I'm trying to understand path tracing. So far, I have only dealt with the very basis - when a ray is launched from each intersection point in a random direction within the hemisphere, then again, and so on recursively, until the ray hits the light source. As a result, this approach leads to the fact that in the case of small light sources, the image is extremely noisy.
The following images show the noise level depending on the number of samples (rays) per pixel.
I am also not sure that i am doing everything correctly, because the "Monte Carlo" method, as far as I understand, implies that several rays are launched from each intersection point, and then their result is summed and averaged. But this approach leads to the fact that the number of rays increases exponentially, and after 6 bounces reaches inadequate values, so i decided that it is better to just run several rays per pixel initially (slightly shifted from the center of the pixel in a random direction), but only 1 ray is generated at each intersection. I do not know whether this approach corresponds to "Monte Carlo" or not, but at least this way the rendering does not last forever..
Bidirectional path tracing
I started looking for ways to reduce the amount of noise, and came across bidirectional path tracing. But unfortunately, i couldn't find a detailed explanation of this algorithm in simple words. All I understood is that the rays are generated from both the camera and the light sources, and then there is a check on the possibility of connecting the endpoints of these paths.
As you can see, if the intersection points of the blue ray from the camera and the white ray from the light source can be freely connected (there are no obstacles in the connection path), then we can assume that the ray from the camera can pass through the points y1, y0 directly to the light source.
But there are a lot of questions:
If the light source is not a point, but has some shape, then the point from which the ray is launched must be randomly selected on the surface of this shape? If you take only the center - then there will be no difference from a point light source, right?
Do i need to build a path from the light source for each path from the camera, or should there be only one path from the light source, while several paths (samples) are built from the camera for one pixel at once?
The number of bounces/re-reflections/refractions should be the same for the path from the camera and the light source? Or not?
But the questions don't end there. I have heard that the bidirectional trace method allows you to model caustics well (in comparison with regular path tracing). But I completely did not understand how the method of bidirectional path tracing can somehow help for this.
Example 1
Here the path will eventually be built, but the number of bounces will be extremely large, so no caustics will work here, despite the fact that the ray from the camera is directed almost to the same point where the path of the ray from the light source ends.
Example 2
Here the path will not be built, because there is an obstacle between the endpoints of the paths, although it could be built if point x3 was connected to point y1, but according to the algorithm (if I understand everything correctly), only the last points of the paths are connected.
Question:
What is the use of such an algorithm, if in a significant number of cases the paths either cannot be built, or are unnecessarily long? Maybe I misunderstand something? I came across many articles and documents where this algorithm was somehow described, but mostly it was described mathematically (using all sorts of magical terms like biased-unbiased, PDF, BSDF, and others), and not.. algorithmically. I am not that strong in mathematics and all sorts of mathematical notation and wording, I would just like to understand WHAT TO DO, how to implement it correctly in the code, how these paths are connected, in what order, and so on. This can be explained in simple words, pseudocode, right? I would be extremely grateful if someone would finally shed some light on all this.
Some references that helped me to understand the Path tracing right :
https://www.scratchapixel.com/ (every rendering student should begin with this)
https://en.wikipedia.org/wiki/Path_tracing
If you're looking for more references, path tracing is used for "Global illumination" wich is the opposite as "Direct illumination" that only rely on a straight line from the point to the light.
What's more caustics is well knowned to be a hard problem, so don't begin with it! Monte Carlo method is a good straightforward method to begin with, but it has its limitations (ie Caustics and tiny lights).
Some advices for rendering newbees
Mathematics notations are surely not the coolest ones. Every one will of course prefer a ready to go code. But maths is the most rigourous way to describe the world. It permits also to modelize a whole physic interaction in a small formula instead of plenty of lines of codes that doesn't fit to the real problem. I suggest you to forget you to try reading what you read better as a good mathematic formula is always detailed. If some variables are not specified, don't loose your time and search another reference.
Related
I am using Poisson surface reconstruction algorithm to reconstruct triangulated mesh surface from points. However, Poisson will always generate a watertight surface, which fills all holes with interpolation.
For some small holes that is the result of data missing, this hole filling is desirable. But for some big holes, I do not want hole filling and just want the surface to remain open.
