I've been reading up on a lot of various articles regarding to ray-marching in GLSL shaders (such as this one article: http://www.iquilezles.org/www/articles/rmshadows/rmshadows.htm) and it raised some questions that I wanted to ask.
In my application, I am rendering a scene with a couple of meshes and I wanted to experiment with shadows. While I seem to somewhat understand the concept of how raymarching works, I don't quite understand how to properly implement this in GLSL. I know how to compute the intersection of a ray and a plane but how would this be handled through GLSL shaders?
According to this thread here: (https://gamedev.stackexchange.com/questions/67719/how-do-raymarch-shaders-work) it mentions that you're measuring the distance between the start of the ray and the 'surface'. Is the surface he's referring to the mesh? Do I need to send an array of planes/points that makes up the mesh to the shader in order to compute the ray intersection test? Do I need to use the depth buffer to determine the distance of the surface?
It's depend of what your shader does vs what your rendering engin does. In pure demo shaders like shadertoy (see its shadow examples ) the whole scene is encoded in the shader so there is no problem shooting secondary rays or more (beside perfs).
If the scene is not managed by your shader, then you need a bit of cooperation from your engine. At least, to produce a shadowmap in a first pass (many different algorithms exists).
Note that with SVO representation, the scene is first converted into sparse voxels, which can then be marched by the shader for secondary rays. Could be even for primary ray, but you do can use regular Z-buffer here, and voxel cone-tracing (for instance) for all kinds of secondary rays ( see *Interactive Indirect Illumination Using Voxel Cone Tracing * here: http://gigavoxels.imag.fr/publications.html (ok, you might find it overkill in your simple application). For soft shadows and depth of field, see the seminal paper GigaVoxels : Ray-Guided Streaming for Efficient and Detailed Voxel Rendering . Note that the tree might even be a regular BSP of triangles, instead of on octree of voxels. But then you loose many advantage of SVO (perfs, increased for soft shadows).
Related
I am given the contour/boundary of a concave polygon which cannot have holes, and I need to apply a heightmap to it. To this end, I need to perform a near-uniform tessellation of the polygon like so:
.
How can I perform this tessellation? Ideally, the solution could parameterize the 'resolution' of the internal vertices to adjust the detail of the tessellation.
I'm using OpenSceneGraph, so if this is somehow possible using its built-in tessellation library that would be ideal, but I'm open to introducing an additional (hopefully lightweight) dependency if necessary.
Very nice ! have you ever considered using a geometry shader for this ? DirectX 11 and OpenGL allow for "on the fly" tesselation using the GPU. It's 20-40 times faster, depending on your hardware.
My favorite starting point for VS2015/C# tesselated animation is
https://github.com/RobyDX/SharpDX_Demo/tree/master/SharpDXTutorial/Tutorial11
If you prefer C/C++ and OpenGL there are many examples too..
http://prideout.net/blog/?p=48
Some of mine, I sample a texture bitmap for the height. On the right, you see some artifacts caused by my small normal errors in my little triangles.. gotta work on that one :)
Above mesh is 32x32 squares and has tesselation depth 7. Here you see the flattened mesh, showing depth 1 tesselation:
This question didn't attract a great deal of interest, but I achieved fairly good results using the difficult-to-Google Triangle library.
If I decided I wanted the triangulation to be strictly uniform on the interior of the polygon, I could probably provide Triangle with a set of vertices to further constrain the tessellation.
I'm trying to develop a high level understanding of the graphics pipeline. One thing that doesn't make much sense to me is why the Geometry shader exists. Both the Tessellation and Geometry shaders seem to do the same thing to me. Can someone explain to me what does the Geometry shader do different from the tessellation shader that justifies its existence?
The tessellation shader is for variable subdivision. An important part is adjacency information so you can do smoothing correctly and not wind up with gaps. You could do some limited subdivision with a geometry shader, but that's not really what its for.
Geometry shaders operate per-primitive. For example, if you need to do stuff for each triangle (such as this), do it in a geometry shader. I've heard of shadow volume extrusion being done. There's also "conservative rasterization" where you might extend triangle borders so every intersected pixel gets a fragment. Examples are pretty application specific.
Yes, they can also generate more geometry than the input but they do not scale well. They work great if you want to draw particles and turn points into very simple geometry. I've implemented marching cubes a number of times using geometry shaders too. Works great with transform feedback to save the resulting mesh.
Transform feedback has also been used with the geometry shader to do more compute operations. One particularly useful mechanism is that it does stream compaction for you (packs its varying amount of output tightly so there are no gaps in the resulting array).
The other very important thing a geometry shader provides is routing to layered render targets (texture arrays, faces of a cube, multiple viewports), something which must be done per-primitive. For example you can render cube shadow maps for point lights in a single pass by duplicating and projecting geometry 6 times to each of the cube's faces.
Not exactly a complete answer but hopefully gives the gist of the differences.
See Also:
http://rastergrid.com/blog/2010/09/history-of-hardware-tessellation/
In my OpenGL project, I want to dynamically create smoothed polygons, similiar like this one:
The problem relies mainly in the smoothing process. My procedure up to this point, is firstly to create a VBO with randomly placed vertices.
