I need to draw a load of cubes and would like them to be white with black stroke. At the moment I am storing all of these cubes in a VBO and I can draw them in wireframe and filled with no outline.
I would like to draw them like the image on the left in this image, stroked only on the sides facing the camera, not like the right.
I am using OpenGL.
What you want is to remove hidden lines.
If you want to draw a wireframe object with hidden lines removed, one approach is to draw the outlines using lines and then fill the interiors of the polygons making up the surface with polygons having the background color.
You need to glEnable(GL_CULL_FACE); in order to get back-face culling of triangles not visible automatically applied. Provided the "winding order" of your triangles is consistent of course (clockwise or anti-clockwise). If they're wound in the opposite direction, you can tell OpenGL which direction to use with glFrontFrace(GL_CW | GL_CCW) and whether to cull front or back facing triangles with glCullFace(GL_BACK | GL_FRONT).
Related
How can I draw a shape like this in OpenGL?
I mean, I know how to draw polygons in OpenGL. I want to know how to make the outline black and the fill color ( for example ) yellow?
You have 5 vertices. Draw a GL_POLYGON with them and then then a GL_LINE_LOOP.
Note that GL_POLYGON is only valid for convex polygons.
I would cheat. I would create the polygons in a 3d suite as meshes, and create different meshes for the borders. And then draw first the polygon and then the border, using the same transforms, with glDepthFunc(GL_LEQUAL).
In this way you can also give the border a nice outfit: you could make them look as if they were drawn by a pen, or sketched with a pencil or whatever. Also, this solution is good to go with modern OpenGL, while using GL_POLYGON is not .
So Im trying to figure out the best way to render a 3D model in OpenGL when some of the textures applied to it have alpha channels.
When I have the depth buffer enabled, and start drawing all the triangles in a 3D model, if it draws a triangle that is in front of another triangle in the model, it will simply not render the back triangle when it gets to it. The problem is when the front triangle has alpha transparency, and should be able to be seen through to the triangle behind it, but the triangle behind is still not rendered.
Disabling the depth buffer eliminates that problem, but creates the obvious issue that if the triangle IS opaque, then it will still render triangles behind it on top if rendered after.
For example, I am trying to render a pine tree that is basically some cones stacked on top of each other that have a transparent base. The following picture shows the problem that arises when the depth buffer is enabled:
You can see how you can still see the outline of the transparent triangles.
The next picture shows what it looks like when the depth buffer is disabled.
Here you can see how some of the triangles on the back of the tree are being rendered in front of the rest of the tree.
Any ideas how to address this issue, and render the pine tree properly?
P.S. I am using shaders to render everything.
If you're not using any partial transparency (everything is either 0 or 255), you can glEnable(GL_ALPHA_TEST) and that should help you. The problem is that if you render the top cone first, it deposits the whole quad into the z-buffer (even the transparent parts), so the lower branches underneath get z-rejected when its their time to be drawn. Enabling alpha testing doesn't write pixels to the z buffer if they fail the alpha test (set with glAlphaFunc).
If you want to use partial transparency, you'll need to sort the order of rendering objects from back to front, or bottom to top in your case.
You'll need to leave z-buffer enabled as well.
[edit] Whoops I realized that those functions I don't believe work when you're using shaders. In the shader case you want to use the discard function in the fragment shader if the alpha value is close to zero.
if(color.a < 0.01) {
discard;
} else {
outcolor = color;
}
You needs to implement a two-pass algorithm.
The first pass render only the back faces, while the second pass render only the front faces.
In this way you don't need to order the triangles, but some artifacts may occour depending whether your geometry is convex or not.
I may be wrong, but this is because when you render in 3d you do no render the backside of triangles using Directx's default settings, when the Z is removed - it draws them in order, with the Z on it doesnt draw the back side of the triangles anymore.
It is possible to show both sides of the triangle, even with Z enabled, however I'm thinking there might be a reason its normally enabled.. such as speed..
Device->SetRenderState(D3DRS_CULLMODE, Value);
value can equal
D3DCULL_NONE - Shows both sides of triangle
D3DCULL_CW - Culls Front side of triangle
D3DCULL_CCW - Default state
I've drawn a simple quad with glBegin and glEnd. With a for-loop I create copies of the quad and rotate it around my y-Axis in 3D space.
Now the problem is that I only see the quads in the front. These in the back are not displayed. I assume that the problem lies within the normal vector, which direction is towards me. Is there a possibility to define two normal vectors for one quad.
Sounds like you need to disable backface culling:
glDisable(GL_CULL_FACE);
These in the back are not displayed. I assume that th problem lies within the normal-vector,
The problem is not the normal vector, but what OpenGL considers front side and backside. What's what is determined by the winding of the vertices on the screen. If the vertices are on screen in counterclockwise order, then by default OpenGL assumes the front face. If back face culling is enables, back faces will not be drawn. You can disable culling, but then you'll get odd lighting results.
The best way is to draw the back side explicitly with it's own set of quads; windings and normals adjusted.
I'm going to have meshes with several coplanar polygons, all lying in a certain plane, that I'm not going to be able to eliminate.
These polygons have a specific draw order. Some polygons are behind other polygons. If I turn off depth testing I'll have the effect I want, but I want to be able to position this mesh in a 3D scene.
I do not trust glPolygonOffset because I'll potentially have several of these overlapping polygons and am am worried about the cumulative effects of the offset.
If I turn off depth testing I'll have the effect I want, but I want to be able to position this mesh in a 3D scene.
Simply disable writing to z-buffer, without disabling depth test.
glDepthMask(GL_FALSE);
Make sure to render all polygons that doesn't require glDepthMask(GL_FALSE) before rendering any polygons with glDepthMask(GL_FALSE); Otherwise object will be incorrectly positioned.
If you can't do that, then you should change your geometry or use texture instead.
glDepthMask documentation
I am rendering 4 vertices (a square) in front of a colored cube. The vertices are colored white, but are blended at 0.5f.
Related: Why does my colored cube not work with GL_BLEND?
Please could someone tell me why the colored cube appears brighter when obscured by the semi-opaque square?
Cube rendered without square in front:
Normal cube http://img408.imageshack.us/img408/2853/normalcube.png
And, rendered with the square:
Cube with square http://img142.imageshack.us/img142/6255/brightsquare.png
Please see the code used to create the colored cube, the code used to actually draw the cube, and the code where the cube and square are rendered.
This is the code in my init function:
glEnable(GL_CULL_FACE);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE);
I'd say it's because your semi-transparent square gets added to the existing pixels, thus incrementing their intensity.
The documentation for glBlendFunc() recommends setting the second parameter to GL_ONE_MINUS_SRC_ALPHA, that is the boilerplate for implementing transparency. Try it.