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n the spring of , we finished writing OpenGL Game Programming. Although the book didn't cover everything we had initially planned, we hoped that it would . Beginning OpenGL Game Programming, Second Edition RLuke Benstead with Dave Astle and Kevin HawkinsCourse Technolog. Merely connect to the internet to obtain this book Beginning OpenGL Game Programming 1st Edition. This is why we suggest you to use and also utilize the.
Luke Luke Benstead Benstead. United Kingdom. Stacy L. Hiquet Associate Director of Marketing: Sarah Panella Manager of Editorial Services: No part of this work covered by the copyright herein may be reproduced, transmitted, stored, or used in any form or by any means graphic, electronic, or mechanical, including but not limited to photocopying, recording, scanning, digitizing, taping, Web distribution, information networks, or information storage and retrieval systems, except as permitted under Section or of the United States Copyright Act, without the prior written permission of the publisher.
Subjects like game architecture, physics, AI, and audio are required in most games, but they are such big topics that they all deserve a book of their own! It would be an unrealistic goal to plan to write code for every platform. For this reason, a decision was made to primarily target the most commonly used operating system, Microsoft Windows.
The Linux versions of the xix xx Introduction source were written, tested, and compiled under Ubuntu 8. OpenGL 2. OpenGL 3. For this reason, the text assumes both OpenGL 3. So, on the CD there are two versions of the code for each platform; one version is designed for OpenGL 3.
The differences between these two versions of the code are minimal: n Chapters —The source code is the same for both versions except for the OpenGL context creation that falls back to an OpenGL 2.
There is only one version of the game that falls back to OpenGL 2. The data will be altered once and accessed multiple times this hint is good for static geometry. The buffer will be modified a lot and accessed many times this is suitable for animated models. The contents will be filled by OpenGL and then subsequently read by the application. The offset is useful if you are storing more than one type of data in the buffer.
In this case, you would use the offset in glColorPointer to indicate to OpenGL where in the array the color data starts. It is defined as follows: You can use a buffer to store indices too. First, before rendering, we initialize an array with a single point in the center of the screen two units back, and then create a vertex buffer object: If you look at the example program for the previous section, you will notice you can barely see the point in the center.
You can increase the size of the point by using glPointSize , which has the following prototype: If point anti-aliasing is disabled which is the default behavior , then the point size is rounded to the nearest integer, which is the pixel size of the point. The point size will never be rounded to less than 1. This can cause the edges of primitives to look jagged. The process of smoothing these jagged edges is known as anti-aliasing.
You can disable point smoothing again by passing the same parameter to glDisable. When point smoothing is enabled, the supported range of point sizes may be limited. If an unsupported size is used, then the point size will be rounded to the nearest supported value. The program displays a series of points that gradually increase in size. First, during initialization we generate a row of 15 points spaced 0. Handling Primitives Figure 3.
During rendering, the only changes from the point drawing is the mode passed to glDrawArrays , and the number of vertices has increased to 2: You can change this value using the aptly named glLineWidth.
Anti-aliasing Lines Anti-aliasing lines works in pretty much the same way as points. Similar to point smoothing, line smoothing is only guaranteed to be available on lines with a width of 1. During initialization, we must generate the vertices that make up the lines: Also, notice that i is incremented by two each iteration so that we draw the next line in the array each time. Drawing Triangles in 3D Although you can do some interesting things armed with points and lines, it is polygons that make up the majority of the scene in a game.
Rendering with triangles has several advantages; most 3D hardware works with triangles internally as they are easier and faster to rasterize interpolation of colors etc. Triangles are always convex and they make non-rendering tasks such as collision detection simpler to calculate. If you have 67 68 Chapter 3 n OpenGL States and Primitives a list of vertices that you want to turn into an arbitrary sided polygon, have no fear! If you refer back to Figure 3.
Neat, huh? Also, rendering four points with a triangle strip will produce a quadrilateral made of two triangles. If you do want to use them, rendering occurs in the same way; you just have to switch the mode passed to glDrawArrays or glDrawElements. We build the vertex list in much the same way: In the example, we show three rotating squares, each made up of a triangle strip and each rotating in a clockwise direction.
