I'm writing a rather complex webgl application in three.js and in order to have more control on my mesh materials I'm defining them through shaders.
However, I'm facing a serious problem for which I could not find any truly satisfying answer. Let's assume the following strong pre-requisites:
my geometry is indexed, i.e. the vertices are NOT duplicated among faces and I don't want to change this
the geometry is stored in a THREE.BufferGeometry object, again I don't want to switch to THREE.Geometry
I don't want to duplicate faces and/or create new geometries
I don't want any two-ways rendering that would drop my framerate
Given the above, let's say I click on a triangle (i.e., shoot a ray and get the closest face intersecting the ray) of my geometry and I want such triangle to be highlighted with a different color.
I'm already performing this in a quite efficient way by assigning a custom vertex attribute, say 1.0, to each vertex belonging to a face to be highlighted.
In the vertex shader I define a varying with such vertex attribute and within the fragment shader I just execute the following (pseudocode):
vec4 frag_color=mix(default_color,highlight_color,(vert_attribute>1-eps));
where eps is a small value (say 1.0e-4).
The idea is that each fragment interpolates its corresponding vertex attributes, say Vatt_1, Vatt_2 and Vatt_3, and if all the three attributes get the value 1.0 thus their interpolation is still nearly 1.0 (not precisely because of some roundoff error, that's why I use a small tolerance eps) and the test is true.
If the test is true I have frag_color=highlight_color.
On the other hand, if at least one Vatt_i (i=1,2,3) is not 1.0 but the default 0.0, the interpolation at the current fragment is <1-eps and the test is false (giving frag_color=default_color).
This seems to work perfectly, but now I have the following problem that looks truly challenging (given the constraints 1 and 2 above):
I don't have any simple way in the fragment shader to know whether or not the current fragment belongs to a specific face (or stays within three specific vertices, which is the same). So, if I select two triangles T1 and T2, and by chance a third triangle T3 has one (or two) vertices shared with T1 and two (or one) vertices shared with T2 I get that T3 gets highlighted too because its three vertices get the attribute 1.0.
Of course, seen from a human perspective this shouldn't happen because the three "highlighted" vertices of T3 should "logically" refer to two different highlighted faces but for obvious reasons the fragment shader highlights also T3.
This is a quite general problem I guess, I read a lot of forums without finding any satisfying possible solution. I understand that "this is how it works" and that the fragment shader does not have any knowledge of the background triangle nor its vertices, but here I'm looking for some clever idea or trick.
Does anybody have any suggestion to face this issue? Sorry for bothering but just to prevent some possible arguments: I consider the four points 1-4 above as strong requirements because otherwise I'd have other problems related to the overall performance and I don't want to pay this price.
Thanks in advance
You can know if a point is inside a triangle using barycentric coordinates. This can be done at vertex or fragment shader level depending on your needs.
Just get the coordinates of the vertices of the triangle that you want to test and convert the coordinates of the current vertex or fragment to barycentric coordinates. After that a simple test of the value of barycentric coordinates will tell you if the point is inside the triangle or not.
Related
Is there any way to tell, from within a gl es vertex shader (that is drawing triangles) which of the three vertices is being processed?
Using gl_VertexID doesn't work for me, because it gives the index of the vertex in the list of vertices, but I use indices to specify a different order to draw the vertices, and so the value I want cannot be determined from gl_VertexID alone.
You can add a vertex attribute to represent the indices 0, 1, 2, but as #matic-oblak noted you may have to replicate some vertices that are shared between triangles. If the mesh is "three-colorable" (in the graph theory sense) then you can assign indices without any replication.
A tetrahedron is not 3-colorable, whereas a cube is 2-colorable, and we can triangulate the faces of a cube and get a 3-colorable mesh. Ordinary vertices have degree 6 in a triangular mesh and are "locally" 3-colorable.
Therefore you can 3-color a mesh as much as possible -- where it fails you will have to replicate vertices. Unfortunately 3-coloring is an NP-complete problem , but with a some simple heuristics I think you can do a fairly reasonable job.
As I commented above, what I was looking for is deliberately not available for pipeline efficiency reasons. See the comment by Alfonse Reinheart at the following page:
https://www.opengl.org/discussion_boards/showthread.php/181822-gl_VertexId-gl_InstanceID-gl_PrimitiveID-but-where-is-gl_IndexID
The other answer, posted by wcochran is interesting, and could be a way to pass less information to the rendering pipeline, although as s/he points out, it comes at the cost of some substantial preprocessing.
I'm creating a 3D globe with a map on it which is supposed to unravel and fill the screen after a few seconds.
