong shading implemented?
• Compute a normal for each vertex of the polygon.
- Using bi-linear interpolation compute a normal for each pixel.
- For each pixel normal, compute an intensity for each pixel of the polygon.
- Paint pixel to the corresponding shade.
• In some modern hardware, variants of this algorithm are called “pixel shading.” It usually means that the lighting calculations can be done per-pixel, and that the lighting variables are interpolated across the polygon.
- What are the effects on light when it hits a surface?
Light incident at a surface = light reflected + light scattered + light absorbed + light transmited
When light hits an opaque surface some is absorbed, the rest is reflected (some can be transmitted/scattered too)
• The reflected light is what we see
• Reflection is not simple and varies with material
– the surface’s micro structure define the details of reflection
– variations produce anything from bright specular reflection (mirrors) to dull male finish (chalk)
- What is a surface normal?
- A surface normal to a flat surface is a vector that is perpendicular to that surface.
- In 3D graphics, a normal is an imaginary line that is perpendicular to the surface of a polygon.
- The normal is ofen used to determine a surface’s orientation toward a light source.
- For a polygon (such as a triangle) a surface normal can be calculated as the vector cross product of two edges of the polygon.
42 What is ambient reflection with relation to local illumination?
- When there are no lights in a scene the picture will be blank.
- By including a small fraction of the surface colour we can simulate the effect of light reflected from around the scene.
- Ambient reflection is a gross approximation of multiple reflections from indirect light sources.
- By itself, ambient reflection produces very little realism.
- Give detailes of Diffuse lighting in local illumination.
• Diffuse lighting is the most significant component of an illumination model.
• Light reflected from a diffuse surface is scattered in all directions.
• Perfect diffuser: follows Lambert’s Cosine Law and surface looks the same from all directions
• To model the effect we assume that a polygon is most brightly illuminated when the incident light strikes the surface at right angles.
• Illumination falls to zero when the beam of light is parallel to the surface.
- What is Lambert’s Cosine Law?
• The reflected energy from a small surface area in a particular direction is proportional to cosine of the angle between that direction and the surface normal.
- An ideal diffuse surface is, at the microscopic level a very rough surface (e.g chalk, cardboard).
- Because of the microscopic variations in the surface, an incoming ray of light is equally likely to be reflected in any direction.
- Describe specular reflection.
- Specular reflection is the direct reflection of light by a surface.
- Most light is reflected in a narrow range of angles.
- Shiny surfaces reflect almost all incident light and therefore have bright specular highlights or hot spots.
- Specular highlights take the colour of the light, not the surface.
- For a perfect mirror the angle of reflection is equal to the angle of incidence.