Each square is given a different polygon mode. Sometimes you will know that the viewer can only see one side of a polygon; for example, the polygons of a solid, opaque box will only ever have the front side visible. In this situation, it is possible to prevent OpenGL from rendering and processing the backside of the primitive. OpenGL can do this automatically through the process known as culling.
The front and back face are 69 70 Chapter 3 n OpenGL States and Primitives determined by polygon winding—the order in which you specify the vertices. Looking at the polygon head-on, you can choose any vertex with which to begin describing it.
By default, OpenGL treats polygons with counterclockwise ordering as front facing and polygons with clockwise ordering as back facing.
The default behavior can be changed using glFrontFace: Anti-aliasing Polygons As with points and lines, you can also choose to anti-alias polygons. As you might expect, it is disabled by default.
Summary In this chapter, you learned a little more about the OpenGL state machine. You have learned about the different types of primitives that can be rendered using OpenGL and also have seen three different methods of rendering them: Primitives are drawn by passing a series of vertices to OpenGL either one at a time using immediate mode or in an array vertex arrays and vertex buffer objects.
You know how to render points, lines, and triangles in OpenGL and how to enable anti-aliasing for each type. You know how to vary the size of points and lines by using glPointSize and glLineWidth , respectively. How is culling enabled? By default, is the front face of a polygon rendered with vertices in a clockwise winding or a counterclockwise winding? What is passed to glEnable to enable polygon smoothing? Write an application that displays a pyramid with four sides excluding the bottom.
The sides of the pyramid should be formed using a triangle-fan and the bottom should be made of a triangle-strip. All polygons should be rendered using vertex buffer objects and each vertex should be a different color. This is a vital ingredient to generating realistic 3D gaming worlds; without it, the 3D scenes you create would be static, boring, and totally non-interactive.
OpenGL makes it easy for the programmer to move objects around using various coordinate transformations, discussed in this chapter. You will also look at how to use your own matrices with OpenGL, which provides you with the power to manipulate objects in many different ways. Look around you. Now, imagine that you have a camera in your hands, and you are taking photographs of your surroundings. They also have some sort of position and orientation in the world space.
You have a position and orientation in world space as well.
The relationship between the positions of these objects around you and your position and orientation determines whether the objects are behind you or in front of you. A zoom lens makes objects appear closer to or farther from your position. Transformations work the same way. They allow you to move, rotate, and manipulate objects in a 3D world, while also allowing you to project 3D coordinates onto a 2D screen.
The modeling transformation moves objects around the scene and moves objects from local coordinates into world coordinates. The viewport transformation maps the clip coordinates into the two-dimensional viewport, or window, on your screen. Understanding Coordinate Transformations Table 4. While these four transformations are standard in 3D graphics, OpenGL includes and combines the modeling and viewing transformation into a single modelview transformation.
Table 4. The modelview transformations execute before the projection transformations. Figure 4. Eye Coordinates One of the most critical concepts to transformations and viewing in OpenGL is the concept of the camera, or eye coordinates. In contrast, OpenGL converts world coordinates to eye coordinates with the modelview matrix. When an object is in eye coordinates, the geometric relationship between the object and the camera is known, which means our objects are positioned relative to the camera position and are ready to be rendered properly.
Essentially, you can use the viewing transformation to move a camera about the 3D world, while the modeling transformation moves objects around the world. In OpenGL, the default camera or viewing matrix transformation is always oriented to look down the negative z-axis, as shown in Figure 4. To give you an idea of this orientation, imagine that you are at the origin and you rotate to the left 90 degrees about the y-axis ; you would then be facing along the negative x-axis.
Similarly, if you were to place yourself in the default camera orientation and rotate degrees, you would be facing in the positive z direction. Viewing Transformations The viewing transformation is used to position and aim the camera.
This is because transformations in OpenGL are applied in reverse order. How do you create the viewing transformation? First, you need to clear the current matrix. After initializing the current matrix, you can create the viewing matrix in several different ways. One method is to leave the viewing matrix equal to the identity matrix.
This results in the default location and orientation of the camera, which would be at the origin and looking down the negative z-axis. Other methods include the following: You can perform these operations one at a time or as a combination of events. A rotation followed by a translation. A translation followed by a rotation. Understanding Coordinate Transformations that you can use on objects: This is where an object is rotated about a vector. This is when you increase or decrease the size of an object.