The figure above shows my idea, the left one is a pointset with normal, the right one is reconstructed surface. I want the top of this surface remains open rather than current watertight result.
Can anyone provide me with some advice, how may I keep these big holes in Poisson surface reconstruction? Or is there any other algorithms that could solve this?
P.S.
Based on the accepted answer to this question, I understand surface reconstruction algorithms could be categorized as explicit ones and implicit ones. Poisson is implicit ones, and explicit ones could naturally handle big hole problem. But since the points data I have are mostly sparse and noisy, I would prefer an implicit one like Poisson.
Your screenshots look like you might be using MeshLab's implementation which is based on an old implementation. This implementation is not capable of trimming the surface.
The latest implementation, however, contains the SurfaceTrimmer that does exactly what you want. Take a look at the examples at the bottom of the page to see how to use it.
To use SurfaceTrimmer program, you have to first use SSDRecon program to reconstruct a mesh surface with --density, then setting a trim value would exactly remove faces under a specific threshold.
Below is a sample usage of that program on the demo eagle data
./SSDRecon --in eagle.points.ply --out eagle.screened.color.ply --depth 10 --density
./SurfaceTrimmer --in eagle.screened.color.ply --out eagle.screened.color.trimmed.ply --trim 7
I have an application where I need to move a number of objects around on the screen in a random fashion and they can not bump into each other. I'm looking for an algorithm that will allow me to generate the paths that don't create collisions and can continue for an indefinite time (i.e.: the objects keep moving around until a user driven event removes them from the program).
I'm not a game programmer but I think this looks like an AI problem and you guys probably solve it with your eyes closed. From what I've read A* seems to be the recommended 'basic idea' but I don't really want to invest a lot of time into it without some confirmation.
Can anyone shed some light on an approach? Anti-gravity movement maybe?
This is to be implemented on iOS, if that is important
New paths need to be generated at the end of each path
There is no visible 'grid'. Movement is completely free in 2D space
The objects are insects that walk around the screen until they are killed
A* is an algorithm to find the shortest path between a start and a goal configuration (in terms of whatever you define as short: common are e.g. euclidean distance, cost or time, angular distance...). Your insects seem not to have a specific goal, they don't even need a shortest path. I would certainly not go for A*. (By the way, since you are having a dynamic environment, D* would have been an idea - still it's meant to find a path from A to B).
I would tackle the problem as follows:
Random Paths and following them
For the random paths I see two methods. The first would be a simple random walk (click here to see a nice 2D animation with explanations), which can suffer from jittering and doesn't look too nice. The second one needs a little bit more detailed explanations.
For each insect generate four random points around them, maybe starting on a sinusoid. With a spline interpolation generate a smooth curve between those points. Take care of having C1 (in 2D) or C2 (in 3D) continuity. (Suggestion: Hermite splines)
With Catmull-Rom splines you can find your configurations while moving along the curve.
An application of a similar approach can be found in this blog post about procedural racetracks, also a more technical (but still not too technical) explanation can be found in these old slides (pdf) from a computer animations course.
When an insect starts moving, it can constantly move between the second and third point, when you always remove the first and append a new point when the insect reaches the third, thus making that the second point.
If third point is reached
Remove first
Append new point
Recalculate spline
End if
For a smoother curve add more points in total and move somewhere in the middle, the principle stays the same. (Personally I only used this in fixed environments, it should work in dynamic ones as well though.)
This can, if your random point generation is good (maybe you can use an approach similar to the one provided in the above linked blog post, or have a look at algorithms on the PCG Wiki), lead to smooth paths all over the screen.
Avoid other insects
To avoid other insects, three different methods come to my mind.
Bug algorithms
Braitenberg vehicles
An application of potential fields
For the potential fields I recommend reading this paper about dynamic motion planning (pdf). It's from robotics, but fairly easy to apply to your problem as well. You can just use the robots next spline point as the goal and set its velocity to 0 to apply this approach. However, it might be a bit too much for your simple game.
A discussion of Braitenberg vehicles can be found here (pdf). The original idea was more of a technical method (drive towards or away from a light source depending on how your motor is coupled with the photo receptor) and is often used to show how we apply emotional concepts like fear and attraction to other objects. The "fear" behaviour is an approach used for obstacle avoidance in robotics as well.
