I have set up a frustum with fov=50, near=0.1,far=1000,aspect ratio=3/2
yet it isn`t coming nice--my object appears to be compressed on far side.suggest something that might help.
A screenshot would help. But I think I understand your problem. What you're observing are the effects of an affine projection. The parameter fov is a bit misleading, as it might suggest you have some angular projection (fisheye) which is not the case. What actually happens is, that the extents of the near projection plane are placed so that it's borders are seen under the fov angle. However the near plane is just that, flat that means, so very large fovs look very unnatural.
I don't know the specs of the iPhone's graphics capabilities from heart, but if I'm not mistaken it supports vertex shaders. So instead of the affine frustum projection method you could implement a totally different kind of projection model, like sterographic or mercator (depending on what your needs are). Operations are then no longer performed in terms of linear matrix multiplication.
As a side note: Those values for near and far distance are too extreme, almost all of the depth buffer precision will be between 0.1 and ~20, due to the nonlinearity of depth buffering in perspective mode.
Related
As I understand it, the standard projection model places an imaginary grid in front of the camera, and for each triangle in the scene, determines which 3 pixels its 3 corners project onto. The color is determined for each of these points, and the fragment shader fills in the rest using interpolation.
My question is this: is it possible to gain control over this projection model? For example, create my own custom distorted uv-grid? Or even just supply my own algorithm:
xyPixelPos_for_Vector3( Vector3 v ) {...}
I'm working in Unity3D, so I think that limits me to cG or openGL.
I did once write a GLES2 shader, but I don't remember ever performing any kind of "ray hits quad" type test to resolve the pixel position of a particular 3D point in space.
I'm going to assume that you want to render 3d images based upon 3d primitives that are defined by vertices. This is not the only way to render images with OpenGL but it is the most common. The technique that you describe sounds much more like Ray-Tracing.
How OpenGL Typically Works:
I wouldn't say that OpenGL creates an imaginary grid. Instead, what it does is take the positions of each of your vertices, and converts them into a different space using linear algebra (Matrices).
If you want to start playing around with this, it would be best to do some reading on Matrices, to understand what the graphics card is doing.
You can easily start warping the positions of Vertices by making a vertex shader. However, there is some setup involved. See the Lighthouse tutorials (http://www.lighthouse3d.com/tutorials/glsl-tutorial/hello-world-in-glsl/) to get started with that! You will also want to read their tutorials on lighting (http://www.lighthouse3d.com/tutorials/glsl-tutorial/lighting/), to create a fully functioning vertex shader which includes a lighting model.
Thankfully, once the shader is set up, you can distort your entire scene to your hearts content. Just remember to do your distortions in the right 'space'. World coordinates are much different than eye coordinates!
I'm currently trying to implement a silhouette algorithm in my project (using Open GLES, it's for mobile devices, primarily iPhone at the moment). One of the requirements is that a set of 3D lines be drawn. The issue with the default OpenGL lines is that they don't connect at an angle nicely when they are thick (gaps appear). Other subtle artifacts are also evident, which detract from the visual appeal of the lines.
Now, I have looked into using some sort of quad strip as an alternative to this. However, drawing a quad strip in screen space requires some sort of visibility detection - lines obscured in the actual 3D world should not be visible.
There are numerous approaches to this problem - i.e. quantitative invisibility. But such an approach, particularly on a mobile device with limited processing power, is difficult to implement efficiently, considering raycasting needs to be employed. Looking around some more I found this paper, which describes a couple of methods for using z-buffer sampling to achieve such an effect. However, I'm not an expert in this area, and while I understand the theory behind the techniques to an extent, I'm not sure how to go about the practical implementation. I was wondering if someone could guide me here at a more technical level - on the OpenGLES side of things. I'm also open to any suggestions regarding 3D line visibility in general.
The technique with z-buffer will be too complex for iOS devices - it needs heavy pixel shader and (IMHO) it will bring some visual artifacts.
If your models are not complex you can find geometric silhouette in runtime - for example by comparing normals of polygons with common edge: if z value of direction in view space has different sings (one normal is directed to camera and other is from camera) then this edge should be used for silhouette.
Another approach is more "FPS friendly": keep extruded version of your model. And render firstly extruded model with color of silhouette (without textures and lighting) and normal model over it. You will need more memory for vertices, but no real-time computations.
PS: In all games I have look at silhouettes were geometric.
I have worked out a solution that works nicely on an iPhone 4S (not tested on any other devices). It builds on the idea of rendering world-space quads, and does the silhouette detection all on the GPU. It works along these lines (pun not intended):
We generate edge information. This consists of a list of edges/"lines" in the mesh, and for each we associate two normals which represent the tris on either side of the edge.
This is processed into a set of quads that are uploaded to the GPU - each quad represents an edge. Each vertex of each quad is accompanied by three attributes (vec3s), namely the edge direction vector and the two neighbor tri normals. All quads are passed w/o "thickness" - i.e. the vertices on either end are in the same position. However, the edge direction vector is opposite for each vertex in the same position. This means they will extrude in opposite directions to form a quad when required.
