MyException - 我的异常网
当前位置:我的异常网» 综合 » I. The Basics-Introduction-Graphics and Renderin

I. The Basics-Introduction-Graphics and Rendering

www.MyException.Cn  网友分享于:2013-11-24  浏览:0次
I. The Basics---Introduction----Graphics and Rendering

http://www.arcsynthesis.org/gltut/Basics/Intro%20Graphics%20and%20Rendering.html

Graphics and Rendering

This is an overview of the process of rendering. Do not worry if you do not understand everything right away; every step will be covered in lavish detail in later tutorials.

Everything you see on your computer's screen, even the text you are reading right now (assuming you are reading this on an electronic display device, rather than a printout) is simply a two-dimensional array of pixels. If you take a screenshot of something on your screen, and blow it up, it will look very blocky.

Figure 8. An Image

An Image

Each of these blocks is a pixel. The word pixel is derived from the term Picture Element. Every pixel on your screen has a particular color. A two-dimensional array of pixels is called an image.

The purpose of graphics of any kind is therefore to determine what color to put in what pixels. This determination is what makes text look like text, windows look like windows, and so forth.

Since all graphics are just a two-dimensional array of pixels, how does 3D work? 3D graphics is thus a system of producing colors for pixels that convince you that the scene you are looking at is a 3D world rather than a 2D image. The process of converting a 3D world into a 2D image of that world is called rendering.

There are several methods for rendering a 3D world. The process used by real-time graphics hardware, such as that found in your computer, involves a very great deal of fakery. This process is called rasterization, and a rendering system that uses rasterization is called a rasterizer.

In rasterizers, all objects that you see are empty shells. There are techniques that are used to allow you to cut open these empty shells, but this simply replaces part of the shell with another shell that shows what the inside looks like. Everything is a shell.

All of these shells are made of triangles. Even surfaces that appear to be round are merely triangles if you look closely enough. There are techniques that generate more triangles for objects that appear closer or larger, so that the viewer can almost never see the faceted silhouette of the object. But they are always made of triangles.

Note

Some rasterizers use planar quadrilaterals: four-sided objects, where all of the points lie in the same plane. One of the reasons that hardware-based rasterizers always use triangles is that all of the lines of a triangle are guaranteed to be in the same plane. Knowing this makes the rasterization process less complicated.

An object is made out of a series of adjacent triangles that define the outer surface of the object. Such series of triangles are often calledgeometry, a model or a mesh. These terms are used interchangeably.

The process of rasterization has several phases. These phases are ordered into a pipeline, where triangles enter from the top and a 2D image is filled in at the bottom. This is one of the reasons why rasterization is so amenable to hardware acceleration: it operates on each triangle one at a time, in a specific order. Triangles can be fed into the top of the pipeline while triangles that were sent earlier can still be in some phase of rasterization.

The order in which triangles and the various meshes are submitted to the rasterizer can affect its output. Always remember that, no matter how you submit the triangular mesh data, the rasterizer will process each triangle in a specific order, drawing the next one only when the previous triangle has finished being drawn.

OpenGL is an API for accessing a hardware-based rasterizer. As such, it conforms to the model for rasterization-based 3D renderers. A rasterizer receives a sequence of triangles from the user, performs operations on them, and writes pixels based on this triangle data. This is a simplification of how rasterization works in OpenGL, but it is useful for our purposes.

Triangles and Vertices. Triangles consist of 3 vertices. A vertex is a collection of arbitrary data. For the sake of simplicity (we will expand upon this later), let us say that this data must contain a point in three dimensional space. It may contain other data, but it must have at least this. Any 3 points that are not on the same line create a triangle, so the smallest information for a triangle consists of 3 three-dimensional points.

A point in 3D space is defined by 3 numbers or coordinates. An X coordinate, a Y coordinate, and a Z coordinate. These are commonly written with parenthesis, as in (X, Y, Z).

