# Drawing in Space: Geometric Primitives and Buffers in OpenGL

This chapter is from the book

## Drawing Triangles in 3D

You've seen how to draw points and lines and even how to draw some enclosed polygons with GL_LINE_LOOP. With just these primitives, you could easily draw any shape possible in three dimensions. You could, for example, draw six squares and arrange them so they form the sides of a cube.

You might have noticed, however, that any shapes you create with these primitives are not filled with any color; after all, you are drawing only lines. In fact, all that arranging six squares produces is a wireframe cube, not a solid cube. To draw a solid surface, you need more than just points and lines; you need polygons. A polygon is a closed shape that may or may not be filled with the currently selected color, and it is the basis of all solid-object composition in OpenGL.

### Triangles: Your First Polygon

The simplest polygon possible is the triangle, with only three sides. The GL_TRIANGLES primitive draws triangles by connecting three vertices together. The following code draws two triangles using three vertices each, as shown in Figure 3.14:

```glBegin(GL_TRIANGLES);
glVertex2f(0.0f, 0.0f);      // V0
glVertex2f(25.0f, 25.0f);    // V1
glVertex2f(50.0f, 0.0f);    // V2

glVertex2f(-50.0f, 0.0f);    // V3
glVertex2f(-75.0f, 50.0f);    // V4
glVertex2f(-25.0f, 0.0f);    // V5
glEnd();```

Figure 3.14 Two triangles drawn using GL_TRIANGLES.

NOTE

The triangles will be filled with the currently selected drawing color. If you don't specify a drawing color at some point, you can't be certain of the result.

### Winding

An important characteristic of any polygonal primitive is illustrated in Figure 3.14. Notice the arrows on the lines that connect the vertices. When the first triangle is drawn, the lines are drawn from V0 to V1, then to V2, and finally back to V0 to close the triangle. This path is in the order that the vertices are specified, and for this example, that order is clockwise from your point of view. The same directional characteristic is present for the second triangle as well.

The combination of order and direction in which the vertices are specified is called winding. The triangles in Figure 3.14 are said to have clockwise winding because they are literally wound in the clockwise direction. If we reverse the positions of V4 and V5 on the triangle on the left, we get counterclockwise winding. Figure 3.15 shows two triangles, each with opposite windings.

Figure 3.15 Two triangles with different windings.

OpenGL, by default, considers polygons that have counterclockwise winding to be front facing. This means that the triangle on the left in Figure 3.15 shows the front of the triangle, and the one on the right shows the back side of the triangle.

Why is this issue important? As you will soon see, you will often want to give the front and back of a polygon different physical characteristics. You can hide the back of a polygon altogether or give it a different color and reflective property (see Chapter 5, "Color, Materials, and Lighting: The Basics"). It's important to keep the winding of all polygons in a scene consistent, using front-facing polygons to draw the outside surface of any solid objects. In the upcoming section on solid objects, we demonstrate this principle using some models that are more complex.

If you need to reverse the default behavior of OpenGL, you can do so by calling the following function:

`glFrontFace(GL_CW);`

The GL_CW parameter tells OpenGL that clockwise-wound polygons are to be considered front facing. To change back to counterclockwise winding for the front face, use GL_CCW.

### Triangle Strips

For many surfaces and shapes, you need to draw several connected triangles. You can save a lot of time by drawing a strip of connected triangles with the GL_TRIANGLE_STRIP primitive. Figure 3.16 shows the progression of a strip of three triangles specified by a set of five vertices numbered V0 through V4. Here, you see the vertices are not necessarily traversed in the same order they were specified. The reason for this is to preserve the winding (counterclockwise) of each triangle. The pattern is V0, V1, V2; then V2, V1, V3; then V2, V3, V4; and so on.

Figure 3.16 The progression of a GL_TRIANGLE_STRIP.

For the rest of the discussion of polygonal primitives, we don't show any more code fragments to demonstrate the vertices and the glBegin statements. You should have the swing of things by now. Later, when we have a real sample program to work with, we resume the examples.

There are two advantages to using a strip of triangles instead of specifying each triangle separately. First, after specifying the first three vertices for the initial triangle, you need to specify only a single point for each additional triangle. This saves a lot of program or data storage space when you have many triangles to draw. The second advantage is mathematical performance and bandwidth savings. Fewer vertices mean a faster transfer from your computer's memory to your graphics card and fewer vertex transformations (see Chapters 2 and 4).

TIP

Another advantage to composing large flat surfaces out of several smaller triangles is that when lighting effects are applied to the scene, OpenGL can better reproduce the simulated effects. You'll learn more about lighting in Chapter 5.

### Triangle Fans

In addition to triangle strips, you can use GL_TRIANGLE_FAN to produce a group of connected triangles that fan around a central point. Figure 3.17 shows a fan of three triangles produced by specifying four vertices. The first vertex, V0, forms the origin of the fan. After the first three vertices are used to draw the initial triangle, all subsequent vertices are used with the origin (V0) and the vertex immediately preceding it (Vn–1) to form the next triangle.

Figure 3.17 The progression of GL_TRIANGLE_FAN.

### InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

## Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

## Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

### Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

### Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

### Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

### Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

### Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

### Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

## Other Collection and Use of Information

### Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

### Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

### Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

### Do Not Track

This site currently does not respond to Do Not Track signals.

## Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

## Children

This site is not directed to children under the age of 13.

## Marketing

Pearson may send or direct marketing communications to users, provided that

• Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
• Such marketing is consistent with applicable law and Pearson's legal obligations.
• Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
• Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

## Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

## Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

## Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

## Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

## Sharing and Disclosure

Pearson may disclose personal information, as follows:

• As required by law.
• With the consent of the individual (or their parent, if the individual is a minor)
• In response to a subpoena, court order or legal process, to the extent permitted or required by law
• To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
• In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
• To investigate or address actual or suspected fraud or other illegal activities
• To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
• To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
• To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.