Then, in my fragment shader, (I'm using the programmable function pipeline) there should happen the smoothing process, or in other words, created the curves out of the previously defined "lines" between the vertices.
And exactly here is the problem: I am not very familiar with thoose complex mathematical algorithms, which would examine, if a point is inside the "smoothed polygon" or not.
First up, you can't really do it in the fragment shader. The fragment shader is limited to setting the final(ish) color of a "pixel" (which is basically, but not exactly, an actual pixel) before it gets written to the screen. It can't create new points on a curve.
This page gives a nice overview of the different algorithms for creating smooth curves.
The general approach is to break a couple of points into multiple points using a geometry shader, and then render them just like a normal polygon. But I don't know the details. Try a google search for bezier geometry shader for example.
Wait, I lie. I found a program here that does it in the fragment shader.
I've been using OpenGL since some time now for making 3D applications, but I never really understood the use of the GL_POINT and GL_LINES primitive drawing types for 3D games in the production phase.
(Where) are point and line primitives in OpenGL still used in modern games?
You know, OpenGL is not just for games and there are other kind of programs than just games. Think CAD programs, or map editors, where wireframes are still very usefull.
GL_POINTS are used in games for point sprites (either via the pointsprite functionality or by generating a quad from a point in the geometry shader) both for "sparkle" effects and volumetric clouds.
They are also used in some special algorithms just when, well... when points are needed. Such as in building histograms in the geometry shader as by the chapter in one of the later GPU Gems books. Or, for GPU instance culling via transform feedback.
GL_LINES have little use in games (mostly useful for CAD or modelling apps). Besides not being needed often, if they are needed, you will normally want lines with a thickness greater than 1, which is not well supported (read as: fast) on all implementations.
In such a case, one usually draws thick lines with triangle strips.
Who ever said those primitives were used in modern games?
GL_LINES is critical for wireframe views in 3D modeling tools.
(Where) are point and line primitives in OpenGL still used in modern games?
Where do you want them to be used?
Under standard methods, points can be used to build point sprites, which are 2D flatcards that always face the camera and are of a particular size. They are always square in window-space. Sadly, the OpenGL specification makes using them somewhat dubious, as point sprites are clipped based on the center of the point, not the size of the two triangles that are used to render it.
Lines are perfectly reasonable for line drawing. Once upon a time, lines weren't available in consumer hardware, but they have been around for many years now. Of course, antialiased line rendering (GL_LINE_SMOOTH) is another matter.
More importantly is the interaction of these things with geometry shaders. You can convert points into a quad. Or a triangle. Or whatever you want, really. Each "point" is just an execution of the geometry shader. You can have points which contain the position and radius of a sphere, and the geometry shader can output a window-aligned quad that is the appropriate size for the fragment shader to do some raytracing logic on it.
GL_POINTS just means "one vertex per geometry shader". GL_LINES means "two vertices per geometry shader." How you use it is up to you.
I'd say for debugging purposes, but that is just from my own perspective.
Some primitives can be used in areas where you don't think they can be applied, such as a particle system.
I agree with Pompe de velo about lines being useful for debugging. They can be useful when debugging AI and collision detection algorithms so that you can visualize the data that is being used by the AI or collision detection. Some example uses for AI, the lines can be used to show AI paths or path meshes. Lines can be used to show steering data that the AI is using. Lines can be used to show what an AI is aiming at. The data that is shown can be displayed in text form but sometimes it is easier to see it in visual form.
In most cases particles are based on GL_POINT, considering that there can be a huge number of particles on the screen it would be very expensive to use 4 vertices per particle, so GL_POINT solves this problem
GL_LINES good for debugging purposes, wireframe mode can be used in various cases. As mentioned above - in CAD apps, but if you're interesed in gamedev use - it's good for a scene editor.
In terms of collision detection, they come in handy when you want to visualize bounding volumes(boxes,spheres,k-dops) and contact manifolds in wireframe mode. Setting the colour of these primitives based on the status of collisions as well is incredibly useful.
I have a program in which I need to apply a 2-dimensional texture (simple image) to a surface generated using the marching-cubes algorithm. I have access to the geometry and can add texture coordinates with relative ease, but the best way to generate the coordinates is eluding me.
Each point in the volume represents a single unit of data, and each unit of data may have different properties. To simplify things, I'm looking at sorting them into "types" and assigning each type a texture (or portion of a single large texture atlas).
My problem is I have no idea how to generate the appropriate coordinates. I can store the location of the type's texture in the type class and use that, but then seams will be horribly stretched (if two neighboring points use different parts of the atlas). If possible, I'd like to blend the textures on seams, but I'm not sure the best manner to do that. Blending is optional, but I need to texture the vertices in some fashion. It's possible, but undesirable, to split the geometry into parts for each type, or to duplicate vertices for texturing purposes.
I'd like to avoid using shaders if possible, but if necessary I can use a vertex and/or fragment shader to do the texture blending. If I do use shaders, what would be the most efficient way of telling it was texture or portion to sample? It seems like passing the type through a parameter would be the simplest way, but possible slow.
