I'm hoping to draw a plane in OpenGL, using C++, with a hole in the center, much like the green of a golf course for example.
I was wondering what the easiest way to achieve this is?
It's fairly simple to draw a circle and a plane (tutorials all over google will show this for those curious), but I was wondering if there is a boolean subtraction technique like you can get when modelling in 3Ds Max or similar software? Where you create both objects, then take the intersection/union etc to leave a new object/shape? In this case subtract the circle from the plane, creating a hole.
Another way I thought of doing it is giving the circle alpha values and making it transparent, but then of course it still leaves the planes surface visible anyway.
Any help or points in the right direction?
I would avoid messing around with transparency, blending mode, and the like. Just create a mesh with the shape you need and draw it. Remember OpenGL is for graphics, not modelling.
There are a couple ways you could do this. The first way is the one you already stated which is to draw the circle as transparent. The caveat is that you must draw the circle first before you draw the plane so that the alpha blending will blend the circle with the background. Then when you render the plane the parts that are covered by the circle will be discarded in the depth test.
The second method you could try is with texture mapping. You could create a texture that is basically a mask with everything set to opaque white except the circle portion which is set to have an alpha value of 0. In your shader you would then multiply your fragment color by this mask texture color so that the portions where the circle is located are now transparent.
Both of these methods would work with shapes other than a circle as well.
I suggest the stencil buffer. Use the stencil buffer to mark the area where you want the hole to be by masking the color and depth buffers and drawing only to the stencil buffer, then unmask your color and depth, avoid drawing to the stencil buffer, and draw your plane with a stencil function telling OpenGL to discard all pixels where the stencil buffer "markings" are.
Related
I want to draw shapes with holes in OpenGL and GLFW3. How can I do this? I don't want to use gluTessBeginPolygon.
this is a rectangle with a rectangle hole in it
If the shape is always the same, then the simplest way is to change how you visualize this. It's not a polygon with a hole, it's 2 (or more) polygons with no holes. Draw that instead:
However, if the shape changes dynamically, calculating this triangulation in code is difficult.
If you can't do this because the hole shape is dynamic then you can use the stencil buffer to prevent OpenGL from drawing where the hole is. Clear the stencil buffer, set the rendering mode so that you only write the stencil, then render the hole. Then set the modes back to normal but set the stencil test so it doesn't draw where the stencil buffer isn't zero, and render the rectangle. Then go back to normal.
If you have a shape with lots of holes (like a chain-link fence) then instead of rendering zillions of vertices, you should use a texture with an alpha channel, and use alpha testing in your shader - use discard; on the transparent pixels so they don't render. The fixed-function version of this is GL_ALPHA_TEST.
If you have a formula to detect whether a pixel is in the hole, you can use discard; as well. Your shader can discard for any reason you like - it doesn't have to be based on the alpha channel of a texture.
What you cannot do is count the number of times you cross the polygon boundary when going from left to right, like a scanline renderer might. That's because OpenGL processes all pixels in parallel - not left-to-right.
I am rendering complex 3d objects. Here is a simple example with a sphere-like object:
Next I am applying a clipping plane to these objects and rendering a texture on this plane, giving the impression you are looking at the inside of the object, as if it was sliced. For example:
The problem is the jagged edge of the texture. It will stick out passed the boundary of the surface. Here's another angle where you can see it sticking out. The surface and the texture both derive from the same source data, but the surface is smoothed and has a higher resolution than the texture.
What I want is to be able to somehow clip the texture, so that it never sticks out past the boundary of the surface. Also, I don't want to simply scale down the texture, since although this might prevent it from sticking outside, it would create interior gaps between the texture edge and the surface edge. I would rather the texture be a little too big and have it clipped so that it sits flush against the edge of the surface.
Here's where I am:
I figured the first step would be to define the intersection of the plane and the surface. So now I have that, as an ordered list of line segments. However, I'm not sure how to proceed with this info (or if this is even the best approach).
I've been reading up on stencil buffers. One approach might be to turn the intersection line into a 2d shape and draw this into a stencil buffer. Then apply this when drawing the texture. (Although I think it's a lot of work since the shapes can be complicated.)
