I want to draw a simple Bézier curve (4 control points) with GLUT in a 3D scene. Now I want to duplicate it, then use quads or polygons to join points from the two curves in order to make a surface.
So to achieve this, I think that I need a way to get some coordinates of the Bézier curve.
How can I get them?
Just to make myself clear, I'm trying to draw a 3D surface based on a Bézier-curve.
Or is there any other way to achieve this?
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I am new to OpenGL.
I want to draw an object which has 4 vertices. It is like a quad object, but for bottom side I need to draw an arc. Other sides are connected with straight lines. I want to fill the object.
Can anybody guide me to do this please?
Triangulate your shape and render those triangles any way you prefer (immediate mode / VBO / VAO).
Convert your arc shape into segments. Number of vertices depends on detalization/smoothness you want to achieve.
Triangulate the shape. With simple shapes, like this one, you can do it manually in code (draw it on paper like I did and write down vertices indexes that form triangles). With more complicated shapes you could use a triangulation algorithms (available on Net). When shapes are even more complicated (i.e. animal outline) - you might need to use special 2D/3D modelling software just to make them, and it will do triangulation in there.
Render the triangles.
I am doing a snake and ladder game in VC++ using opengl.
I think i can draw a snake using arc and line. But i don't have an idea of how to do it in opengl. Can anyone give me some guidance?
Arc must be broken into small line segments manually. GL can render points, lines, triangles, quads and polygons only.
You might want to take a look at spline interpolation, e.g. Bezier splines, B-splines etc.
I'm using OpenGL and was told I should draw circles at each vertex of my outline to get smoothness. I tried this and it works great. The problem is speed. It crippled my application to draw a circle at each vertex. I'm not sure how else to fix the anomaly of my outlines other than circles, but using display lists and trying with vertex array both were brutally slow. Thanks
see: Edges on polygon outlines not always correct
One (perhaps too fancy) alternative is to draw a single polygon that bounds the circle (say, a quad), and then use a fragment program to discard the fragments. This would not be entirely trivial to write, but I would bet it's the fastest way.
You would simply pass the circle parameters to the fragment program and discard the fragment if the distance from the fragment center to the center of the circle is bigger than the desired radius.
Have you seen this article?
..or if you have access to the GL utility library, you could use gluDisk
I need to draw a circle on some arbitrary non plane surface, but this circle should lay on surface and follow surface's irregular form. In other words ( that is actuially can be one of possible solutions) want to have a "shadow" like projection on non plane surface near the mouse pointer. Do I need to create in memory a sphere and project it on the surface ? Are there some other techniques to achieve the same goal?
Thank you in advance.
There are two ways to do this. First would be, to create a cylinder and intersect it with the surface, get the intersection segments, and draw them. If you already have a math library which you can leverage, and if you don't have to do intersections every frame, then this might be a good idea. You will get accurately what you want.
The other option, as you already suggested would be to use a projection. I am not sure though that, you will be able to see clearly the shadow of a circle on a surface. If however, you have parametric texture coordinates for your surface, you can create a texture with the circle imprinted on it, and apply this texture to the surface.
I have enjoyed learning to use OpenGL under the context of games programming, and I have experimented with creating small shapes. I'm wondering if there are any resources or apps that will generate code similar to the following with a simple paint-like interface.
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_LINE_STRIP);
glVertex2f(1, 0);
glVertex2f(2, 3);
glVertex2f(4, 5);
glEnd();
I'm having trouble thinking of the correct dimensions to generate shapes and coming up with the correct co-ordinates.
To clarify, I'm not looking for a program I can just freely draw stuff in and expect it to create good code to use. Just more of a visual way of representing and modifying the sets of coordinates that you need.
I solved this to a degree by drawing a shape in paint and measuring the distances between the pixels relative to a single point, but it's not that elegant.
It sounds like you are looking for a way to import 2d geometry into your application. The best approach in my opinion would be to develop a content pipeline. It goes something like this:
You would create your content in a 3d modeling program like Google's Sketchup. In your case you would draw 2d shapes using polygons.
You need a conversion tool to get the data out of the original format and into a format that your target application can understand. One way to get polygon and vertex data out of Sketchup is to export to Collada and have your tool read and process it. (The simplest format would be a list of triangles or lines.)
Write a geometry loader in your code that reads the data created by your conversion tool. You need to write opengl code that uses vertex arrays to display the geometry.
The coordinates you'll use just depend on how you define your viewport and the resolution you're operating in. In fact, you might think about collecting the coordinates of the mouse clicks in whatever arbitrary coordinate system you want and then mapping those coordinates to opengl coordinates.
What kind of library are you expecting?
something like
drawSquare(dx,dy);?
drawCircle(radius);?
drawPoly(x1,y1,x2,y2....);?
Isn't that exactly the same as glVertex but with a different name? Where is the abstraction?
I made one of these... it would take a bitmap image, and generate geometry from it. try looking up triangulation.
the first step is generating the edge of the shape, converting it from pixels to vertices and edges, find all the edge pixels and put a vertex at each one, then based on either the distance between vertices, or (better) the difference in gradient between edges to cull out vertices and reduce the poly count of the mesh.
if your shape drawing program works with 'vector graphics' rather than pixels, i.e. plotting points and having lines drawn between them, then you can skip that first step and you just need to do triangulation.
the second step, once you have your edges and vertices is triangulation, in order to generate triangles, ear clipping is a simple method for instance.
as for the coordinates to use? that’s entirely up to you as others have said, to keep it simple, Id just work in pixel coordinates.
you can then scale and translate as needed to transform the shape for use.