Category: Opengl line width

  • Shop now. 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.

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    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.

    opengl line width

    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.

    opengl line width

    See All Related Store Items. 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.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

    Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. In my OpenGL app, it won't let me draw a line greater then ten pixels wide.

    Drawing Antialiased Lines with OpenGL

    Is there a way to make it draw more than ten pixels? You could try drawing a quad. Make it as wide as you want your line to be long, and tall as the line width you need, then rotate and position it where the line would go. I recommend to use a Shaderwhich generates triangle primitives along a line strip or even a line loop. That means to avoid computation of polygons on the CPU as well as geometry shaders or tessellation shaders. Each segment of the line consist of a quad represented by 2 triangle primitives respectively 6 vertices.

    Create an array with the corners points of the line strip. The array has to contain the first and the last point twice. Of course it would be easy, to identify the first and last element of the array by comparing the index to 0 and the length of the array, but we don't want to do any extra checks in the shader.

    If a line loop has to be draw, then the last point has to be add to the array head and the first point to its tail. The array of points is stored to a Shader Storage Buffer Object. We use the benefit, that the last variable of the SSBO can be an array of variable size.

    The shader doesn't need any vertex coordinates or attributes.

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    All we have to know is the index of the line segment. The coordinates are stored in the buffer. We have to create an "empty" Vertex Array Object without any vertex attribute specification :. For the coordinate array in the SSBO, the data type vec4 is used Pleas believe me, you don't want to use vec3 :. Compute the index of the line segment, where the vertex coordinate belongs too and the index of the point in the 2 triangles:. The coordinates have to be transformed from model space to window space.

    Don't forget the perspective divide. The drawing of the line will even work at perspective projection. The miter calculation even works if the predecessor or successor point is equal to the start respectively end point of the line segment. In this case the end of the line is cut normal to its tangent:.Using a line width other than 1 has different effects, depending on whether line antialiasing is enabled. Line antialiasing is initially disabled.

    If line antialiasing is disabled, the actual width is determined by rounding the supplied width to the nearest integer. If the rounding results in the value 0, it is as if the line width were 1. Otherwise, i pixels are filled in each row that is rasterized. If antialiasing is enabled, line rasterization produces a fragment for each pixel square that intersects the region lying within the rectangle having width equal to the current line width, length equal to the actual length of the line, and centered on the mathematical line segment.

    The coverage value for each fragment is the window coordinate area of the intersection of the rectangular region with the corresponding pixel square. This value is saved and used in the final rasterization step. Not all widths can be supported when line antialiasing is enabled. If an unsupported width is requested, the nearest supported width is used.

    Only width 1 is guaranteed to be supported; others depend on the implementation. Likewise, there is a range for aliased line widths as well. Clamping and rounding for aliased and antialiased lines have no effect on the specified value. Nonantialiased line width may be clamped to an implementation-dependent maximum.

    In OpenGL 1. The old names are retained for backward compatibility, but should not be used in new code. From OpenGL Wiki. Jump to: navigationsearch. Navigation menu Personal tools Create account Log in. Namespaces Page Discussion. Views Read View source View history. This page was last edited on 29 Aprilat The following error codes can be retrieved by the glGetError function. The glLineWidth function specifies the rasterized width of both aliased and antialiased lines.

    Using a line width other than 1. If line antialiasing is disabled, the actual width is determined by rounding the supplied width to the nearest integer.

    If the rounding results in the value 0. Otherwise, i pixels are filled in each row that is rasterized. If antialiasing is enabled, line rasterization produces a fragment for each pixel square that intersects the region lying within the rectangle having width equal to the current line width, length equal to the actual length of the line, and centered on the mathematical line segment.

    The coverage value for each fragment is the window coordinate area of the intersection of the rectangular region with the corresponding pixel square.

    opengl line width

    This value is saved and used in the final rasterization step. Not all widths can be supported when line antialiasing is enabled. If an unsupported width is requested, the nearest supported width is used.

    Only width 1. Clamping and rounding for aliased and antialiased lines have no effect on the specified value. Non-antialiased line width may be clamped to an implementation-dependent maximum. Although this maximum cannot be queried, it must be no less than the maximum value for antialiased lines, rounded to the nearest integer value.

    Skip to main content. Contents Exit focus mode. The default is 1. Return value This function does not return a value. Error codes The following error codes can be retrieved by the glGetError function. Remarks The glLineWidth function specifies the rasterized width of both aliased and antialiased lines. Yes No. Any additional feedback? Skip Submit.

    glLineWidth function

    Is this page helpful? The function was called between a call to glBegin and the corresponding call to glEnd.It does give you a straight line, but a very ugly one. To improve this, most people would enable GL line smoothing:. This article focuses on 2D rendering in sub pixel accuracy. Make sure you view all images in their original size. The first function line gives you all the functionality. You can choose not to use alpha blending by setting alphablend to false ; in this case, you will get color fading to the background.

    In no- alpha- blending mode, you still get good results when the background is solid and lines are not dense. It is also useful when doing overdraw. You can optionally use alpha blend; otherwise, it assumes the background is white. I provide this in case you do not need all the functionalities.

    If you copy only part of the code, make sure you also copy the function. You just need to know a little bit of OpenGL. Look at the hello world OpenGL program below.

    It merely draws a triangle with different colors on each vertex. What do you observe? The above observation is sufficient to enable us to do what we want. Now let's draw a parallelogram which changes color from white to red.

    The right side is still jaggy. The left side is smooth. Can you now think of anything? Now let's draw two parallelograms which change color from white to red then to white again. Let's call this the 'fade polygon technique': draw a thin quadrilateral to render the core part of a line, then draw two more beside the original one that fade in color.