I've managed to create the globe using three.js and webGL, but I'm having trouble finding any information on being able to animate a shape change. Can anyone provide any help? Is it even possible?
(Abstract Algorithm's and Kevin Reid's answers are good, and only one thing is missing: some actual Three.js code.)
You basically need to calculate where each point of the original sphere will be mapped to after it flattens out into a plane. This data is an attribute of the shader: a piece of data attached to each vertex that differs from vertex to vertex of the geometry. Then, to animate the transition from the original position to the end position, in your animation loop you will need to update the amount of time that has passed. This data is a uniform of the shader: a piece of data that remains constant for all vertices during each frame of the animation, but may change from one frame to the next. Finally, there exists a convenient function called "mix" that will linearly interpolate between the original position and the end/goal position of each vertex.
I've written two examples for you: the first just "flattens" a sphere, sending the point (x,y,z) to the point (x,0,z).
http://stemkoski.github.io/Three.js/Shader-Attributes.html
The second example follows Abstract Algorithm's suggestion in the comments: "unwrapping the sphere's vertices back on plane surface, like inverse sphere UV mapping." In this example, we can easily calculate the ending position from the UV coordinates, and so we actually don't need attributes in this case.
http://stemkoski.github.io/Three.js/Sphere-Unwrapping.html
Hope this helps!
In 3D, anything and everything is possible. ;)
Your sphere geometry has it's own vertices, and basically you just need to animate their position, so after animation they are all sitting on one planar surface.
Try creating sphere and plane geometry, with same number of vertices, and animating sphere's vertices with interpolated values of sphere's and plane's original values. That way, on the start you would have sphere shape and in the end, plane shape.
Hope this helps, tell me if you need more directives how to do it.
myGlobe.geometry.vertices[index].position = something_calculated;
// myGlobe is instance of THREE.Mesh and something_calculated would be THREE.Vector3 instance that you can calculate in some manner (sphere-plane interpolation over time)
(Abstract Algorithm's answer is good, but I think one thing needs improvement: namely using vertex shaders.)
You make a set of vertices textured with the map image. Then, design a calculation for interpolating between the sphere shape and the flat shape. It doesn't have to be linear interpolation — for example, one way that might be good is to put the map on a small portion of an sphere of increasing radius until it looks flat (getting it all the way will be tricky).
Then, write that calculation in your vertex shader. The position of each vertex can be computed entirely from the texture coordinates (since that determines where-on-the-map the vertex goes and implies its position) and a uniform variable containing a time value.
Using the vertex shader will be much more efficient than recomputing and re-uploading the coordinates using JavaScript, allowing perfectly smooth animation with plenty of spare resources to do other things as well.
Unfortunately, I'm not familiar enough with Three.js to describe how to do this in detail, but all of the above is straightforward in basic WebGL and should be possible in any decent framework.
I am trying to create a terrain solution in ThreeJS and I'm running into some trouble with the generation of the normals. I am approaching the problem by creating a number of mesh objects using the THREE.PlaneGeometry class. Once all of the tiles have been created I go through each and set the UV's so that each tile represents a part of the whole. I also generate a height value of the vertex Y positions to create some hills. I then call the geometry functions
geometry.computeFaceNormals();
geometry.computeVertexNormals();
This is just so that I have some default face and vertex normals for each tile.
I then go through each tile and try to average out the normals on each corner.
The problem is (I think) with the normals, but I don't really know what to call this problem. Each of the normals on the plane's corners point in the same direction as the face when created. This makes the terrain look like a flat shaded object. To prevent this I thought perhaps what I needed to do was make sure each vertext normal (each corner) had the same averaged normal as its immediate neighbours normals. I.E each corner of each tile has the same normal as all the immediate normals around it from the adjacent planes.
figure A
Here I am visualising each of the 4 normals on the mesh. You can see that at each corner the normals are the same (On top of eachother)
figure B
EDIT
figure C
EDIT
Figure D
Except even when the verts all share the same normals it still comes up all blocky <:/
I don't know how to do this... I think my understanding of what needs to be done is incorrect...?
Any help would be greatly appreciated.
You're basically right about what should happen. The shading you're getting is not consistent with continuous normals. If each all the vertex-faces at a given location have the same normal you should not see the clear shading discontinuities in your second image. However the image doesn't look like simple face normals either, at least not to my eye.
A couple of things to look at:
1) I note that your quads themselves are not planar. Is it possible your algorithm is assuming that they are? the non-planar quad meshes don't have real 'face normal' to use as a base.
2) Are your normalized normalized after you average them? That is, do they have a vector length of 1?
