Is there a way to draw a triangle with line only ?
I think GL_TRIANGLES option make triangle filled with color.
Set the fill mode with glPolygonMode(face, model):
glPolygonMode(GL_FRONT_AND_BACK,GL_LINE);
You have to set this every frame
Is there a way to draw a triangle with line only?
Use GL_LINES, GL_LINE_STRIP , or GL_LINE_LOOP (difference see here) with the same vertices that you use for GL_TRIANGLES.
if you are only rendering a single triangle at a time you can use GL_LINE_LOOP. it will connect the first and last, so if you have more than one triangle, it won't work.
Related
I want to keep scale when i rotate verticles. But it is getting smaller after rotation. Picture 3 and 4:
How can i keep it same size ?
Rotate is the wrong tool, if you look directly at the circular face after rotating it will remain circular, where the result you are after will become an oval.
The tool you want is called shear, which will move the top vertices the opposite way to the bottom vertices without altering the height. You can find shear in Mesh->Transform->Shear or the shortcut Ctrl-Alt-Shift-S. The shear tool transforms based on the view so you will want to be in side view when you use it.
Another option would be to use the knife tool to cut a line through your tube.
You can also position a plane over the tube and use intersect (Knife).
I am actually trying to develop a web application that would visualize a Finite Element mesh. In order to do so, I am using WebGl. Right now I have a page with all the code necessary to draw the mesh in the viewport using triangles as primitives (each quad element of the mesh was splitted into two triangles to draw it). The problem is that, when using triangles, all the piece is "continuous" and you cant see the separation between triangles. In fact, what I would like to achieve is to add lines between the nodes so that, around each quad element (formed by two triangles) we have these lines in black, and so the mesh can actually be shown.
So I was able to define the lines in my page, but since one shader just can have one type of primitive, if I add the code for the line buffers and bind them it just show the lines, not the element (as they were the last buffers binded).
So the closest solution I have found is using multiple shaders, and managing them with multiple programs, but this solution would just enable me whether to plot the geometry with trias or to draw just the lines, depending on which program is currently selected.
Could any of you help me about how to approach this issue? I have seen a windows application that shows FE meshes using OpenGL and it is able to mix the triangles with points and lines, apart from using different layers, illumination etc. So I am aware that this may be complicated, but I assume that if it is possible somehow with OpenGl it should be as well with webGL.
Please if you provide any solution I would appreciate a lot that it contains some code as an example, for instance drawing a single triangle but including three black lines at its borders and maybe three points at the vertices.
setup()
{
<your current code here>
Additional step - Unbind the previous textures, upload and bind one 1x1 black pixel as a texture. Let this texture object be borderID;
}
Draw loop()
{
Unbind the previous textures, bind your normal textures, and draw the mesh like your current setup. This will fill the entire area with different colours, without border (the current case)
Bind the borderID texture, and draw the same vertices again except this time, use context.LINE_STRIP instead of context.TRIANGLES. This will draw lines with the black texture, and will appear as border, on top of the previously drawn colors for each triangle. You can have something like below
if(currDrawMode==0)
context3dStore.bindTexture(context3dStore.TEXTURE_2D, meshTextureObj[bindId]); else context3dStore.bindTexture(context3dStore.TEXTURE_2D, borderTexture1pixObj[bindId]);
context3dStore.drawElements((currDrawMode == 0) ? context3dStore.TRIANGLES: context3dStore.LINE_LOOP, indicesCount[bindId], context3dStore.UNSIGNED_SHORT, 0); , where currDrawMode toggles between drawing the border and drawing the meshfill.
Since the line texture appears as a border over the flat colors you had earlier, this should solve your need
}
How to detect vertical and horizontal lines ? i know opencv provide the function based on hough transform to detect all direction lines,but it`s low_performance to compute all direction lines because i just want to dectect Ver and Hon lines .Any suggestion, thanks!
HoughLinesP is the easiest approach. You can filter out the angles that aren't horizontal or vertical.
However, if you want to use a potentially faster approach that only detects certain angles, you can try cv::Sobel. There's some sample code here that detects vertical and horizontal edges:
http://subversion.assembla.com/svn/CoffeeWatcher/trunk/ImageUtils.cpp
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. That's great, but I can't find anyone that actually does this emulation and/or has a description of the steps involved.
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. Using LINE_STRIP, this doesn't happen, and the lines remain a constant width in the map.
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 3.5 of the GL spec).
There are two ways to get around this:
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.
You can project the texture in eye space (e.g. glTexGen).
Texture coordinate generation is of course not available in GLES 1.1, but if you are using vertices by array, you can fake it by:
Setting your texture coordinate array to be your vertex coordinate array and
Using the texture matrix to "transform" the vertices.
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.
If all you want is to draw dashed lines, just change from GL_LINE_STRIP to GL_LINES. That way, open GL will connect vertices 1&2, 3&4, 5&6, but not 3&4, or 4&5 leaving spaces there. It will in essence be a half/half ratio dotted line-- the rough equivalent of glLineStipple(1, 0101);
IE: in the vertex array
[0,0,1,1,2,2,3,3,4,4,5,5,6,6]
OpenGL will connect (0,0) to (1,1), but will not connect (1,1) to (2,2) [whereas it would with GL_LINE_STRIP]. It will then connect (3,3) to (4,4), but NOT (4,4) to (5,5). The final connection will be (5,5) to (6,6).
This is what it looks like when I did it:
Dotted Lines on Android
The line is not 50/50 dotted/empty because in my case, it represents the position of a game entity each game frame-- and game frames are not necessarily all of equal length, thus the inconsistent line:space ratio.
The code looks like this to draw it:
public void draw(GL10 gl)
{
gl.glDisable(GL10.GL_TEXTURE_2D);
gl.glColor4f(color[0], color[1], color[2], color[3] / fade);
//pointsBuffer holds the points on the line
gl.glVertexPointer(2, GL10.GL_FLOAT, 0, pointsBuffer);
gl.glDrawArrays(GL10.GL_LINES, 0, points.length/2);
gl.glEnable(GL10.GL_TEXTURE_2D);
}
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. :/
Is there a way to draw a line along a curved path with a gradient that varies in a direction perpendicular to the direction of the line? I am using the GDI+ framework for my graphics.
The simple answer is no. You can create a GraphicsPath in order to describe what you would like to draw, using AddPoint/AddLine/AddBezier and so forth as needed to describe the complex path of what you want to draw. When you draw the path you can provide a Brush which can be something like LinearGradientBrush or RadialGradientBrush. Neither of those gradient brushes reacts to the actual path being drawn in the sense of changing direction as the drawing occurs. You have to specify the angles etc as constant for the entire gradient area.
One possible method you can use is to set the clip region of the Graphics object to be that of the line only. Then draw a Linear Gradient over the extremes of the line e.g.
GraphicsPath gp = new GraphicsPath();
gp.AddArc(); // etc...
graphics.SetClip( gp );
graphics.FillRectangle( myLinearGradientBrush, gp.GetBounds());
The above code might give you what you are looking for.