The third and probably simplest method are bug algorithms (pdf). I always have problems with the boundary following, which is a bit tricky. But to avoid another insect, these algorithms - no matter which one you use (I suggest Bug 1 or Tangent Bug) - should do the trick. They are very simple: Move towards your goal (in this application with the catmull-rom splines) until you have an obstacle in front. If the obstacle is close, change the insect's state to "obstacle avoidance" and run your bug algorithm. If you give both "colliding" insects the same turn direction, they will automatically go around each other and follow their original path.
As a variation you could just let them turn and recalculate a new spline from that point on.
Conclusion
Path finding and random path generation are different things. You have to experiment around what looks best for your insects. A* is definitely meant for finding shortest paths, not for creating random paths and following them.
You cannot plan the trajectories ahead of time for an indefinite duration !
I suggest a simpler approach where you just predict the next collision (knowing the positions and speeds of the objects allows you to tell if they will collide and when), and resolve it by changing the speed or direction of either objects (bounce before objects touch).
Make sure to redo a check for collisions in case you created an even earlier collision !
The real challenge in your case is to efficiently predict collisions among numerous objects, a priori an O(N²) task. You will accelerate that by superimposing a coarse grid on the play field and look at objects in neighboring cells only.
It may also be possible to maintain a list of object pairs that "might interfere in some future" (i.e. considering their distance and relative speed) and keep it updated. Checking that a pair may leave the list is relatively easy; efficiently checking for new pairs needing to enter the list is not.
Look at this and this Which described an AI program to auto - play Mario game.
So in this link, what the author did was using a A* star algorithm to guide Mario Get to the right border of the screen as fast as possible. Avoid being hurt.
So the idea is for each time frame, he will have an Environment which described the current position of other objects in the scene and for each action (up, down left, right and do nothing) , he calculate its cost function and made a decision of the next movement based on this.
Source: http://www.quora.com/What-are-the-coolest-algorithms
For A* you would need a 2D-Grid even if it is not visible. If I get your idea right you could do the following.
Implement a pathfinding (e.g. A*) then just generate random destination points on the screen and calculate the path. Once your insect reaches the destination, generate another destination point/grid-cell and proceed until the insect dies.
As I see it A* would only make sence if you have obstacles on the screen the insect should navigate around, otherwise it would be enough to just calculate a straight vector path and maybe handle collision with other insects/objects.
Note: I implemented A* once, later I found out that Lee's Algorithm
pretty much does the same but was easier to implement.
Consider a Hamiltonian cycle - the idea is a route that visits all the positions on a grid once (and only once). If you construct the cycle in advance (i.e. precalculate it), and set your insects off with some offset between them, they will never collide, simply because the path never intersects itself.
Also, for bonus points, Hamiltonian paths tend to 'wiggle about', and because it's a loop you can predict (and precalculate) the path into the indefinite future.
You can always use the nodes of the grid as knot points for a spline to smooth the movement, or even randomly shift all the points away from their strict 2d grid positions, until you have the desired motion.
Example Hamiltonian cycle from Wikimedia:
On a side note, if you want to generate such a path, consider constructing a loop through many points and just moving the points around in such a manner that they never intersect an existing edge. With some encouragement to move into gaps and away from each other, they should settle into some long, never-intersecting path. Store the result and use for your loop.
Many certain resources about raytracing tells about:
"shoot rays, find the first obstacle to cut it"
"shoot secondary rays..."
"or, do it reverse and approximate/interpolate"
I didnt see any algortihm that uses a diffusion algorithm. Lets assume a point-light is a point that has more density than other cells(all space is divided into cells), every step/iteration of lighting/tracing makes that source point to diffuse into neighbours using a velocity field and than their neighbours and continues like that. After some satisfactory iterations(such as 30-40 iterations), the density info of each cell is used for enlightment of objects in that cell.
Point light and velocity field:
But it has to be a like 1000x1000x1000 size and this would take too much time and memory to compute. Maybe just computing 10x10x10 and when finding an obstacle, partitioning that area to 100x100x100(in a dynamic kd-tree fashion) can help generating lighting/shadows for acceptable resolution? Especially for vertex-based illumination rather than triangle.
Has anyone tried this approach?
Note: Velocity field is here to make light diffuse to outwards mostly(not %100 but %99 to have some global illumination). Finite-element-method can make this embarassingly-parallel.