I am wondering if I can somehow use the already drawn surface (in conjunction with a stencil buffer or some other technique) to somehow clip the texture -- without having to go through the extra trouble of deriving the intersection line, etc.
What's the best approach here? (Any online examples you can point me to would also be really helpful.)
If you're clipping convex objects and know coordinates of clipped points, you can create polygonal "cap" yourself - just draw clipped points in proper order using GL_TRIANGLE_FAN, and that's it. Won't work with non-convex object - that would require triangulation algorithm. You could use glu tesselators to triangulate polygons, but that can be tricky/difficult.
If clipped area can be defined by formula, you can write a shader that'll precisely clip pixels over certain distance (i.e. if x^2+x^2+z^2 > r^2 do not draw pixel).
You could also draw back-facing faces with a shader that would draw every back facing pixel as if it were on on clip-plane using simple raytracing. That's complicated, and might be overkill in your case. Dead Rising used similar technique in their game engine.
Also you can use stencil buffer.
Draw back-facing faces first with GL_INCR (glStencilOp(GL_KEEP, GL_INCR, GL_INCR)), then draw front-facing surfaces with GL_DECR (glStencilOp(GL_KEEP, GL_DECR, GL_DECR)). Then draw texture only where stencil is non-zero. (glStencilFunc(GL_GREATER, 0, 0xff); glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);). If you have many overlapping shapes, however, you'll need to take special care of them.
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However, I'm not sure how to proceed with this info (or if this is even the best approach).
Draw it as a triangle fan. For convex objects, that's all you need. For non-convex objects that won't work.
ve been reading up on stencil buffers. One approach might be to turn the intersection line into a 2d shape
No, it won't work like that. Region you want to fill with texture should hold certain stencil value. That's how stencil clipping works.
to somehow clip the texture
In OpenGL you have 6(?) clip planes. If you need more than that, you'll need advanced techniques - stencil, deriving intersection line, shaders, or triangulation.
Any online examples you can point me to would also be really helpful
Drawing Filled, Concave Polygons Using the Stencil Buffer
I have an assignment to render a terrain from a greyscale 8bit bmp and get colors to the terrain from a texture 24bit bmp. I managed to get a proper landscape with heights and so on, and also I get the colors from the texture bitmap. The problem is that the full color rendered terrain is very "blocky", it shows right colors and height but it's so blocky. I use glShadeModel(GL_SMOOTH) but it still looks so blocky, almost like I can see the pixels from the bitmap. So any hints are appreciated.
Do you use the bitmap as texture, or do you set vertex colours from the bitmap? I suggest you use a texture, using the planar vertex position as texture coordinate.
One thing you have to take into consideration is when you are rendering are you using GL_TRIANGLES or GL_TRIANGLESTRIPS this makes a difference on performance, second if you are using lighting you have to define your normals and each triangle or each vertex of each triangle, the problem then becomes tricky because almost every triangle is on a different plane. Not having proper normals would make it look blocky. The third thing that makes a difference is how big or small the triangles are; the smaller the triangles or the more divisions in your [x,z] plane increases you resolution thus increases the visual quality, but also slows down your frame rate. You have to find a good balance between the two.
I am currently using glLogicOp() with a cube, which i render twice: with glFrontFace(GL_CW) and then with glFrontFace(GL_CCW). This allows me to see which area of the other 3d object my cube is overlapping with.
But i want to change the negative color to something else, lets say 0.5f transparent blue color.
How this can be done? Sorry about the title, i dont know the name of this method.
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Also, i am having problem with being inside the cube with my camera: i need to fill the screen with negative coloring, is there any other way than swithing to 2d mode and drawing a quad with glLogicOp() enabled ? Also the problem is that theres a chance to see bugged rendering if i am at the edge of the cube surface, any ideas for preventing this perfectly?
You should look into the "Carmack's reverse" algorithm and the stencil shadow algorithms in general, as your problem is closely related to them (your cube being a shadow volume object). You will not get away with using glLogicOp() if you want other colors than black and white.
