Category: Gl line stipple example

Gl line stipple example

Orders delivered to U. Learn more. If you've ever had a chemistry class and probably even if you haven'tyou know that all matter consists of atoms and that all atoms consist of only three things: protons, neutrons, and electrons. Although this explanation is a little oversimplified for almost anyone beyond the third or fourth grade, it demonstrates a powerful principle: With just a few simple building blocks, you can create highly complex and beautiful structures.

The connection is fairly obvious. Objects and scenes that you create with OpenGL also consist of smaller, simpler shapes, arranged and combined in various and unique ways. This chapter explores these building blocks of 3D objects, called primitives. All primitives in OpenGL are one- or two-dimensional objects, ranging from single points to lines and complex polygons. In this chapter, you learn everything you need to know to draw objects in three dimensions from these simpler shapes.

When you first learned to draw any kind of graphics on any computer system, you probably started with pixels. A pixel is the smallest element on your computer monitor, and on color systems, that pixel can be any one of many available colors. This is computer graphics at its simplest: Draw a point somewhere on the screen, and make it a specific color.

Then build on this simple concept, using your favorite computer language to produce lines, polygons, circles, and other shapes and graphics. Perhaps even a GUI With OpenGL, however, drawing on the computer screen is fundamentally different. You're not concerned with physical screen coordinates and pixels, but rather positional coordinates in your viewing volume.

You let OpenGL worry about how to get your points, lines, and everything else projected from your established 3D space to the 2D image made by your computer screen.

This chapter and the next cover the most fundamental concepts of OpenGL or any 3D graphics toolkit. In the upcoming chapter, we provide substantial detail about how this transformation from 3D space to the 2D landscape of your computer monitor takes place, as well as how to transform rotate, translate, and scale your objects.

For now, we take this ability for granted to focus on plotting and drawing in a 3D coordinate system. This approach might seem backward, but if you first know how to draw something and then worry about all the ways to manipulate your drawings, the material in Chapter 4, "Geometric Transformations: The Pipeline," is more interesting and easier to learn.

When you have a solid understanding of graphics primitives and coordinate transformations, you will be able to quickly master any 3D graphics language or API. See All Related Store Items. FREE U. Wright and Benjamin Lipchak Sep 24, Richard Wright and Benjamin Lipchak explain how simple shapes in a 3D space can be combined to form amazing 3D renderings using OpenGL in this sample chapter.

This chapter is from the book. Related Resources Store Articles. Join Sign In. All rights reserved.Join Stack Overflow to learn, share knowledge, and build your career. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I have an application that draws 3-d map view marked up lines that show various features. I am porting the map over to an OpenGL-ES architecture, but am having a bit of trouble working out how to display dashed lines.

Doing a lot of googling, I've found many references to the idea that drawing dashed lines and polygons were removed from OpenGL-ES as they can be easily emulated using textures triangles. An example, of one problem I have encountered trying to prototype out this concept is perspective squeezes my lines to invisible as they go towards the horizon. Any advice on how to achieve dashed constant width lines in a perspective view would be much appreciated. I believe you can apply a texture to a line, not just a triangle.

You'll need to set texture coordinates for each end of the line; the texture will be interpolated linearly along the line. The effectiveness of this solution should be invariant of whether you use lines or line strips - line strips are just a way to create lines with fewer vertices. There is one other problem: the tendency of the texture pattern to become compact as a line goes away from the camera.

This happens because texture coordinate interpolation is perspective-correct even for lines see section 3. If you can calculate a "q" coordinate for your texture that undoes the perspective, you can restore screen-space texturing. This technique is probably too performance-expensive. Texture coordinate generation is of course not available in GLES 1. The disadvantage of this technique is that the texture pattern will be in fixed screen space - that is, the texture won't run across the lines.

It will then connect 3,3 to 4,4but NOT 4,4 to 5,5. The final connection will be 5,5 to 6,6. Dotted Lines on Android. An alternative idea to create a more intentionally patterned stipple would be to skip certain vertices by instead drawing with an indices array glDrawElements. However, I don't have an example of that to show you.

Learn more. Asked 11 years, 1 month ago. Active 6 years, 11 months ago. Viewed 9k times.Specifies a multiplier for each bit in the line stipple pattern.

If factor is 3, for example, each bit in the pattern is used three times before the next bit in the pattern is used. Specifies a bit integer whose bit pattern determines which fragments of a line will be drawn when the line is rasterized. Bit zero is used first; the default pattern is all 1's. Line stippling masks out certain fragments produced by rasterization; those fragments will not be drawn. The masking is achieved by using three parameters: the bit line stipple pattern patternthe repeat count factorand an integer stipple counter s.

