advanced computer graphics - Rendering

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Transcript of advanced computer graphics - Rendering

  • February 5, 2015

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    ADVANCED COMPUTER GRAPHICS

    Rendering

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    The computer screen is a flat surface. When image data is stored in the

    computer it is in the form of a data structure consisting of coordinate

    points. a data structure of image points cannot be displayed directly onto

    a flat computer screen.

    In the same way that an engineer must use a rendering scheme in order

    to represent a solid object onto the surface of the drawing paper, the

    programmer must find a way of converting a data structure of coordinates

    into an image on the computer monitor.

    Introduction

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    generating 2D images of a 3D world represented in a computer, building 2D images from 3D models ,Rendering is the conversion of a

    scene into an image:

    Scenes are composed of models in three-dimensional space, Models

    are composed of primitives supported by the rendering system, Models

    entered by hand or created by a program, The image is drawn on

    monitor, printed on laser printer, or written to a raster in memory or a

    file.

    Rendering

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    Classically, "model" to scene" to image" conversion broken into finer steps, called the graphics pipeline. The basic forward projection

    pipeline looks like:

    Rendering

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    Each stage refines the scene, converting primitives in modeling space to

    primitives in device space, where they are converted to pixels

    (rasterized).

    Rendering

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    Three conceptual stages of the pipeline:

    Application (executed on the CPU) Geometry Rasterizer

    Rendering Pipeline

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    The APPLICATION stage

    Executed on the CPU, Means that the programmer decides what

    happens

    Most important task: send rendering primitives (e.g. triangles) to the

    graphics hardware

    The GEOMETRY stage

    geometrical operations on the input data (e.g. triangles) Allows:

    Move objects (matrix multiplication) Move the camera (matrix multiplication) Compute lighting at vertices of triangle Project onto screen (3D to 2D) Clipping (avoid triangles outside screen) Map to window

    Rendering Pipeline

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    The RASTERIZER stage

    Main task: take output from GEOMETRY and turn into visible pixels

    on screen

    Also, add textures and various other per-pixel operations

    Rendering Pipeline

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    Coordinate Transformation

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    Coordinate Transformation

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    Coordinate Transformation

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    Coordinate Transformation

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    A common interpretation of the rendering process is to consider it as a

    series of transformations that take the object from the coordinate

    system in which it is encoded, into the coordinate system of the

    display surface. This process, sometimes referred to as the rendering

    pipeline, is described as a series of spaces through which the object

    transform from database to screen.

    A waterfall model of the rendering pipeline is shown

    Coordinate Transformation

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    Local Space

    Objects are usually easier to model if they are conveniently positioned in the

    coordinate plane. For example, when we place the bottom-left vertex of a

    cube at the origin of the coordinate system, the coordinates are all positive

    values

    Coordinate Transformation

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    World Space

    The coordinate system of the scene is called the world space, or world

    coordinate system.

    Objects modeled in local space usually have to be transformed into

    world space at the time they are placed in a scene. For example, a

    particular scene may require a cube placed so that its left-bottom vertex

    is at coordinates x = 2, y = 3, z = 0. The process requires applying a

    translation transformation to the cube as it was originally defined in local

    space. Furthermore, lighting conditions are usually defined in world

    space. Once the light sources are specified and located, then shading

    and other rendering transformations can be applied to the polygons so

    as to determine how the object appears under the current illumination.

    Surface attributes of the object, such as texture and color, may affect

    the shading process. Figure shows the world space transformation of a

    cube under unspecified illumination conditions and with undefined texture

    and color attributes.

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    Coordinate Transformation

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    A vector space is a mathematical structure that

    is defined by a given number of linearly

    independent vectors, also called base vectors

    (for example in Figure, there are three base

    vectors);

    Every model in the game lives in its own Model

    Space and if you want them to be in any spatial

    relation (like if you want to put a teapot over a

    table) you need to transform them into a

    common space (which is what is often called

    World Space).

    Coordinate Transformation

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    Now let's say that we start with an active space, call it Space A, that

    contains a teapot. We now want to apply a transformation that moves

    everything in Space A into a new position; but if we move Space A we then

    need to define a new "active" space to represent the transformed Space

    A. Let's call the new active space Space B

    Coordinate Transformation

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    The first step when we want to render a 3D scene is to put all the models

    in the same space, the World Space. Since every object will be in its own

    position and orientation in the world, every one has a different Model to

    World transformation matrix.

    Three teapots each one in its own model space

    Coordinate Transformation

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    Three teapots set in World Space

    Coordinate Transformation

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    With all the objects at the right place we now need to project them to the screen. This is usually done in two steps. The first step moves all the

    object in another space called the View Space. The second step performs

    the actual projection using the projection matrix. This last step is a bit

    different from the others and we will see it in detail in a moment.

    Why do we need a View Space? The View Space is an auxiliary space that we use to simplify the math and keep everything elegant and

    encoded into matrices. The idea is that we need to render to a camera,

    which implies projecting all the vertices onto the camera screen that can

    be arbitrarily oriented in space.

    Coordinate Transformation

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    Now, if you imagine you want to put the camera in World Space you would use

    a transformation matrix that is located where the camera is and is oriented so

    that the Z axis is looking to the camera target.

    two teapots and a camera in

    World Space

    everything is transformed into View

    Space (World Space is represented

    only to help visualize the

    transformation)

    Coordinate Transformation

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    The scene is now in the most friendly space possible for a projection, the

    View Space. All we have to do now is to project it onto the imaginary screen

    of the camera. Before flattening the image, we still have to move into

    another, final space, the Projection Space. This space is a cuboid which

    dimensions are between -1 and 1 for every axis. This space is very handy

    for clipping (anything outside the 1:-1 range is outside the camera view

    area) and simplifies the flattening operation (we just need to drop the z

    value to get a flat image).

    Coordinate Transformation

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    Projection Space obtained from the teapot

    in previous Figure

    Coordinate Transformation

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    Eye Space

    The eye space, or camera coordinate system, introduces the

    necessary transformations to improve rendering to any desired

    degree. Perspective transformations requires knowledge of the

    camera position and the projection plane.

    Backface Elimination or Culling

    One of the most important rendering problems that must be solved at

    this stage of the pipeline is the elimination of the polygonal faces

    that are not visible from the eye position, In the simplest case,

    entire polygons that are not visible are removed at this time.

    This operation is known as culling.

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    A solid object composed of polygonal surfaces that completely enclose

    its volume is called a polyhedron. the polygons whose normals point

    away from the eye or camera can be assumed to be blocked by other,

    closer polygons, and are thus invisible.

    A single mathematical test can be used to determine if a polygonal face

    is visible.

    The geometric normal to the polygonal face is compared with a vector

    from the polygon to the camera or eye position. This is called the line-of-

    sight vector. If the resulting angle is greater than 90 degrees, then the

    polygonal surface faces away from the camera and can be culled. Figure

    shows the use of polygonal surface and line-of-sight vectors in culling.

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    Once the position of the camera

    is determined in the scene, it is

    possible to perform the backface

    elimination.

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    Screen space is defined in terms of the viewport. The final transformation

    in the rendering pipeline consists of eliminating those elements of the eye

    space that fall outside the boundaries of the screen space. This

    transformation is known as clipping.

    The perspective and clipping transformations are applied as the image

    reaches the last stage of the rendering pipeline.

    Screen Space

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    Types of rendering

    a. Wireframe

    b. Smooth shading

    c. Ray tracing

    d. Radiosity

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