I want to make a game with Worms-like destructible terrain in 2D, using OpenGL.
What is the best approach for this?
Draw pixel per pixel? (Uh, not good?)
Have the world as a texture and manipulate it (is that possible?)
Thanks in advance
Thinking about the way Worms terrain looked, I came up with this idea. But I'm not sure how you would implement it in OpenGL. It's more of a layered 2D drawing approach. I'm posting the idea anyway. I've emulated the approach using Paint.NET.
First, you have a background sky layer.
And you have a terrain layer.
The terrain layer is masked so the top portion isn't drawn. Draw the terrain layer on top of the sky layer to form the scene.
Now for the main idea. Any time there is an explosion or other terrain-deforming event, you draw a circle or other shape on the terrain layer, using the terrain layer itself as a drawing mask (so only the part of the circle that overlaps existing terrain is drawn), to wipe out part of the terrain. Use a transparent/mask-color brush for the fill and some color similar to the terrain for the thick pen.
You can repeat this process to add more deformations. You could keep this layer in memory and add deformations as they occur or you could even render them in memory each frame if there aren't too many deformations to render.
I guess you'd better use texture-filled polygons with the correct mapping (a linear one that doesn't stretch the texture to use all the texels, but leaves the cropped areas out), and then reshape them as they get destroyed.
I'm assuming your problem will be to implement the collision between characters/weapons/terrain.
As long as you aren't doing this on opengl es, you might be able to get away with using the stencil buffer to do per-pixel collision detection and have your terrain be a single modifyable texture.
This page will give an idea:
http://kometbomb.net/2007/07/11/hardware-accelerated-2d-collision-detection-in-opengl/
The way I imagine it is this:
a plane with the texture applied
a path( a vector of points/segments ) used for ground collisions.
When something explodes, you do a boolean operation (rectangle-circle) for the texture(revealing the background) and for the 'walkable' path.
What I'm trying to say is you do a geometric boolean operation and you use the result to update the texture(with an alpha mask or something) and update the data structure you use to keep track of the walkable area(which ever that might be).
Split things up, instead of relying only on gl draw methods
I think I would start by drawing the foreground into the stencil buffer so the stencil buffer is set to 1 bits anywhere there's foreground, and 0 elsewhere (where you want your sky to show).
Then to draw a frame, you draw your sky, enable the stencil buffer, and draw the foreground. For the initial frame (before any explosion has destroyed part of the foreground) the stencil buffer won't really be doing anything.
When you do have an explosion, however, you draw it to the stencil buffer (clearing the stencil buffer for that circle). Then you re-draw your data as before: draw the sky, enable the stencil buffer, and draw the foreground.
This lets you get the effect you want (the foreground disappears where desired) without having to modify the foreground texture at all. If you prefer not to use the stencil buffer, the alternative that seems obvious to me would be to enable blending, and just manipulate the alpha channel of your foreground texture -- set the alpha to 0 (transparent) where it's been affected by an explosion. IMO, the stencil buffer is a bit cleaner approach, but manipulating the alpha channel is pretty simple as well.
I think, but this is just a quick idea, that a good way might be to draw a Very Large Number of Lines.
I'm thinking that you represent the landscape as a bunch of line segments, for each column of the screen you have 0..n vertical lines, that make up the ground:
12 789
0123 6789
0123456789
0123456789
In the above awesomeness, the column of "0":s makes up a single line, and so on. I didn't try to illustrate the case where a single pixel column has more than one line, since it's a bit hard in this coarse format.
I'm not sure this will be efficient, but it at least makes some sense since lines are an OpenGL primitive.
You can color and texture the lines by enabling texture-mapping and specifying the desired texture coordinates for each line segment.
Typically the way I have seen it done is to have each entity be a textured quad, then update the texture for animation. For a destructible terrain it might be best to break the train into tiles then you only have to update the ones that have changed. Don't use GLdrawpixels it is probably the slowest approach possible (outside of reloading textures from disk every frame though it would be close.)