With scaling, you can specify different values for different axes. This gives you the ability to stretch and shrink objects non-uniformly. For example, as shown in Figure 4. Then you rotate the arrow 30 degrees about the z-axis. After the translation, the arrow would be located at 5, 0. When you apply the rotation transformation, the arrow would still be located at 5, 0 , but it would be pointing at a degree angle from the x-axis. It is performed after the modeling and viewing transformations. You can think of the projection transformation as determining which objects belong in the viewing volume and how they should look.
It is very much like choosing a camera lens that is used to look into the world. OpenGL offers two types of projections: This type of projection shows 3D worlds exactly as you see things in real life.
With perspective projection, objects 79 80 Chapter 4 n Transformations and Matrices that are farther away appear smaller than objects that are closer to the camera. This type of projection shows objects on the screen in their true size, regardless of their distance from the camera.
Viewport Transformations The last transformation is the viewport transformation. Transformations in OpenGL rely on the matrix for all mathematical computations. As you will soon see, OpenGL has what is called the matrix stack, which is useful for constructing complicated models composed of many simple objects. You will be taking a look at each of the transformations and look more into the matrix stack in this section. Before we begin, it is worth noting that the matrix stack was marked as deprecated in OpenGL 3.
Still, the current matrix functionality will be around for a while yet and it is utilized in most of the code that is available at the time of writing. Also, the concept of the matrix stack and the different matrices we will be discussing are vital for 3D computer graphics.
Vertices are transformed by multiplying a Fixed-Function OpenGL and Matrices vertex vector by the modelview matrix, resulting in a new vertex vector that has been transformed. Before calling any transformation commands, you must specify whether you want to modify the modelview matrix or the projection matrix. Doing this looks like: To do this, you call the glLoadIdentity function, which loads the identity matrix as the current modelview matrix, thereby positioning the camera at the world origin and default orientation.
For example, if you execute the command glTranslatef 3. Suppose you want to move a cube from the origin to the position 5, 5, 5. You then perform the translation transformation on the current matrix to position 5, 5, 5 before calling your renderCube function.
How about a translation example? Fixed-Function OpenGL and Matrices because the world coordinate system is being translated, the square plane appears to be moving into and away from the view. Here is the code from the prepare function, which performs the oscillation logic: Making transformations dependant on time keeps everything running at the same speed regardless of the power of the computer. This code in the prepare function is called prior to the render function, which looks like this: After clearing the color and depth buffers, we load the identity matrix to initialize to the default world position and orientation; we then bind our vertex buffers and translate along the z-axis using the value determined in the prepare function, and then draw the square.
The resulting execution shows a plane that moves back and forth along the z-axis. A screenshot is shown in Figure 4. For example, if you wanted to rotate around the y-axis degrees in the counterclockwise direction, you would use the following: If you wanted to rotate clockwise, you would set the angle of rotation as a negative number. To rotate around the y-axis degrees in the clockwise direction, you use the following code: You can accomplish this by specifying the arbitrary axis vector in the x, y, and z parameters.
By drawing a line from the relative origin to the point represented by x, y, z , you can see the arbitrary axis around which you will rotate. For instance, if you rotate 90 degrees 85 86 Chapter 4 n Transformations and Matrices Figure 4. In code, this looks like the following: The order in which you specify rotations is very important when doing this because each rotation you apply changes the local coordinate system of the rotations.
However, the second rotation about the y-axis will not be in the context of the world coordinate system. A screenshot of this example is shown in Figure 4. If you build and run the example, you will see the same plane we created in the Translation example, except this time it is rotating about the origin instead of translating along the z-axis.
The important part of this example is the following lines in the render method: Then when we rotate about the y-axis, the rotation occurs in the new orientation that has been created as a result of the x-axis rotation.
Hopefully, through this example you can see how much the order of rotation about different axes matters.
In other words, when using scaling operations, vertex coordinates for an object are multiplied by a scaling factor for each axis. This means that if you would normally place a vertex at the location 1, 1, 1 without scaling, then applying a scaling factor of 2.
For example, this line applies a scaling factor of 2. You would use the following: Well, because the scaling factors are each multiplied by the vertices, you simply choose a value less than one, like this: This line will shrink an object by half its original size.