3) Are you confident that the normal averaging code is actually using the correct normals to average? The shading in this does not look like completely flat shaded image where each vertex-face normal in a quad is the same - if that were the case you'd get consistent shading across each quad although the quads would not be continuous. This it possible your original vertex-face normals are not in fact lined up with the face normals?
4) Try turning off the bump maps to debug. Depending on how the bump is being done in your shader you may have incorrect binormals/bitangents rather than bad vert normals.
Instead of averaging at each vertex / corner the neighborhood normals you should average the four normals that each vertex has (4 tiles meet at each vertex).
I want to create a shader to outline 2D geometry. I'm using OpenGL ES2.0. I don't want to use a convolution filter, as the outline is not dependent on the texture, and it is too slow (I tried rendering the textured geometry to another texture, and then drawing that with the convolution shader). I've also tried doing 2 passes, the first being single colorded overscaled geometry to represent an oultine, and then normal drawing on top, but this results in different thicknesses or unaligned outlines. I've looking into how silhouette's in cel-shading are done but they are all calculated using normals and lights, which I don't use at all.
I'm using Box2D for physics, and have "destructable" objects with multiple fixtures. At any point an object can be broken down (fixtures deleted), and I want to the outline to follow the new outter counter.
I'm doing the drawing with a vertex buffer that matches the vertices of the fixtures, preset texture coordinates, and indices to draw triangles. When a fixture is removed, it's associated indices in the index buffer are set to 0, so no triangles are drawn there anymore.
The following image shows what this looks like for one object when it is fully intact.
The red points are the vertex positions (texturing isn't shown), the black lines are the fixtures, and the blue lines show the seperation of how the triangles are drawn. The gray outline is what I would like the outline to look like in any case.
This image shows the same object with a few fixtures removed.
Is this possible to do this in a vertex shader (or in combination with other simple methods)? Any help would be appreciated.
Thanks :)
Assuming you're able to do something about those awkward points that are slightly inset from the corners (eg, if you numbered the points in English-reading order, with the first being '1', point 6 would be one)...
If a point is interior then if you list all the polygon edges connected to it in clockwise order, each pair of edges in sequence will have a polygon in common. If any two edges don't have a polygon in common then it's an exterior point.
Starting from any exterior point you can then get the whole outline by first walking in any direction and subsequently along any edge that connects to an exterior point you haven't visited yet (or, alternatively, that isn't the edge you walked along just now).
Starting from an existing outline and removing some parts, you can obviously start from either exterior point that used to connect to another but no longer does and just walk from there until you get to the other.
You can't handle this stuff in a shader under ES because you don't get connectivity information.
I think the best you could do in a shader is to expand the geometry by pushing vertices outward along their surface normals. Supposing that your data structure is a list of rectangles, each described by, say, a centre, a width and a height, you could achieve the same thing by drawing each with the same centre but with a small amount added to the width and height.
To be completely general you'd need to store normals at vertices, but also to update them as geometry is removed. So there'd be some pushing of new information from the CPU but it'd be relatively limited.
I'm trying to create an inner glow effect for a triangle fan primitive using GLSL ES 2.0 - though only the outer edges are to be subject to the effect at hand. I guess there are many ways to do this, but haven't found any description so far.
There is the technique described in Make the edges of a textured polygon glow in OpenGL ES 2.0, however, this doesn't work for me as I'm working purely with primitive at this stage.
My initial thought was to somehow calculate the distance to the nearest edge in the fragment shader, and then set the color according to wether or not the distance falls within the bounds of some threshold value or not. (Of course, the color and alpha is to be a function of the distance from the nearest edge - the exact gradient profile is not important at this point.)
This approach poses two problems:
1) How do I calculate the distance from a fragment to the nearest edge?
2) How do I exclude common edges in this process, i.e. edges that are common to two (or more) triangles?
Is this a sensible approach, and if so: how do I resolve my two issues? Suggestions for alternative approaches are also greatly appreciated. (For instance, I've been reading that texture data need not be an image, and that it may be utilized for custom purposes. Could a non-image texture be part of the solution?) :)
To answer your two questions, I don't think there is any glsl magic that will do this for you. By the time you get to the fragment shader, there is no longer any information available about edges, especially trying to segregate true edges from internal edges.
What I recommend is to add more vertices to your fan, and use a new custom attribute to define the 'glow level'. See image for example, I would put a row of vertices around the edge, define these (and the center of the fan) to have maximum glow, and then define the edges to have zero glow, and then you can get an interpolated glow value between the edge and the new vertices.