Rasterization Overview

The rasterization pipeline, particularly for modern hardware, is very complex. This is a very simplified overview of this pipeline. It is necessary to have a simple understanding of the pipeline before we look at the details of rendering things with OpenGL. Those details can be overwhelming without a high level overview.

光栅化管线,特别是现在的硬件,是非常复杂的.这里只是一个简单的概括.在我们开始讲使用Opengl进行render之前,有必要对管线有个简单的了解.这不需要高深的知识.

Clip Space Transformation. The first phase of rasterization is to transform the vertices of each triangle into a certain region of space. Everything within this volume will be rendered to the output image, and everything that falls outside of this region will not be. This region corresponds to the view of the world that the user wants to render.

Clip Space Transformation.光栅化的第一个阶段是将每个三角形的顶点转换到一个确定的空间中.在这个volume中的每件物体都会被render到输出画面,并且不会render volume外面的任何物体.这个区域对应这user想要render的世界.

The volume that the triangle is transformed into is called, in OpenGL parlance, clip space. The positions of the triangle's vertices in clip space are called clip coordinates.

在opengl用法中,这个即将把三角形转换到的volume,我们称之为clip space. 在clip space中的三角形的坐标称为 clip坐标.

Clip coordinates are a little different from regular positions. A position in 3D space has 3 coordinates. A position in clip space has fourcoordinates. The first three are the usual X, Y, Z positions; the fourth is called W. This last coordinate actually defines what the extents of clip space are for this vertex.

 clip坐标与普通的positions有些不同.3D空间中的position有3个坐标轴.而在clip space 中有四个坐标轴.前面三个与position的X,Y,Z相同,第四个为W.最后的这个坐标定义了what the extents of clip space are for this vertex(不知道怎么翻译,谷歌为:其实这最后的坐标定义为这个顶点剪辑空间的范围是什么。).

Clip space can actually be different for different vertices within a triangle. It is a region of 3D space on the range [-W, W] in each of the X, Y, and Z directions. So vertices with a different W coordinate are in a different clip space cube from other vertices. Since each vertex can have an independent W component, each vertex of a triangle exists in its own clip space.

其实一个三角形中的不同顶点可以有不同的Clip space.于是顶点的W不同,那么它的clip space就不同.因为每个顶点有独立的W,所以三角形的每个顶点存在顶点自己的clip space中.

In clip space, the positive X direction is to the right, the positive Y direction is up, and the positive Z direction is away from the viewer.

The process of transforming vertex positions into clip space is quite arbitrary. OpenGL provides a lot of flexibility in this step. We will cover this step in detail throughout the tutorials.

Because clip space is the visible transformed version of the world, any triangles that fall outside of this region are discarded. Any triangles that are partially outside of this region undergo a process called clipping. This breaks the triangle apart into a number of smaller triangles, such that the smaller triangles are all entirely within clip space. Hence the name clip space.

因为clip space是转换后的可见世界,所以区域外的任何三角形都会被丢弃.该区域外的三角形经历的操作称之为clipping. 它将三角形break成许多小的三角形,这样的话小三角形都会出在clip space中. 因此称为 clip space.

Normalized Coordinates. Clip space is interesting, but inconvenient. The extent of this space is different for each vertex, which makes visualizing a triangle rather difficult. Therefore, clip space is transformed into a more reasonable coordinate space: normalized device coordinates.

This process is very simple. The X, Y, and Z of each vertex's position is divided by W to get normalized device coordinates. That is all.

The space of normalized device coordinates is essentially just clip space, except that the range of X, Y and Z are [-1, 1]. The directions are all the same. The division by W is an important part of projecting 3D triangles onto 2D images; we will cover that in a future tutorial.

Figure 9. Normalized Device Coordinate Space

Normalized Device Coordinate Space

The cube indicates the boundaries of normalized device coordinate space.

Window Transformation. The next phase of rasterization is to transform the vertices of each triangle again. This time, they are converted from normalized device coordinates to window coordinates. As the name suggests, window coordinates are relative to the window that OpenGL is running within.

