Changes between Version 6 and Version 7 of Processing/lagdevelopersfaq


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Timestamp:
Jul 5, 2012 5:16:26 PM (7 years ago)
Author:
jaho
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  • Processing/lagdevelopersfaq

    v6 v7  
    55[#q4 4. What are LAS files?] \\
    66
    7 [#q4.1 4.1 What's the deal with LAS 1.3 files? \\
     7[#q4.1 4.1 What's the deal with LAS 1.3 files?] \\
    88
    99[#q5 5. What do I need to know about laslib?] \\
     
    2727
    2828[#q23 23. What are main ideas for further LAG development?] \\
    29 [#q24 24. What are some major issues with LAG that need fixing?] \\
    30 [#q25 25. What are some additional features that can be added to LAG?] \\
    31 [#q26 26. What tools are there to help me with LAG development?] \\
     29[#q24 24. What are the unfinished development stuff?] \\
     30[#q25 25. What is lag memory issue?] \\
     31[#q26 26. What are some major issues with LAG that need fixing?] \\
     32[#q27 27. What are some additional features that can be added to LAG?] \\
     33[#q28 28. What tools are there to help me with LAG development?] \\
     34[\q29 29. How do I use profiling tools to optimise code?] \\
    3235
    3336[#q5 Question X?] \\
     
    236239  A flat data structure which actually holds the points. Each quadtree node holds a pointer to a PointBucket which holds the points in an array of LidarPoints. The PointBucket is also responsible for caching and uncaching of data.
    237240PointData.cpp
    238   A struct to hold LAS 1.3+ point attributes which are stored separately.
     241  A struct to hold LAS 1.3+ point attributes which are stored separately, outside the quadtree.
    239242CacheMinder.cpp
    240243  A class to keep track of how many points are currently loaded into memory.
     