    This gives us the effect of anti-aliasing. This article focuses on 2D line drawing so the meaning of "perfect quality" is with respect to 2D graphics. Let's see a picture from his article. The above picture shows lines with thickness starting from 0. Using triangles to approximate line segments in the correct dimension is not easy. I did it by experiment and hand calibrated the drawing code:.

    It is not perfect though, the end points are not sharp enough, and so I say "nearly perfect". Their difference is subtle, so make sure you flip them in a slideshow program to observe. I have made one for you here. It is seen that Cairo draws thin lines a little bit thicker than it should look. But you see, the horizontal line is a 2px grey line.

    But there is no guarantee in sub- pixel coordinates, other colors, and orientations. Ideal 1px black lines should look very close to aliased raw 1px lines, but just smoother.Maps are mostly made up of lines, as well as the occasional polygon thrown in.

    Unfortunately, drawing lines is a weak point of OpenGL.

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    As an alternative to native lines, we can tessellate the line to polygons and draw it as a shape. A few months ago, I investigated various approaches to line rendering and experimented with one that draws six triangles per line:.

    Two pairs of triangles form a quadrilateral gradient on each sides, and a quadrilateral in the middle makes up the actual line. The gradients provide antialiasing, so that the line fades out at the edges.

    When scaled down, this produces high quality lines:. Unfortunately, generating six triangles per line segment means generating eight vertices per line segment, which requires a lot of memory.

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    I worked on an experiment that uses only two vertices per line segment, but this way of drawing lines requires three draw calls per line. To maintain a good framerate we need to minimize the number of draw calls per frame. First, a list of vertices is passed to the vertex shader. The vertex shader is basically a small function that transforms every vertex in the model coordinate system to a new position the screen coordinate systemso that you can reuse the same array of vertices for every frame, but still do things like rotate, translate, or scale the objects.

    Three consecutive vertices form a triangle. All pixels in that area are then processed by the fragment shaderalso called the pixel shader. While the vertex shader is run once for every vertex in the source array, the fragment shader is run once for every pixel in a triangle to decide what color to assign to that pixel. In the simplest case, it might assign a constant color, like this:. The color order is RGBAso this example renders all fragments as opaque black.

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    When transforming vertices in the vertex shader, OpenGL allows us to assign attributes to every vertex, for example:. These attributes are then passed on to the pixel shader. This interpolation produces gradients between the vertices. When drawing lines, we have a couple of requirements:. Since we want to change the line width dynamically, we cannot perform the complete tessellation at setup time. Instead, we repeat the same vertex twice, so that for a line segment, we end up with four vertices marked in our array:.

    In addition, we calculate the normal unit vector for the line segment and assign it to every vertex, with the first vertex getting a positive unit vector and the second a negative unit vector. The unit vectors are the small arrows you see in this picture:. The vertex shader looks something like this:. In the main function, we multiply the normal unit vector with the line width to scale it to the actual line width.

    Finally, we multiply by the projection matrix to get the vertex position in projection space in our case, we use a parallel projection so there is not much going on, except for scaling the screen space coordinates to the range of In the vertex shader, we just pass through the normal unit vectors to the pixel shader.I am a computer graphics researcher at Yale Univerisity.

    I enjoy language learning, visiting new places and occasionally do rock climbing. This tutorial is more expanded version of an answer on my stackoverflow question. To summarize the goal, we want to be able to draw lines in 3D that satisfy the next conditions:.

    The default OpenGL drawing of line strip geometry does not allow to render smooth non-broken lines, as well as to render them of customary thicker than default width:. One of the ways to solve the problem is to represent each line segment as a set of triangles. Adjacent triangles or quads are drawn without any gaps between them. In this case we have to deal with two problems:.

    To address the problem 1, we have to make sure the geometry is always facing the camera, i. For the problem 2, the solution is similar - re-adjust the ribbon width with the change of viewport. A very effective way to achieve the desired effect is to use GLSL shaders. Assuming the familiarity of the aforementioned programs, we will move directly to the implementation details. The presented code is heavily based on the Cinder library discussion threadand the main principle of triangle coordinates calculation is taken from there as well.

    The vertex shader is what helps to transform the 3D world coordinates into screen coordinates. Simply speaking, this is where we deal with lines being always faced towards the camera.

    In order to implement it, we have to use model-view-projection MVP matrix which is the matrix that is updated on every view change.

    Global

    The position of each vertex of the triangle is calculated in relation towards the viewport of the widget which displays the whole scene.

    This allows the lines to have a constant thickness in spite of the their location in 3D world. Refer to the source code for more details on shader implementation. The fragment shader is a simple pass-through shader. It takes the incoming color and assigns it to each fragment:.

    For debugging purposes, I set up the color in the shader to green, so that to verify all the previous steps of shader program has completed successfully. We need to provide two uniform s: for MVP matrix and the Viewport. When using OSG, the best way to do it is by using callbacks. In this case we need to derive from osg::Uniform::Callback. Below are the code snippets for each of the callbacks:. Of course, we need to pass the pointer on a camera that is attached to the viewer that displays the scene.

    In a similar way we define the callback for viewport:. By following the OSG tutorials on how to set up and use shaders withing an OSG program, we create an osg::Program instance and attach to it the created shaders.

    This is how it can be done in OpenSceneGraph:. After we need to add the necessary uniforms, including the MVP matrix and viewport. And finally connect the shader program to the state set of the geometry. Note: in order to avoid an aliased look of the shadered lines, we have to enable multi-sampling. Some screenshots of the result lines. Note how the connection between the anchor point does not look broken compared to the red line.

    For this case we turned on the multi-sampling. The demonstration of ability to produce much thicker lines.


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