Window Transformation. 光栅化的下一个阶段是再次转换每个三角形.这次,是将三角形从normalized device coordinates转换到window coordinates. 像名称所表示的,窗口坐标与OpenGL正在运行的窗口有关.

Even though they refer to the window, they are still three dimensional coordinates. The X goes to the right, Y goes up, and Z goes away, just as for clip space. The only difference is that the bounds for these coordinates depends on the viewable window. It should also be noted that while these are in window coordinates, none of the precision is lost. These are not integer coordinates; they are still floating-point values, and thus they have precision beyond that of a single pixel.

The bounds for Z are [0, 1], with 0 being the closest and 1 being the farthest. Vertex positions outside of this range are not visible.

Note that window coordinates have the bottom-left position as the (0, 0) origin point. This is counter to what users are used to in window coordinates, which is having the top-left position be the origin. There are transform tricks you can play to allow you to work in a top-left coordinate space if you need to.

The full details of this process will be discussed at length as the tutorials progress.

Scan Conversion. After converting the coordinates of a triangle to window coordinates, the triangle undergoes a process called scan conversion. This process takes the triangle and breaks it up based on the arrangement of window pixels over the output image that the triangle covers.

Scan Conversion.在将三角形转换到窗口坐标后,接下来的操作称为scan conversion.这个操作根据窗口像素的排列将三角形所cover的图像将三角形break up.

Figure 10. Scan Converted Triangle

Scan Converted Triangle

The center image shows the digital grid of output pixels; the circles represent the center of each pixel. The center of each pixel represents a sample: a discrete location within the area of a pixel. During scan conversion, a triangle will produce a fragment for every pixel sample that is within the 2D area of the triangle.

中间的图像表示了输出像素网格,圆环代表每个像素的中心.每个像素的中心代表一个 sample:一个像素区域内的不连续的location.在scan conversion阶段,一个三角形将会为2D三角形区域中的每个像素的 sample产生一个 fragment .

The image on the right shows the fragments generated by the scan conversion of the triangle. This creates a rough approximation of the triangle's general shape.

右图展示了一个三角形scan conversion阶段产生的fragment.这将产生一个粗略的近似三角形形状.

It is very often the case that triangles are rendered that share edges. OpenGL offers a guarantee that, so long as the shared edge vertex positions are identical, there will be no sample gaps during scan conversion.

经常会发生render三角形时有共同的边.OpenGL提供一个保证,如果共同的边相同,那么在 scan conversion过程中不会产生sample gaps.

Figure 11. Shared Edge Scan Conversion

Shared Edge Scan Conversion

To make it easier to use this, OpenGL also offers the guarantee that if you pass the same input vertex data through the same vertex processor, you will get identical output; this is called the invariance guarantee. So the onus is on the user to use the same input vertices in order to ensure gap-less scan conversion.

为了更容易使用,OpenGL同样提出了一个保证,如果你给相同的顶点processor提供相同的顶点数据,那么你将会得到相同的输出,这称为 invariance guarantee. 这样用户的负担就是要使用相同的输入顶点来使产生gap-less scan conversion.

Scan conversion is an inherently 2D operation. This process only uses the X and Y position of the triangle in window coordinates to determine which fragments to generate. The Z value is not forgotten, but it is not directly part of the actual process of scan converting the triangle.

Scan conversion是一个固有的2D操作.这个过程只使用窗口坐标系中的position的X,Y坐标来决定产生哪些 fragment.Z值没有被忘记,但是它不是scan converting三角形过程中的直接参与部分.

The result of scan converting a triangle is a sequence of fragments that cover the shape of the triangle. Each fragment has certain data associated with it. This data contains the 2D location of the fragment in window coordinates, as well as the Z position of the fragment. This Z value is known as the depth of the fragment. There may be other information that is part of a fragment, and we will expand on that in later tutorials.

scan convert一个三角形的结果是产生一系列的fragment,这些fragment覆盖了一个三角形的形状.每个fragment都有与之相连的数据.这个数据包括2D窗口坐标系中的位置,同样也包括fragment的Z position.这个Z值被认为是 fragment的深度值.还有其他的一些fragment的信息,接下来会讲到.