Edit: any object that is hit by a positive-density will be an obstacle to generate a new velocity field around the surface of it. So light cannot go through that object but can be mirrored to another direction.(if it is a lens object than light diffuse harder through it) So the reflection of light can affect other objects with a higher iteration limit
Same kd-tree can be used in object-collision algorithms :)
Just to take as a grain of salt: a neural-network can be trained for advection&diffusion in a 30x30x30 grid and that can be used in a "gpu(opencl/cuda)-->neural-network ---> finite element method --->shadows" way.
There's a couple problems with this as it stands.
The first problem is that, fundamentally, a photon in the Newtonian sense doesn't react or change based on the density of other photons around. So using a density field and trying to light to follow the classic Navier-Stokes style solutions (which is what you're trying to do, based on the density field explanation you gave) would result in incorrect results. It would also, given enough iterations, result in complete entropy over the scene, which is also not what happens to light.
Even if you were to get rid of the density problem, you're still left with the the problem of multiple photons going different directions in the same cell, which is required for global illumination and diffuse lighting.
So, stripping away the problem portions of your idea, what you're left with is a particle system for photons :P
Now, to be fair, sudo-particle systems are currently used for global illumination solutions. This type of thing is called Photon Mapping, but it's only simple to implement a direct lighting solution using it :P
I am hacking a vacuum cleaner robot to control it with a microcontroller (Arduino). I want to make it more efficient when cleaning a room. For now, it just go straight and turn when it hits something.
But I have trouble finding the best algorithm or method to use to know its position in the room. I am looking for an idea that stays cheap (less than $100) and not to complex (one that don't require a PhD thesis in computer vision). I can add some discrete markers in the room if necessary.
Right now, my robot has:
One webcam
Three proximity sensors (around 1 meter range)
Compass (no used for now)
Wi-Fi
Its speed can vary if the battery is full or nearly empty
A netbook Eee PC is embedded on the robot
Do you have any idea for doing this? Does any standard method exist for these kind of problems?
Note: if this question belongs on another website, please move it, I couldn't find a better place than Stack Overflow.
The problem of figuring out a robot's position in its environment is called localization. Computer science researchers have been trying to solve this problem for many years, with limited success. One problem is that you need reasonably good sensory input to figure out where you are, and sensory input from webcams (i.e. computer vision) is far from a solved problem.
If that didn't scare you off: one of the approaches to localization that I find easiest to understand is particle filtering. The idea goes something like this:
You keep track of a bunch of particles, each of which represents one possible location in the environment.
Each particle also has an associated probability that tells you how confident you are that the particle really represents your true location in the environment.
When you start off, all of these particles might be distributed uniformly throughout your environment and be given equal probabilities. Here the robot is gray and the particles are green.
When your robot moves, you move each particle. You might also degrade each particle's probability to represent the uncertainty in how the motors actually move the robot.
When your robot observes something (e.g. a landmark seen with the webcam, a wifi signal, etc.) you can increase the probability of particles that agree with that observation.
You might also want to periodically replace the lowest-probability particles with new particles based on observations.
To decide where the robot actually is, you can either use the particle with the highest probability, the highest-probability cluster, the weighted average of all particles, etc.
If you search around a bit, you'll find plenty of examples: e.g. a video of a robot using particle filtering to determine its location in a small room.
Particle filtering is nice because it's pretty easy to understand. That makes implementing and tweaking it a little less difficult. There are other similar techniques (like Kalman filters) that are arguably more theoretically sound but can be harder to get your head around.
A QR Code poster in each room would not only make an interesting Modern art piece, but would be relatively easy to spot with the camera!
If you can place some markers in the room, using the camera could be an option. If 2 known markers have an angular displacement (left to right) then the camera and the markers lie on a circle whose radius is related to the measured angle between the markers. I don't recall the formula right off, but the arc segment (on that circle) between the markers will be twice the angle you see. If you have the markers at known height and the camera is at a fixed angle of inclination, you can compute the distance to the markers. Either of these methods alone can nail down your position given enough markers. Using both will help do it with fewer markers.