A value of 0. If you set a scaling factor to 1. As you might have guessed from this, scaling is equivalent to multiplying by the scaling factor. Values between 0. So naturally, now you are going to see an example for scaling of an object shrinking and expanding. Taking a look at the prepare function in the Example class, you will see: We pass the scaling factor to the glScale function in the render function below: It then calls the glScale function, passing the scale factor to all three axis parameters.
The result is a plane that increases and decreases in size with the value of the scale factor. There are four types of matrix stacks in OpenGL: The texture matrix stack is used for the transformation of texture coordinates, and the color matrix can be used to modify colors. The modelview matrix stack allows you to save the current state of the transformation matrix, perform other transformations, and then return to the saved transformation matrix without having to store or calculate the transformation matrix on your own.
The projection, texture, and color matrix stacks allow you to do the same thing. Using the modelview matrix stack essentially allows you to transform from one coordinate system to another while being able to revert to the original coordinate 91 92 Chapter 4 n Transformations and Matrices Figure 4.
For instance, if we position ourselves at the point 10, 5, 7 , and we then push the current modelview matrix onto the current stack, then our current transformation matrix is reset to the local coordinate system centered around the point 10, 5, 7. This means that any transformations we do are now based on the coordinate system at 10, 5, 7.
So if we then translate 10 units down the positive x axis with glTranslate When the matrix stack is popped, we revert to the original transformation matrix and therefore the original coordinate system, which means we are again positioned at 10, 5, 7. Two functions allow you to push and pop the matrix stacks: The modelview matrix stack is guaranteed to have a stack depth of at least 32, and all of the other matrix stacks have a depth of at least 2.
The glPopMatrix function pops off the top matrix on the stack and discards its contents. All other matrices in the stack are moved up one position. The robot is constructed of cubes that you scale to different shapes and sizes to give the robot arms, legs, feet, a torso, and a head. Take special note of these functions as you trace through the source code.
There are two functions that you should focus on as you browse through the source code. The method that does the main rendering work is probably the most interesting.
This is the render method of the Robot class: These methods are renderHead , renderTorso , renderArm , and renderLeg. By setting a projection transformation, you are, in effect, creating a viewing volume, which serves two purposes. Objects that are outside this volume are not rendered. Projections The second purpose of the viewing volume is to determine how objects are drawn.
This depends on the shape of the viewing volume, which is the primary difference between orthographic and perspective projections. Before specifying any kind of projection transformation, though, you need to make sure that the projection matrix is the currently selected matrix stack. As you saw earlier with the modelview matrix, this is done with a call to glMatrixMode: Unlike with the modelview matrix, it is rare to make many changes to the projection matrix.
Orthographic As we mentioned before, orthographic, or parallel, projections are those that involve no perspective correction. In other words, no adjustment for distance from the camera is made; objects appear the same size onscreen whether they are close or far away.
Although this may not look as realistic as perspective projections, it has a number of uses. Traditionally, orthographic projections are included in OpenGL for applications such as CAD, but they can also be used for 2D games or isometric games. OpenGL provides the glOrtho function to set up orthographic projections: Together, these coordinates specify a box-shaped viewing volume. More precisely, opposite planes are parallel to each other, and adjacent planes are perpendicular.
Using gluOrtho2D is equivalent to calling glOrtho with near set to —1. In perspective projections, as an object gets farther from the viewer, it appears smaller on the screen—an effect commonly referred to as foreshortening.
The viewing volume for a perspective projection is a frustum, which looks like a pyramid with the top cut off, with the narrow end toward the viewer. That the far end of the frustum is larger than the near end is what creates the foreshortening effect.
The way this works is that OpenGL transforms the frustum so that it becomes a cube. This transformation affects the objects inside the frustum as well, so objects at the wide end of the frustum are compressed more than objects at the narrow end. The greater the ratio between the wide and narrow ends, the more an object is shrunk. There are a couple of ways you can set up the view frustum, and thus the perspective projection. Thus, the top-left corner of the near clipping plane is at left, top, —near , and the bottom-right corner is at right, bottom, —near.
The corners of the far clipping plane are determined by casting a ray from the viewer through the corners of the near clipping plane and intersecting them with the far clipping plane. So, the closer the viewer is to the near clipping plane, the larger the far clipping plane is, and the more foreshortening is apparent. In addition, thinking about what the viewer can see in terms of a frustum is not particularly intuitive.