My volumes are relatively small, 8-16 points in each dimension (I'm keeping them smaller to speed up generation, but there are many on-screen at a given time). I briefly considered making the isosurface twice the resolution of the volume, so each point has more vertices (8, in theory), which may simplify texturing. It doesn't seem like that would make blending any easier, though.
To build the surfaces, I'm using the Visualization Library for OpenGL and its marching cubes and volume system. I have the geometry generated fine, just need to figure out how to texture it.
Is there a way to do this efficiently, and if so what? If not, does anyone have an idea of a better way to handle texturing a volume?
Edit: Just to note, the texture isn't simply a gradient of colors. It's actually a texture, usually with patterns. Hence the difficulty in mapping it, a gradient would've been trivial.
Edit 2: To help clarify the problem, I'm going to add some examples. They may just confuse things, so consider everything above definite fact and these just as help if they can.
My geometry is in cubes, always (loaded, generated and saved in cubes). If shape influences possible solutions, that's it.
I need to apply textures, consisting of patterns and/or colors (unique ones depending on the point's "type") to the geometry, in a technique similar to the splatting done for terrain (this isn't terrain, however, so I don't know if the same techniques could be used).
Shaders are a quick and easy solution, although I'd like to avoid them if possible, as I mentioned before. Something usable in a fixed-function pipeline is preferable, mostly for the minor increase in compatibility and development time. Since it's only a minor increase, I will go with shaders and multipass rendering if necessary.
Not sure if any other clarification is necessary, but I'll update the question as needed.
On the texture combination part of the question:
Have you looked into 3d textures? As we're talking marching cubes I should probably immediately say that I'm explicitly not talking about volumetric textures. Instead you stack all your 2d textures into a 3d texture. You then encode each texture coordinate to be the 2d position it would be and the texture it would reference as the third coordinate. It works best if your textures are generally of the type where, logically, to transition from one type of pattern to another you have to go through the intermediaries.
An obvious use example is texture mapping to a simple height map — you might have a snow texture on top, a rocky texture below that, a grassy texture below that and a water texture at the bottom. If a vertex that references the water is next to one that references the snow then it is acceptable for the geometry fill to transition through the rock and grass texture.
An alternative is to do it in multiple passes using additive blending. For each texture, draw every face that uses that texture and draw a fade to transparent extending across any faces that switch from one texture to another.
You'll probably want to prep the depth buffer with a complete draw (with the colour masks all set to reject changes to the colour buffer) then switch to a GL_EQUAL depth test and draw again with writing to the depth buffer disabled. Drawing exactly the same geometry through exactly the same transformation should produce exactly the same depth values irrespective of issues of accuracy and precision. Use glPolygonOffset if you have issues.
On the coordinates part:
Popular and easy mappings are cylindrical, box and spherical. Conceptualise that your shape is bounded by a cylinder, box or sphere with a well defined mapping from surface points to texture locations. Then for each vertex in your shape, start at it and follow the normal out until you strike the bounding geometry. Then grab the texture location that would be at that position on the bounding geometry.
I guess there's a potential problem that normals tend not to be brilliant after marching cubes, but I'll wager you know more about that problem than I do.
This is a hard and interesting problem.
The simplest way is to avoid the issue completely by using 3D texture maps, especially if you just want to add some random surface detail to your isosurface geometry. Perlin noise based procedural textures implemented in a shader work very well for this.
The difficult way is to look into various algorithms for conformal texture mapping (also known as conformal surface parametrization), which aim to produce a mapping between 2D texture space and the surface of the 3D geometry which is in some sense optimal (least distorting). This paper has some good pictures. Be aware that the topology of the geometry is very important; it's easy to generate a conformal mapping to map a texture onto a closed surface like a brain, considerably more complex for higher genus objects where it's necessary to introduce cuts/tears/joins.
You might want to try making a UV Map of a mesh in a tool like Blender to see how they do it. If I understand your problem, you have a 3D field which defines a solid volume as well as a (continuous) color. You've created a mesh from the volume, and now you need to UV-map the mesh to a 2D texture with texels extracted from the continuous color space. In a tool you would define "seams" in the 3D mesh which you could cut apart so that the whole mesh could be laid flat to make a UV map. There may be aliasing in your texture at the seams, so when you render the mesh it will also be discontinuous at those seams (ie a triangle strip can't cross over the seam because it's a discontinuity in the texture).
I don't know any formal methods for flattening the mesh, but you could imagine cutting it along the seams and then treating the whole thing as a spring/constraint system that you drop onto a flat surface. I'm all about solving things the hard way. ;-)
Due to the issues with texturing and some of the constraints I have, I've chosen to write a different algorithm to build the geometry and handle texturing directly in that as it produces surfaces. It's somewhat less smooth than the marching cubes, but allows me to apply the texcoords in a way that works for my project (and is a bit faster).
For anyone interested in texturing marching cubes, or just blending textures, Tommy's answer is a very interesting technique and the links timday posted are excellent resources on flattening meshes for texturing. Thanks to both of them for their answers, hopefully they can be of use to others. :)