It is incremented after each fragment of a unit width aliased line segment is generated or after each i fragments of an i width line segment are generated. The i fragments associated with count s are masked out if. Bit zero of pattern is the least significant bit. Whether rectangle s is rasterized or not depends on the fragment rule described for aliased lines, counting rectangles rather than groups of fragments.

When enabled, the line stipple pattern is applied as described above. When disabled, it is as if the pattern were all 1's. Initially, line stippling is disabled. Light Dark. OpenGL 2. Parameters factor Specifies a multiplier for each bit in the line stipple pattern. Description Line stippling masks out certain fragments produced by rasterization; those fragments will not be drawn. Think you can improve this page? Edit this page on GitHub.

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All other content is in the public domain.Abstract : We learn more of the line rendering capabilities of OpenGL. OpenGL only supports a few basic primitive geometrical objects: points, lines, polygons, and surfaces described by arrays of small quadrilaterals or triangles.

The main idea behind OpenGL's simplicity is that it is up to the developer to implement from this simple objects more complex geometrical models. OpenGL contains a number of commands to control the details of points, lines and polygons.

By the default the size of points is 1. Notice that the size of a point is specified by a float number; fractional point and line sizes are allowed.


OpenGL interprets fractional pixel sizes according to the rendering context. If the anti-aliasing mode is enable then OpenGL modifies the neighboring pixels to the line in question in order to give the appearance of a fractional width. Anti-aliasing is a technique also used to eliminate the ugly stars the straight lines show on computer screens at low monitor resolution.

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If anti-aliasing is not enable then glPointSize will round off size to the closets integer. The physical size of a pixel actually is device dependent. So for example at low monitor resolution a pixel appears wider. Similarly on very high resolution devices, like a plotter, the default 1 pixel line can appear almost invisible.

To estimate the real width of your lines you must know the actual physical dimensions of pixels on the output device. The width of lines is specified by the glLineWidth function, that must be invoked before the glBegin - glEnd pair that draws the line s. Here is the full syntax of the command:. OpenGL implementations may limit the width of nonantialiased lines to its maximum antialiased line width, rounded to the nearest integer value. Also keep in mind that line widths are measured not perpendicularly to the line but in the y-direction if the absolute value of the slope is less than 1; if greater in the x-direction.

This month we have prepared another simple but hopefully useful 2D animation that shows you how to use various kinds of line widths in your OpenGL applications example2. I chose an example from Quantum Physics, a quantum particle trapped in a double well potential. Umm actually I forget.

Anyway I figure that it would be useful for physics and engineering students to see how to integrate the time dependent Schroedinger equation, others may just enjoy watching the non-intuitive nature of quantum mechanics.

A particle in QM is not represented by a position and a velocity but by a "wave" a quantum wave solid purple line in our animation whose absolute square value represents the probability of observing the particle at a given position dash white line :. This method is far more accurate and rapid than any finite difference method.

It is applicable to nonlinear wave propagation; the time evolution operator is split to second or higher order into operators only dependent on either position and momentum frequencythen the wavefunction is evolved in time by successively applying these operators switching back and forth between the position and momentum frequency space. The body of the source code can be used for many other applications. You can swap my quantum simulation with your own time dependent function and get a nice animation of your system.

You could also try to write a simplified OpenGL-based gnuplot for plotting functions and data files. If the reader has followed the previous articles on GLUT and OpenGL this source code will appear too simple and easy to understand of course, quantum mechanics aside.

There is nothing extraordinary going here. In the main we open a single window in double-buffer mode, then we pass a display and idle callback functions that take care of plotting the wavefunction and integrating the wave equation respectively.

Again, do mind what goes on in the idle function, although it is a very beautiful trick is not necessary to fully understand it to grasp the content of this article. The really new OpenGL stuff is in the display callback function:. The first thing done is to clear the color buffer bit, this gives us a clean black drawing board. Then we add a footnote using glRasterPos and glutBitmapCharacter drawstring is nothing but a wrapper for the clut utility.

In future lessons glRasterPos will appear again as an auxiliary function for texture rendering.A display list is a group of OpenGL commands that have been stored for later execution. When a display list is invoked, the commands in it are executed in the order in which they were issued.