    319322  }}}
    320323}}}
    321 
    322 
    323 
    324 [#q22 22. How does rendering work?] \\
    325 
    326 [#q23 23. What are main ideas for further LAG development?] \\
    327 [#q24 24. What are some major issues with LAG that need fixing?] \\
    328 [#q25 25. What are some additional features that can be added to LAG?] \\
    329 [#q26 26. What tools are there to help me with LAG development?] \\
    330 
     324When 1.3 or higher format is recognised //LoadWorker::load_points_wf()// method is used which creates //LidarPoint// objects to be inserted into the quadtree as well as //PointData// objects which hold the remaining attributes. A temporary file is created for each flightline and //PointData// objects are serially written to it. The //point_data_paths// is used to map a flightline number to a name of a temporary file where //PointData// objects are stored and //point_number// vector holds the number of points currently loaded per flightline. This number is also assigned to //LidarPoint::data_index// so the exact location of its data in the temporary file can be retrieved by using //LidarPoint::data_index// x //sizeof(PointData)//. Not that the actual waveform data is not touched during the process.\\
     325When LAS 1.3 points are saved the location of temporary file is retrieved inside {{{SaveWorker::save_points_wf()}}} method from {{{LoadWorker::point_data_paths.find(flightline_number)}}}. Then for each saved point it corresponding {{{PointData}}} is retrieved from the temporary file with like this:
     326{{{
     327#!div style="font-size: 100%"
     328  {{{#!c++ 
     329  char* buffer = new char[sizeof(PointData)];
     330  PointData* temp;
     331  int s = sizeof(PointData);
     332 
     333  fseek(datafile, points[k].getDataindex() * s, SEEK_SET);
     334  fread(buffer, s, 1, datafile);
     335  temp = (PointData*) buffer;
     336  }}}
     337}}}
     338Because laslib library doesn't support storing waveform data in the same file as point records it automatically sets {{{start_of_waveform_data_packet_record}}} field in the header to 0. We need to manually change it to the correct value:
     339{{{
     340#!div style="font-size: 100%"
     341  {{{#!c++
     342  fseek(fileout, 227, SEEK_SET);
     343  fwrite(&reader->header.start_of_waveform_data_packet_record, sizeof(I64), 1, fileout);
     344  }}}
     345}}}
     346Finally we copy over the waveform data manually  and attach it to the end of the output file. Note that this requires for LAS 1.3 files to be saved to a separate file after modification. Unfortunately there is no way around this with how laslib library currently works.
     347
     348=== 22. How does rendering work?] [=#q22] ===
     349
     350It's difficult to tell. It's quite a mess and it definitely needs some refactoring for maintainability. It does work however so I haven't changed it, focusing on other things. \\ \\
     351
     352Once the files have been loaded, a pointer to the quadtree is passed to {{{LagDisplay}}}, {{{TwoDeeOverview}}} and {{{Profile}}}. First {{{LagDisplay::prepare_image()}}} method is called which sets up OpenGl and gets all the buckets from the quadtree with {{{advSubset()}}} method. Based on the ranges of z and intensity values it then sets up arrays for coloring and brightness of the points in {{{LagDisplay::coloursandshades()}}}. Then {{{resetview()}}} method is called which sets up orthographic projection and conversion of world coordinates to screen coordinates. \\ \\
     353After that a {{{drawviewable()}}} method is called in {{{TwoDeeOverview}}} and/or {{{Profile}}} which checks what part of the data is currently being viewed on the screen and gets the relevant buckets from the quadtree. It also uses a good number of boolean variables which are changed all over the place to make things harder to understand.If you were wondering how NOT to do threading in C++, here's a great example. Eventually we create a new thread and call {{{TwoDeeOverview::mainimage()}}} method passing it the buckets we've got. \\ \\
     354
     355In {{{mainimage()}}} we call {{{drawpointsfrombuckets()}}} which prepares the points for being drawn. For each bucket from the quadtree it determines the resolution level needed (sub-bucket) based on the current zoom and viewing area. Then for each point in that bucket it creates vertices with attributes of the point to be drawn (location, color, brightness) and passes them to {{{DrawGLToCard()}}} method in the main thread which draws the contents of the vertices to the framebuffer. Finally {{{FlushGLToScreen()}}} method draws the contens of framebuffer to the screen. \\ \\
     356
     357Simple as that.
     358
     359=== 23. What are main ideas for further LAG development? [=#q23] ===
     360
     361There are few categories of things that need to be done:
     362  * things that have been started but haven't been quite finished
     363  * issues that need fixing
     364  * additional features
     365
     366To add to that there's still plenty of refactoring to be done (possibly a good thing to start with to understand the code better). Particularly the rendering, which works, but is kind of scattered between different classes and methods and its threading which is just bad. Ideally there should be a separate Renderer/RenderingWorker class with all the code in one place.
     367
     368=== 24. What is the unfinished development stuff? [=#q24] ===
     369
     370That's mainly sorting out the threading. While LoadWorker and SaveWorker have been fully implemented, the ProfileWorker and ClassifyWorker need some improvement (so the job is actually done inside these classes). To add to that a progress indication for classifying points and loading a profile could be added. \\ \\
     371
     372Add more things here.
     373
     374=== 25. What are some major issues with LAG that need fixing? [=#q25] ===
     375
     376Luckily there's not so many any more, however the major one is the memory consumption. This is quite a difficult problem, but it has to be looked at before lag can be released to the public. \\
     377Another one is handling of the LAS 1.3 files. \\
     378Also there is an issue with rendering/loading thread where forcing ON_EXPOSE event while loading a file at the same time may sometimes cause a segfault. For example moving some window on top of the TwoDeeOverview window while file is being loaded may cause this. This is most likely cause by both loading and rendering thread trying to access the quadtree at the same time and I believe sorting out the rendering thread should solve this problem.
     379
     380=== 26. What is lag memory issue? [=#q26] ===
     381
     382It's an unknown source of high memory consumption of LAG. It looks like either a memory leak, memory fragmentation or both. The reason to suspect memory fragmentation is that the memory usage keeps growing to some point then stays constant. It's easy to see when loading the same file multiple times. Also when using a heap profiler it will show correct and constant memory usage and will not grow as high, possibly because of custom memory pool from the profiler.
     383
     384Probably the best way to fix it would be to create a [http://en.wikipedia.org/wiki/Allocator_(C%2B%2B)#Custom_allocators custom memory allocator] which is an interesting project on its own.
     385
     386=== 27. What are some additional features that can be added to LAG? [=#q27] ===
     387
     388Here are some ideas:
     389  * A 3D overview. Not so useful for processing, but pretty cool for visualisation and possibly attractive for other users outside ARSF (once LAG is released to the public).
     390  * Additional Loading/Saving features. For example an option to use shape files to filter out points, ability to import/export different file formats, support for more projections etc. Also improved support for waveform data.
     391  * Plugins or scripting support. Probably scripting would be easier to implement, for example using boost::python. The idea is to provide an interface for lag/quadtree to a scripting language like python so even users that don't know C++ could add their modifications. For example classify_las could then be made a script or tool inside lag.
     392  * Further GUI improvements. Eg. progress bars for loading a profile and classifying points, tool tips and other user-friendly stuff.
     393  * Waveform visualisation.
     394  * Multi-platform support.
     395
     396=== 28. What tools are there to help me with LAG development? [=#q28] ===
     397
     398I'd recommend the following: \\
     399[http://sources.redhat.com/gdb/ Gdb] - best debugging tool on Linux.\\
     400[http://valgrind.org/ Valgrind] - an excellent set of tools for profiling, optimisation, memory leak detection etc. \\
     401[http://code.google.com/p/gperftools/ gperftools] - a set of performance analysis tools from Google. \\
     402
     403=== 29. How do I use profiling tools to optimise code? [=q29] ===
     404
     405Profiling is better then your intuition when it comes to optimising code and you should do it often. It lets you easily identify bottlenecks and see how your changes actually affect the performance. It also lets you pinpoint functions that cause high memory consumption and find possible memory leaks. \\ \\
     406To be able to use all features of profiling tools you'll have to compile your code with {{{-g}}} and {{{-pg}}} flags. In case of lag and quadtree in means adding them to Makefile.am AM_CXXFLAGS. They should be removed before rolling out a new release though as they negatively affect both performance and size of the executables. \\ \\
     407To get an idea about call times and frequencies of the functions you can use valgrind tool called callgrind. You run it like this:
     408{{{#!sh
     409valgrind --tool=callgrind lag
     410}}}
     411The lag will start as usual however it will run extremely slow. Run some operations (possibly such that you can repeat with no variations, like loading a single file) and close the program. The callgrind will have generated a call-graph file called callgrind.out.xxxxx which you can open in {{{kcachegrind}}} for visualisation.
     412