This function is as follows: For a realistic perspective, something around 45—90 degrees usually works well. You know that the viewport transformation happens after the projection transformation, so now is as good a time as any to discuss it. It is set using glViewport: Although the viewport generally 99 Chapter 4 n Transformations and Matrices matches your window size, there is nothing requiring it to be the same size.
There may be times when you want to limit rendering to a sub-region of your window, and setting a smaller viewport is one way to do this. The demo starts off with a perspective projection; pressing the spacebar enables you to toggle between orthographic shown in Figure 4.
The relevant portion of this demo is in the updateProjection and onResize methods of the Example class, which are listed here for convenience: Projections Figure 4. The second option is to use a combination of the glTranslate and glRotate functions to orient and position the viewpoint. For instance, you might want the viewpoint to be oriented through the polar coordinate system. The value 0, 0, 0 would naturally specify the origin. From this, OpenGL can determine the forward direction of the camera.
The last set of parameters upx, upy, upz is a vector that tells which direction is the up direction. Manipulating the Viewpoint Figure 4. Here is a short code snippet that uses the gluLookAt function. By manipulating the parameters, you can move the camera to any position and orientation that you want. One solution is to simply use the glRotate and glTranslate modeling-transformation functions as discussed earlier in this Chapter 4 n Transformations and Matrices chapter.
The code below uses the modeling functions to produce the same effect on the camera as the previous gluLookAt code. Moving the world 10 units along the negative z-axis effectively moves the camera to the position 0. But if you were orienting the camera at an odd angle, you would need to use the glRotate function as well, which leads to the next way of manipulating the camera: Using the modeling-transformation functions, you could create the following function to create the viewing transformation: This is just one of the uses of your own customized routines.
The greatest degree of camera control can be obtained by manually constructing and loading your own matrices, which will be covered in the next section. Eventually, though, you may want to create some advanced effects that are possible only by directly affecting the matrices. Loading Your Matrix Before you can load a matrix, you need to specify it.
For example, to access the bottom-left element of the matrix in Figure 4. The nth element in the array corresponds to element mn in Figure 4. As an example, if you want to specify the identity matrix you could use: This is done by calling glLoadMatrix: Multiplying Matrices In addition to loading new matrices onto the matrix stack and thus losing whatever information was previously in it , you can multiply the contents of the active matrix by a new matrix.
What You Have Learned Summary In this chapter, you learned how to manipulate objects in your scene by using transformations. You now have the means to place objects in a 3D world, to move and animate them, and to move around the world. What You Have Learned n Transformations allow you to move, rotate, and manipulate objects in a 3D world, while also allowing you to project 3D coordinates onto a 2D screen. Objects that are farther away appear smaller than objects that are closer to the camera.
How do you store the current matrix so that it can be restored later? Write the line of code that will position an object at 10, 5, 0. Name three different matrix stacks in OpenGL.
Write the line of code that will halve the size of the objects that are rendered after. Which command allows you to rotate an object?
On Your Own 6. Which command allows you to load your own matrix in to the current matrix stack? How do you restore a matrix that was previously pushed onto the stack? Write a program that renders a pyramid that rotates constantly around the y-axis and moves backwards and forwards along the z-axis.
This document describes, in detail, some extra functionality that allows you to render faster, in better quality or more easily. For example: Table 5. Functions and Tokens Table 5. Silicon Graphics Inc.
If the extension provides new functions, you will need to get a pointer to them before you can use them. Any new tokens must follow the consistent naming from the rest of OpenGL. Instead, you can just download the latest glext.
There are also wglext. As the implementations are unavailable when you compile your code, you need to link to them dynamically at runtime. This involves querying the graphics drivers for a function pointer to the function you want to use. First, you must declare a pointer to a function.
Fortunately, the previously mentioned headers provide us with some typedefs to make the function pointers a little more readable. If the function pointer is NULL after this call, then the extension that provides the function is not available. Now you can use the function pointer in the same way as a regular function.
After including glext. The next step is to get the address of the glGetStringi function and assign it to our pointer. You should check this before attempting to use the function pointer; otherwise, your program may crash. On platforms besides Windows, this is still the case.