Unfortunately, those methods are imperfect due to measurement errors. You get around this by using a Kalman estimator to incorporate multiple noisy measurements to arrive at a good position estimate - you can then feed in some dead reckoning information (which is also imperfect) to refine it further. This part is goes pretty deep into math, but I'd say it's a requirement to do a great job at what you're attempting. You can do OK without it, but if you want an optimal solution (in terms of best position estimate for given input) there is no better way. If you actually want a career in autonomous robotics, this will play large in your future. (
Once you can determine your position you can cover the room in any pattern you'd like. Keep using the bump sensor to help construct a map of obstacles and then you'll need to devise a way to scan incorporating the obstacles.
Not sure if you've got the math background yet, but here is the book:
http://books.google.com/books/about/Applied_optimal_estimation.html?id=KlFrn8lpPP0C
This doesn't replace the accepted answer (which is great, thanks!) but I might recommend getting a Kinect and use that instead of your webcam, either through Microsoft's recently released official drivers or using the hacked drivers if your EeePC doesn't have Windows 7 (presumably it does not).
That way the positioning will be improved by the 3D vision. Observing landmarks will now tell you how far away the landmark is, and not just where in the visual field that landmark is located.
Regardless, the accepted answer doesn't really address how to pick out landmarks in the visual field, and simply assumes that you can. While the Kinect drivers may already have feature detection included (I'm not sure) you can also use OpenCV for detecting features in the image.
One solution would be to use a strategy similar to "flood fill" (wikipedia). To get the controller to accurately perform sweeps, it needs a sense of distance. You can calibrate your bot using the proximity sensors: e.g. run motor for 1 sec = xx change in proximity. With that info, you can move your bot for an exact distance, and continue sweeping the room using flood fill.
Assuming you are not looking for a generalised solution, you may actually know the room's shape, size, potential obstacle locations, etc. When the bot exists the factory there is no info about its future operating environment, which kind of forces it to be inefficient from the outset.
If that's you case, you can hardcode that info, and then use basic measurements (ie. rotary encoders on wheels + compass) to precisely figure out its location in the room/house. No need for wifi triangulation or crazy sensor setups in my opinion. At least for a start.
Ever considered GPS? Every position on earth has a unique GPS coordinates - with resolution of 1 to 3 metres, and doing differential GPS you can go down to sub-10 cm range - more info here:
http://en.wikipedia.org/wiki/Global_Positioning_System
And Arduino does have lots of options of GPS-modules:
http://www.arduino.cc/playground/Tutorials/GPS
After you have collected all the key coordinates points of the house, you can then write the routine for the arduino to move the robot from point to point (as collected above) - assuming it will do all those obstacles avoidance stuff.
More information can be found here:
http://www.google.com/search?q=GPS+localization+robots&num=100
And inside the list I found this - specifically for your case: Arduino + GPS + localization:
http://www.youtube.com/watch?v=u7evnfTAVyM
I was thinking about this problem too. But I don't understand why you can't just triangulate? Have two or three beacons (e.g. IR LEDs of different frequencies) and a IR rotating sensor 'eye' on a servo. You could then get an almost constant fix on your position. I expect the accuracy would be in low cm range and it would be cheap. You can then map anything you bump into easily.
Maybe you could also use any interruption in the beacon beams to plot objects that are quite far from the robot too.
You have a camera you said ? Did you consider looking at the ceiling ? There is little chance that two rooms have identical dimensions, so you can identify in which room you are, position in the room can be computed from angular distance to the borders of the ceiling and direction can probably be extracted by the position of doors.
This will require some image processing but the vacuum cleaner moving slowly to be efficiently cleaning will have enough time to compute.
Good luck !
Use Ultra Sonic Sensor HC-SR04 or similar.
As above told sense the walls distance from robot with sensors and room part with QR code.
When your are near to a wall turn 90 degree and move as width of your robot and again turn 90deg( i.e. 90 deg left turn) and again move your robot I think it will help :)
I am thinking of implement a image processing based solution for industrial problem.
The image is consists of a Red rectangle. Inside that I will see a matrix of circles. The requirement is to count the number of circles under following constraints. (Real application : Count the number of bottles in a bottle casing. Any missing bottles???)
The time taken for the operation should be very low.
I need to detect the red rectangle as well. My objective is to count the
items in package and there are no
mechanism (sensors) to trigger the
camera. So camera will need to capture
the photos continuously but the
program should have a way to discard
the unnecessary images.