Most OpenGL commands can be either stored in a display list or issued in immediate modewhich causes them to be executed immediately. You can freely mix immediate-mode programming and display lists within a single program.

The programming examples you've seen so far have used immediate mode. This chapter discusses what display lists are and how best to use them. It has the following major sections:. Display lists may improve performance since you can use them to store OpenGL commands for later execution.

It is often a good idea to cache commands in a display list if you plan to redraw the same geometry multiple times, or if you have a set of state changes that need to be applied multiple times. To see how you can use display lists to store geometry just once, consider drawing a tricycle. The two wheels on the back are the same size but are offset from each other. The front wheel is larger than the back wheels and also in a different location.

An efficient way to render the wheels on the tricycle would be to store the geometry for one wheel in a display list then execute the list three times.

glLineStipple function

You would need to set the modelview matrix appropriately each time before executing the list to calculate the correct size and location for the wheels. When running OpenGL programs remotely to another machine on the network, it is especially important to cache commands in a display list. In this case, the server is a different machine than the host.

Since display lists are part of the server state and therefore reside on the server machine, you can reduce the cost of repeatedly transmitting that data over a network if you store repeatedly used commands in a display list. When running locally, you can often improve performance by storing frequently used commands in a display list. Some graphics hardware may store display lists in dedicated memory or may store the data in an optimized form that is more compatible with the graphics hardware or software.

See "Display-List Design Philosophy" for a detailed discussion of these optimizations. A display list is a convenient and efficient way to name and organize a set of OpenGL commands.

For example, suppose you want to draw a torus and view it from different angles. The most efficient way to do this would be to store the torus in a display list. Then whenever you want to change the view, you would change the modelview matrix and execute the display list to draw the torus.

Example illustrates this. Let's start by looking at init. It creates a display list for the torus and initializes the viewing matrices and other rendering state. Note that the routine for drawing a torus torus is bracketed by glNewList and glEndListwhich defines a display list. The argument listName for glNewList is an integer index, generated by glGenListsthat uniquely identifies this display list. Whenever this happens, the callback function keyboard is called, which concatenates a degree rotation matrix about the x- or y-axis with the current modelview matrix.

Then glutPostRedisplay is called, which will cause glutMainLoop to call display and render the torus after other events have been processed. The display function is very simple: It clears the window and then calls glCallList to execute the commands in the display list. If we hadn't used display lists, display would have to reissue the commands to draw the torus each time it was called. A display list contains only OpenGL commands.

In Exampleonly the glBeginglVertexand glEnd calls are stored in the display list. The parameters for the calls are evaluatedand their values are copied into the display list when it is created.

All the trigonometry to create the torus is done only once, which should increase rendering performance. However, the values in the display list can't be changed later. And once a command has been stored in a list it is not possible to remove it. Neither can you add any new commands to the list after it has been defined.Join Stack Overflow to learn, share knowledge, and build your career.

Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. What is it replaced with?


If not replaced, how can I get a similar effect? I don't want to use a compatibility profile of course Sorry, it hasn't been replaced with anything. The first idea coming to my mind for emulating it would be the geometry shader. You feed the geometry shader with a line, compute its screen space length and based on that you generate a variable number of sub lines between its start and end vertex. EDIT: Perhaps you could also use a 1D texture with the alpha or red channel encoding the pattern as 0.

You can facilitate the geometry shader to generate your line texCoords, as otherwise you need different vertices for every line. This way you can also make the texCoord dependent on the screen space length of the line.

Then you have to do the triangle-line transformation yourself in the geometry shader, putting in triangles and putting out lines that could also be a reason for deprecating polygon mode in the future, if it hasn't already. It's just a minimal solution, feel free to add custom features yourself.

I haven't tested it because I lack the neccessary hardware, but you should get the point:. In the geometry shader we take a line and compute its screen space length in pixels. This is taken as 1D texture coordinate of the second line end point. Note that this texture coordinate has to be interpolated linearly noperspective interpolation specifier. The usual perpective-correct interpolation would cause the stipple pattern to "squeeze together" on farther away parts of the line, whereas we are explicitly working with screen-space values.

The fragment shader now just performs a simple alpha test using the value from the pattern texture, which contains a 1 for line and a 0 for no line.

So to emulate the fixed function stipple you would have a 16 pixel 1-component 1D texture instead of a 16bit pattern. But as said earlier, if you want to render triangles using glPolygonModeit won't work this way. Instead you have to adapt the geometry shader to accept triangles and generate 3 lines for each triangle. EDIT: In fact OpenGL 3's direct support for integer operations in shaders allows us to completely drop this whole 1D-texture approach and work straight-forward with an actual bit-pattern.