We determine whether a vertex is part of a visible edge in the vertex shader by performing two dot products between each tri norm and the view vector and checking if they have opposite signs. (see standard silhouette algorithms around the net for details)
For vertices that are part of visible edges, we take the cross product of the edge direction vector with the view vector to get a screen-oriented "extrusion" vector. We add this vector to the vertex, but divided by the w value of the projected vertex in order to create a constant thickness quad.
This does not directly resolve the gaps that can appear between neighbor edges but is far more flexible when it comes to combating this. One solution may involve bridging the vertices between large angled lines with another quad, which I am exploring at the moment.
I'm using texture-based DOT3 lighting to achieve bump mapping on older iPhones, and I'm wondering if there's a sneaky way to make it look correct even when the viewpoint changes.
With "real" lighting, normals get transformed by the inverse model-view matrix. With texture-based DOT3 lighting, there's no transform.
With shader-based bump mapping, the normal map is in tangent space. With DOT3 lighting, the normal map needs to be in eye space. This is okay only if you have a fixed viewpoint, or if your geometry happens to be a flat plane.
Should I just accept this as another limitation of DOT3 lighting? Since it can't handle specular either, I'm starting to question how useful it really is.
Your normal map isn’t required to be in any particular space, as long as the light vector is in the same coordinate space. For DOT3 lighting on fixed-function hardware, it’s probably best to keep a normal map in object space, rather than eye space. From here, you just need to make sure that when you render each object, you apply the appropriate transformation to move your directional light vector from world space to object space.
can someone point me to a paper/algorithm/resource/whatever that tells me how to implement a texture minification filter (applies when texels are smaller than pixels) in a raytracer?
thanks!
Since you are using ray tracing I suspect you are looking for a high quality filtering that changes sampling dynamically based on the amount of "error". Based on this assumption I would say take a look at "ray differentials". There's a nice paper on this here: http://graphics.stanford.edu/papers/trd/ and it takes effects like refraction and reflection into account.
Your answer to yourself sounds like the right approach, but since others may stumble across the page I'll add a resource link as requested. In addition to discussing mipmapping (ripmapping is basically more advanced mipmapping), they discuss the effects of reflection and refraction on derivatives and mip-level selection.
Homan Igehy. "Tracing Ray Differentials." 1999. Proceedings of SIGGRAPH. http://graphics.stanford.edu/papers/trd/
Upon closer reading I see that Rehno Lindeque mentioned this paper. At first didn't realize that it was the right reference because he says that the method samples dynamically based on the error of the sampling, which is incorrect. Filtering is done based on the size of the pixel's footprint and uses only one ray, just as you described.
Edit:
Another reference that might be useful ( http://www.cs.unc.edu/~awilson/class/238/#challenges ). Scroll to the section "Derivatives of Texture Coordinates." He suggests backward mapping of texture derivatives from the surface to the screen. I think this would be incorrect for reflected and refracted rays, but is possibly easier to implement and should be okay for primary rays.
I think you mean mipmap'ing.
Here is an article talking about using them.
But nether say how to chose which mipmap to use, but they are often blended (the bigger and smaller mipmap).
Here's a one more article about how Google Earth works, and it talks about how they mipmapping the earth.
thank you guys for your answers, but since I didn't find any appropriate techinque i created something myself which turned out to work very well:
i assume my ray to be a cone with a coneradius of half a pixel on the imageplane. when the ray hits a surface, i calculate the ellipse which is projected onto the surface (the ellipse from the plane-cone intersection). Then, using the texturecoordinate derivatives at the intersection point, i project this ellipse into texturespace. now i know which part of the texture lies under my pixel and can subsample this area
I Also use RipMaps to improve the quality - and i chose the RipMap level based on the size of the ellipse in Texturespace
The Screen-to-world problem on the iPhone
I have a 3D model (CUBE) rendered in an EAGLView and I want to be able to detect when I am touching the center of a given face (From any orientation angle) of the cube. Sounds pretty easy but it is not...
The problem:
How do I accurately relate screen-coordinates (touch point) to world-coordinates (a location in OpenGL 3D space)? Sure, converting a given point into a 'percentage' of the screen/world-axis might seem the logical fix, but problems would arise when I need to zoom or rotate the 3D space. Note: rotating & zooming in and out of the 3D space will change the relationship of the 2D screen coords with the 3D world coords...Also, you'd have to allow for 'distance' in between the viewpoint and objects in 3D space. At first, this might seem like an 'easy task', but that changes when you actually examine the requirements. And I've found no examples of people doing this on the iPhone. How is this normally done?
An 'easy' task?:
Sure, one might undertake the task of writing an API to act as a go-between between screen and world, but the task of creating such a framework would require some serious design and would likely take 'time' to do -- NOT something that can be one-manned in 4 hours...And 4 hours happens to be my deadline.
The question:
What are some of the simplest ways to
know if I touched specific locations
in 3D space in the iPhone OpenGL ES
world?
You can now find gluUnProject in http://code.google.com/p/iphone-glu/. I've no association with the iphone-glu project and haven't tried it yet myself, just wanted to share the link.