Processing should be realtime.
There may be a "noise" in image capturing. You may see ovals instead of circles.
My questions are as follows,
What is the best edge detection algorithm that matches with the given
scenario?
Are there any other mechanisms that I can use other than the edge
detection?
Is there a big impact between the language I use and the performance of
the system?
AHH - YOU HAVE NOW TOLD US THE BOTTLES ARE IN FIXED LOCATIONS!
IT IS AN INCREDIBLY EASIER PROBLEM.
All you have to do is look at each of the 12 spots and see if there is a black area there or not. Nothing could be easier.
You do not have to do any edge or shape detection AT ALL.
It's that easy.
You then pointed out that the box might be rotatated, things could be jiggled. That the box might be rotated a little (or even a lot, 0 to 360 each time) is very easily dealt with. The fact that the bottles are in "slots" (even if jiggled) massively changes the nature of the problem. You're main problem (which is easy) is waiting until each new red square (crate) is centered under the camera. I just realised you meant "matrix" literally and specifically in the sentence in your original questions. That changes everything totally, compared to finding a disordered jumble of circles. Finding whether or not a blob is "on" at one of 12 points, is a wildly different problem to "identifying circles in an image". Perhaps you could post an image to wrap up the question.
Finally I believe Kenny below has identified the best solution: blob analysis.
"Count the number of bottles in a bottle casing"...
Do the individual bottles sit in "slots"? ie, there are 4x3 = 12 holes, one for each bottle.
In other words, you "only" have to determine if there is, or is not, a bottle in each of the 12 holes.
Is that correct?
If so, your problem is incredibly easier than the more general problem of a pile of bottles "anywhere".
Quite simply, where do we see the bottles from? The top, sides, bottom, or? Do we always see the tops/bottoms, or are they mixed (ie, packed top-to-tail). These issues make huge, huge differences.
Surf/Sift = overkill in this case you certainly don't need it.
If you want real time speed (about 20fps+ on a 800x600 image) I recommend using Cuda to implement edge detection using a standard filter scheme like sobel, then implement binarization + image closure to make sure the edges of circles are not segmented apart.
The hardest part will be fitting circles. This is assuming you already got to the step where you have taken edges and made sure they are connected using image closure (morphology.) At this point I would proceed as follows:
run blob analysis/connected components to segment out circles that do not touch. If circles can touch the next step will be trickier
for each connected componet/blob fit a circle or rectangle using RANSAC which can run in realtime (as opposed to Hough Transform which I believe is very hard to run in real time.)
Step 2 will be much harder if you can not segment the connected components that form circles seperately, so some additional thought should be invested on how to guarantee that condition.
Good luck.
Edit
Having thought about it some more, I feel like RANSAC is ideal for the case where the circle connected components do touch. RANSAC should hypothetically fit the circle to only a part of the connected component (due to its ability to perform well in the case of mostly outlier points.) This means that you could add an extra check to see if the fitted circle encompasses the entire connected component and if it does not then rerun RANSAC on the portion of the connected component that was left out. Rinse and repeat as many times as necessary.
Also I realize that I say circle but you could just as easily fit an ellipse instead of circles using RANSAC.
Also, I'd like to comment that when I say CUDA is a good choice I mean CUDA is a good choice to implement the sobel filter + binirization + image closing on. Connected components and RANSAC are probably best left to the CPU, but you can try pushing them onto CUDA though I don't know how much of an advantage a GPU will give you for those 2 over a CPU.
For the circles, try the Hough transform.
other mechanisms: dunno
Compiled languages will possibly be faster.
SIFT should have a very good response to circular objects - it is patented, though. GLOHis a similar algorithm, but I do not know if there are any implementations readily available.
Actually, doing some more research, SURF is an improved version of SIFT with quite a few implementations available, check out the links on the wikipedia page.
Sum of colors + convex hull to detect boundary. You need, mostly, 4 corners of a rectangle, and not it's sides?
No motion, no second camera, a little choice - lot of math methods against a little input (color histograms, color distribution matrix). Dunno.
Java == high memory consumption, Lisp == high brain consumption, C++ == memory/cpu/speed/brain use optimum.
If the contrast is good, blob analysis is the algorithm for the job.