Fragment Processing. This phase takes a fragment from a scan converted triangle and transforms it into one or more color values and a single depth value. The order that fragments from a single triangle are processed in is irrelevant; since a single triangle lies in a single plane, fragments generated from it cannot possibly overlap. However, the fragments from another triangle can possibly overlap. Since order is important in a rasterizer, the fragments from one triangle must all be processed before the fragments from another triangle.

This phase is quite arbitrary. The user of OpenGL has a lot of options of how to decide what color to assign a fragment. We will cover this step in detail throughout the tutorials.

Fragment Processing. 这个阶段将一个fragmentscan converted 后三角形取出来,赋给它一个或多个颜色值和一个单独的深度值.从scan converted 后三角形中取出fragment的顺序是不相干的.因为一个特定三角形只能处于一个平面,所以三角形产生的 fragments不会重叠.但是从其他三角形产生的 fragments会与上个三角形产生的 fragments 重叠.光栅化过程的顺序很重要,一个三角形产生的fragments必须在其他三角形产生fragments之前全部产生完.

Direct3D Note

Direct3D prefers to call this stage pixel processing or pixel shading. This is a misnomer for several reasons. First, a pixel's final color can be composed of the results of multiple fragments generated by multiple samples within a single pixel. This is a common technique to remove jagged edges of triangles. Also, the fragment data has not been written to the image, so it is not a pixel yet. Indeed, the fragment processing step can conditionally prevent rendering of a fragment based on arbitrary computations. Thus a pixel in D3D parlance may never actually become a pixel at all.

Fragment Writing. After generating one or more colors and a depth value, the fragment is written to the destination image. This step involves more than simply writing to the destination image. Combining the color and depth with the colors that are currently in the image can involve a number of computations. These will be covered in detail in various tutorials.

Fragment Writing.在产生颜色值和深度值之后,fragment会被写入到目的图片.这步骤不仅只是写入到目的图片.图片中颜色值和深度值的结合会对许多结果产生影响.接下来会讲到.

Colors

Previously, a pixel was stated to be an element in a 2D image that has a particular color. A color can be described in many ways.

In computer graphics, the usual description of a color is as a series of numbers on the range [0, 1]. Each of the numbers corresponds to the intensity of a particular reference color; thus the final color represented by the series of numbers is a mix of these reference colors.

The set of reference colors is called a colorspace. The most common color space for screens is RGB, where the reference colors are Red, Green and Blue. Printed works tend to use CMYK (Cyan, Magenta, Yellow, Black). Since we're dealing with rendering to a screen, and because OpenGL requires it, we will use the RGB colorspace.

在计算机图形学里,我们通常使用一些列0到1范围内的数字来描述颜色.每个数值对应相应的颜色值的强度,最后的结果是这些值的混合.

reference colors 的几何称作color space. 屏幕的最常见的颜色空间是RGB,红色绿色和蓝色.打印工作长使用CMYK颜色空间.我们使用RGB模式.

Note

You can play some fancy games with programmatic shaders (see below) that allow you to work in different colorspaces. So technically, we only have to output to a linear RGB colorspace.

So a pixel in OpenGL is defined as 3 values on the range [0, 1] that represent a color in a linear RGB colorspace. By combining different intensities of this 3 colors, we can generate millions of different color shades. This will get extended slightly, as we deal with transparency later.

Shader

shader is a program designed to be run on a renderer as part of the rendering operation. Regardless of the kind of rendering system in use, shaders can only be executed at certain points in that rendering process. These shader stages represent hooks where a user can add arbitrary algorithms to create a specific visual effect.

In term of rasterization as outlined above, there are several shader stages where arbitrary processing is both economical for performance and offers high utility to the user. For example, the transformation of an incoming vertex to clip space is a useful hook for user-defined code, as is the processing of a fragment into final colors and depth.