How would you use such a function? This PDF mentions that:
The Utility Library routine gluUnProject() performs this reversal of the transformations. Given the three-dimensional window coordinates for a location and all the transformations that affected them, gluUnProject() returns the world coordinates from where it originated.
int gluUnProject(GLdouble winx, GLdouble winy, GLdouble winz,
const GLdouble modelMatrix[16], const GLdouble projMatrix[16],
const GLint viewport[4], GLdouble *objx, GLdouble *objy, GLdouble *objz);
Map the specified window coordinates (winx, winy, winz) into object coordinates, using transformations defined by a modelview matrix (modelMatrix), projection matrix (projMatrix), and viewport (viewport). The resulting object coordinates are returned in objx, objy, and objz. The function returns GL_TRUE, indicating success, or GL_FALSE, indicating failure (such as an noninvertible matrix). This operation does not attempt to clip the coordinates to the viewport or eliminate depth values that fall outside of glDepthRange().
There are inherent difficulties in trying to reverse the transformation process. A two-dimensional screen location could have originated from anywhere on an entire line in three-dimensional space. To disambiguate the result, gluUnProject() requires that a window depth coordinate (winz) be provided and that winz be specified in terms of glDepthRange(). For the default values of glDepthRange(), winz at 0.0 will request the world coordinates of the transformed point at the near clipping plane, while winz at 1.0 will request the point at the far clipping plane.
Example 3-8 (again, see the PDF) demonstrates gluUnProject() by reading the mouse position and determining the three-dimensional points at the near and far clipping planes from which it was transformed. The computed world coordinates are printed to standard output, but the rendered window itself is just black.
In terms of performance, I found this quickly via Google as an example of what you might not want to do using gluUnProject, with a link to what might lead to a better alternative. I have absolutely no idea how applicable it is to the iPhone, as I'm still a newb with OpenGL ES. Ask me again in a month. ;-)
You need to have the opengl projection and modelview matrices. Multiply them to gain the modelview projection matrix. Invert this matrix to get a matrix that transforms clip space coordinates into world coordinates. Transform your touch point so it corresponds to clip coordinates: the center of the screen should be zero, while the edges should be +1/-1 for X and Y respectively.
construct two points, one at (0,0,0) and one at (touch_x,touch_y,-1) and transform both by the inverse modelview projection matrix.
Do the inverse of a perspective divide.
You should get two points describing a line from the center of the camera into "the far distance" (the farplane).
Do picking based on simplified bounding boxes of your models. You should be able to find ray/box intersection algorithms aplenty on the web.
Another solution is to paint each of the models in a slightly different color into an offscreen buffer and reading the color at the touch point from there, telling you which brich was touched.
Here's source for a cursor I wrote for a little project using bullet physics:
float x=((float)mpos.x/screensize.x)*2.0f -1.0f;
float y=((float)mpos.y/screensize.y)*-2.0f +1.0f;
p2=renderer->camera.unProject(vec4(x,y,1.0f,1));
p2/=p2.w;
vec4 pos=activecam.GetView().col_t;
p1=pos+(((vec3)p2 - (vec3)pos) / 2048.0f * 0.1f);
p1.w=1.0f;
btCollisionWorld::ClosestRayResultCallback rayCallback(btVector3(p1.x,p1.y,p1.z),btVector3(p2.x,p2.y,p2.z));
game.dynamicsWorld->rayTest(btVector3(p1.x,p1.y,p1.z),btVector3(p2.x,p2.y,p2.z), rayCallback);
if (rayCallback.hasHit())
{
btRigidBody* body = btRigidBody::upcast(rayCallback.m_collisionObject);
if(body==game.worldBody)
{
renderer->setHighlight(0);
}
else if (body)
{
Entity* ent=(Entity*)body->getUserPointer();
if(ent)
{
renderer->setHighlight(dynamic_cast<ModelEntity*>(ent));
//cerr<<"hit ";
//cerr<<ent->getName()<<endl;
}
}
}
Imagine a line that extends from the viewer's eye
through the screen touch point into your 3D model space.
If that line intersects any of the cube's faces, then the user has touched the cube.
Two solutions present themselves. Both of them should achieve the end goal, albeit by a different means: rather than answering "what world coordinate is under the mouse?", they answer the question "what object is rendered under the mouse?".
One is to draw a simplified version of your model to an off-screen buffer, rendering the center of each face using a distinct color (and adjusting the lighting so color is preserved identically). You can then detect those colors in the buffer (e.g. pixmap), and map mouse locations to them.
The other is to use OpenGL picking. There's a decent-looking tutorial here. The basic idea is to put OpenGL in select mode, restrict the viewport to a small (perhaps 3x3 or 5x5) window around the point of interest, and then render the scene (or a simplified version of it) using OpenGL "names" (integer identifiers) to identify the components making up each face. At the end of this process, OpenGL can give you a list of the names that were rendered in the selection viewport. Mapping these identifiers back to original objects will let you determine what object is under the mouse cursor.
Google for opengl screen to world (for example there’s a thread where somebody wants to do exactly what you are looking for on GameDev.net). There is a gluUnProject function that does precisely this, but it’s not available on iPhone, so that you have to port it (see this source from the Mesa project). Or maybe there’s already some publicly available source somewhere?