However, on the Windows platform, you must use extensions to access any functionality after OpenGL 1. To understand why, we need to look at what is required to use OpenGL in your applications. To use any new functionality you must use the extension mechanism. The only thing left to do is to grab the extension strings one by one and store them in an array: Then you can use the function to query the available extensions.
We can bring all these steps together to create a function that returns an array of all supported extensions: Determining whether or not an extension is supported just consists of iterating over the array looking for the extension string you require. The isExtensionSupported function below does just that. To get access to this function, you will need to use wglGetProcAddress and check that the returned function pointer is not NULL before using it. The format of this function is as follows: Defining Tokens If you are using an up-to-date version of glext.
However, when you begin using many extension functions they very quickly become hard to manage, and obtaining the function pointers clutters up your initialization code. Fortunately, there are existing libraries that transparently initialize function pointers, specify tokens, and give you an easy way of checking if an extension is supported.
There are two options for using GLee in your applications: GLee internally holds a boolean variable for each extension that stores whether or not the extension is available. For example, if you wanted to see if the vertex buffer object extension is available which will always be the case in a GL 3. Figure 5. The program renders a simple heightmapbased terrain, which we will build on in later chapters to make more realistic.
One version of the application checks for and loads extensions manually; the other uses GLee to manage the extensions. On all platforms they provide access to the cutting-edge functionality on the latest graphics cards.
What are extensions for? GLSL and shader techniques are a massive topic so this chapter will provide a brief introduction rather than a complete reference. Unfortunately, the API has become very large, with a whole array of options to choose from for each particular rendering task.
Here is a list of the main functionality deprecated in Version 3. Most of the above functionality is now implemented using shaders, and some parts such as the matrix stack can be implemented in separate libraries. There are two main types of shaders: Figure 6. Vertex Shaders As we have already covered, OpenGL takes a series of vertices to build geometric primitives. These are then transformed, clipped, and rasterized to produce the pixels in the framebuffer as output.
One important thing to note is that a vertex shader only knows about a single vertex at a time; it is not possible to extract information on neighboring vertices. The only output that is required by a vertex shader is the position. The shader can output other variables, which can then be used as inputs to the fragment shader. These outputs are normally interpolated across the surface of the primitive and each fragment receives the interpolated value that corresponds with its position on the surface.
If the shader operates on two neighboring vertices and outputs 0. This is how colors, texture coordinates, etc. Once the vertex processing stage is complete, the graphics card takes back control until the fragment processing stage. When using a vertex shader, you will need to manually handle the following: Geometry shaders are still relatively new and currently only available as an extension.
They are generally used for advanced effects and are beyond the scope of this book. Fragment shaders take the outputs from the vertex shader which may have been interpolated as input variables; you can use these inputs to color or texture the fragment, or achieve more advanced effects such as bump mapping and per-pixel lighting. When using a fragment shader you must handle the following parts of the pipeline: We will be covering version 1. Variables that are passed into or out of a shader must be declared in global scope.
It is normal for a list of these variables to be declared at the top of the shader before the main function. The full list of preprocessor directives and their functions are shown in Table 6. A variable can exist in local scope between two curly braces , global scope at the top of the shader , or a variable can exist across more than one shader program scope.
Table 6. Each variable that is declared must specify a data type that it will hold throughout the lifetime of the variable. These data types include vectors and matrices with various dimensions. Samplers allow access to the pixel data of the texture and are required for many different effects. We will discuss textures and samplers in detail in the next chapter.
You can initialize a variable when it is declared like so: For vectors, these elements can be addressed individually by adding a period. For vectors, the component names that are available are as follows: Matrix components are accessed using the array-style notation: Arrays must be indexed with a zero-based integer constant. Negative indexes are illegal.
A structure must contain at least one data member. On types such as vectors that are made up of several components e. The one exception is multiplications involving matrices, which work using standard linear algebra rules. If you want to mix types, you need to use constructors to mimic the behavior of casting.
Centroid interpolation, which appears in the table, is sometimes used to avoid artifacts when using multisampling. Uniforms cannot be the target of an assignment inside the shader. Their value can only be set by the application.
They can be accessed by all stages of the shader program if the variable is declared identically in each shader.
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