Thus the geometry shader is slightly changed to put out the actual screen-size pattern coordinate, without normalization:. In the fragment shader we then just take a bit pattern as unsigned integer though bit in contrast to glLineStipple 's bit value and the stretch factor of the pattern and just take the texture coordinate well, no texture anymore actually, but nevermind modulo 32 to get it's position on the pattern those explicit uint s are annoying, but my GLSL compiler says implicit conversions between int and uint are evil :.

To answer this question, we've to investigate first, what glLineStipple actually does.

gl line stipple example

When using line segments, the stipple pattern started at each segment. The pattern is restarted at each primitive. When using a line strip, then the stipple pattern is applied seamless to the entire polygon. A pattern seamlessly continuous beyond vertex coordinates. Be aware that the length of the pattern is stretched at the diagonals. This is possibly the key to the implementation.Although you can draw complex and interesting pictures using OpenGL, they're all constructed from a small number of primitive graphical items.

This shouldn't be too surprising - look at what Leonardo da Vinci accomplished with just pencils and paintbrushes. At the highest level of abstraction, there are three basic drawing operations: clearing the window, drawing a geometric object, and drawing a raster object.

gl line stipple example

Raster objects, which include such things as two-dimensional images, bitmaps, and character fonts, are covered in Chapter 8. In this chapter, you learn how to clear the screen and to draw geometric objects, including points, straight lines, and flat polygons. You might think to yourself, "Wait a minute. I've seen lots of computer graphics in movies and on television, and there are plenty of beautifully shaded curved lines and surfaces.

How are those drawn, if all OpenGL can draw are straight lines and flat polygons? It turns out that all the curved lines and surfaces you've seen are approximated by large numbers of little flat polygons or straight lines, in much the same way that the globe on the cover is constructed from a large set of rectangular blocks.

The globe doesn't appear to have a smooth surface because the blocks are relatively large compared to the globe.

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Later in this chapter, we show you how to construct curved lines and surfaces from lots of small geometric primitives. One thing to keep in mind as you read the rest of this chapter is that with OpenGL, unless you specify otherwise, every time you issue a drawing command, the specified object is drawn.

This might seem obvious, but in some systems, you first make a list of things to draw. When your list is complete, you tell the graphics hardware to draw the items in the list. The first style is called immediate-mode graphics and is the default OpenGL style. In addition to using immediate mode, you can choose to save some commands in a list called a display list for later drawing.

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Immediate-mode graphics are typically easier to program, but display lists are often more efficient. Chapter 7 tells you how to use display lists and why you might want to use them. This section explains how to clear the window in preparation for drawing, set the color of objects that are to be drawn, and force drawing to be completed.

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None of these subjects has anything to do with geometric objects in a direct way, but any program that draws geometric objects has to deal with these issues. Drawing on a computer screen is different from drawing on paper in that the paper starts out white, and all you have to do is draw the picture. On a computer, the memory holding the picture is usually filled with the last picture you drew, so you typically need to clear it to some background color before you start to draw the new scene.

The color you use for the background depends on the application. For a word processor, you might clear to white the color of the paper before you begin to draw the text. If you're drawing a view from a spaceship, you clear to the black of space before beginning to draw the stars, planets, and alien spaceships.

Sometimes you might not need to clear the screen at all; for example, if the image is the inside of a room, the entire graphics window gets covered as you draw all the walls.

gl line stipple example

At this point, you might be wondering why we keep talking about clearing the window - why not just draw a rectangle of the appropriate color that's large enough to cover the entire window? First, a special command to clear a window can be much more efficient than a general-purpose drawing command.

In addition, as you'll see in Chapter 3OpenGL allows you to set the coordinate system, viewing position, and viewing direction arbitrarily, so it might be difficult to figure out an appropriate size and location for a window-clearing rectangle.

Finally, on many machines, the graphics hardware consists of multiple buffers in addition to the buffer containing colors of the pixels that are displayed. These other buffers must be cleared from time to time, and it's convenient to have a single command that can clear any combination of them.

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See Chapter 10 for a discussion of all the possible buffers. You must also know how the colors of pixels are stored in the graphics hardware known as bitplanes.

There are two methods of storage. Either the red, green, blue, and alpha RGBA values of a pixel can be directly stored in the bitplanes, or a single index value that references a color lookup table is stored.

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