Shaders for OpenGL are run on the actual rendering hardware. This can often free up valuable CPU time for other tasks, or simply perform operations that would be difficult if not impossible without the flexibility of executing arbitrary code. A downside of this is that they must live within certain limits that CPU code would not have to.

There are a number of shading languages available to various APIs. The one used in this tutorial is the primary shading language of OpenGL. It is called, unimaginatively, the OpenGL Shading Language, or GLSL. for short. It looks deceptively like C, but it is very much not C.


文章评论

漫画:程序员的工作
漫画:程序员的工作
程序猿的崛起——Growth Hacker
程序猿的崛起——Growth Hacker
60个开发者不容错过的免费资源库
60个开发者不容错过的免费资源库
Java程序员必看电影
Java程序员必看电影
老美怎么看待阿里赴美上市
老美怎么看待阿里赴美上市
10个调试和排错的小建议
10个调试和排错的小建议
那些争议最大的编程观点
那些争议最大的编程观点
“懒”出效率是程序员的美德
“懒”出效率是程序员的美德
亲爱的项目经理,我恨你
亲爱的项目经理,我恨你
Java 与 .NET 的平台发展之争
Java 与 .NET 的平台发展之争
程序员和编码员之间的区别
程序员和编码员之间的区别
团队中“技术大拿”并非越多越好
团队中“技术大拿”并非越多越好
当下全球最炙手可热的八位少年创业者
当下全球最炙手可热的八位少年创业者
做程序猿的老婆应该注意的一些事情
做程序猿的老婆应该注意的一些事情
如何区分一个程序员是“老手“还是“新手“?
如何区分一个程序员是“老手“还是“新手“?
我是如何打败拖延症的
我是如何打败拖延症的
一个程序员的时间管理
一个程序员的时间管理
什么才是优秀的用户界面设计
什么才是优秀的用户界面设计
我跳槽是因为他们的显示器更大
我跳槽是因为他们的显示器更大
程序员周末都喜欢做什么?
程序员周末都喜欢做什么?
旅行,写作,编程
旅行,写作,编程
编程语言是女人
编程语言是女人
科技史上最臭名昭著的13大罪犯
科技史上最臭名昭著的13大罪犯
程序员的一天:一寸光阴一寸金
程序员的一天:一寸光阴一寸金
10个帮程序员减压放松的网站
10个帮程序员减压放松的网站
不懂技术不要对懂技术的人说这很容易实现
不懂技术不要对懂技术的人说这很容易实现
如何成为一名黑客
如何成为一名黑客
写给自己也写给你 自己到底该何去何从
写给自己也写给你 自己到底该何去何从
程序员应该关注的一些事儿
程序员应该关注的一些事儿
要嫁就嫁程序猿—钱多话少死的早
要嫁就嫁程序猿—钱多话少死的早
“肮脏的”IT工作排行榜
“肮脏的”IT工作排行榜
中美印日四国程序员比较
中美印日四国程序员比较
代码女神横空出世
代码女神横空出世
Web开发人员为什么越来越懒了?
Web开发人员为什么越来越懒了?
程序员最害怕的5件事 你中招了吗?
程序员最害怕的5件事 你中招了吗?
 程序员的样子
程序员的样子
程序员的鄙视链
程序员的鄙视链
我的丈夫是个程序员
我的丈夫是个程序员
为什么程序员都是夜猫子
为什么程序员都是夜猫子
Web开发者需具备的8个好习惯
Web开发者需具备的8个好习惯
程序员必看的十大电影
程序员必看的十大电影
十大编程算法助程序员走上高手之路
十大编程算法助程序员走上高手之路
总结2014中国互联网十大段子
总结2014中国互联网十大段子
老程序员的下场
老程序员的下场
5款最佳正则表达式编辑调试器
5款最佳正则表达式编辑调试器
软件开发程序错误异常ExceptionCopyright © 2009-2015 MyException 版权所有