[gmx-developers] Help needed with tng_io library

Tue Aug 12 10:17:12 CEST 2014

Hi Semen,

There are currently no bullet-proof way of counting the number of frames 
containing a specific type of data, but I'm working on adding that now.
To get the positions I would say that the easiest way is to call 
tng_util_particle_data_next_frame_read() in a loop. That way you will 
read until you find the next frame containing data of the requested type 
(e.g. positions). When there is no more data you should get a TNG_FAILURE.

The bug #2 is the same as I mentioned as being fixed below, i.e. the 
same bug that made VMD read forever. That should be fixed in the TNG 
repository, otherwise I'll have to check what's wrong. I haven't checked 
what's going on with problem #3 yet. I'll see when I have time to look 
into that. Hopefully you can at least get further now. Otherwise just 
ask me for more assistance and I can hopefully point you in the right 
direction.

Cheers,

Magnus

On 08/08/2014 11:43 AM, Semen Esilevsky wrote:
> Dear Magnus,
> Thank you very much for explanations!
>
> So,  is there any boolet-proof  way of finding out how many position sets are in the trajectory?
>
>
> Another question is which function should I preferebly use to get the positions - tng_util_particle_data_next_frame_read() or tng_util_pos_read_range()? The later gives me an array of floats, which I need, but I suspect that it is much slower because of seeking for frame. Is it so?
>
> I'm looking forward to see my stupid mistake, which leads to problems #2 and #3.
>
> Regards,
> Semen
>
>
>
>
>
> On Friday, August 8, 2014 12:07 PM, Magnus Lundborg <magnus.lundborg at scilifelab.se> wrote:
>
>
>>
>> Hi Semen,
>>
>> I found some extra time, so I will try to clarify some things below.
>>
>> On 2014-08-07 12:16, Semen Esilevsky wrote:
>>> Dear All,
>>> I'm currently trying to add support for tng files to my code and I realised that I can't understand the logic of the library at all. I'm definetely doing something wrong, but can't figure out a mistake.
>>> The tng code is got from the latest git. Test trajectory is generated by Gromacs 5.0.
>>>
>>>
>>> I can open the tng file and can extract, for example, position exponential correctly, but after that...
>>>
>>>
>>> Problem #1: getting number of frames.
>>>
>>> int64_t n1,n2;
>>> tng_num_frames_get(trj, &n1);
>>> tng_num_frame_sets_get(trj, &n2);
>>>
>>> This gets
>>> n1 = 10761
>>> n2 = 1086
>>>
>>> The correct number of frames is 1086.
>>>
>>> What on Earth is returned from tng_num_frames_get()?
>> One thing that is slightly difficult with TNG is that different kinds of
>> data can be written at different intervals, e.g. it can have positions
>> written every 100th frame, forces every 200th frame, velocities every
>> 250th frame etc. To make this a bit more graspable in GROMACS every md
>> step is considered a frame when writing the TNG file (it would be
>> possible to let the n steps between frames be the greatest common
>> denominator of the written data types as well). The default when writing
>> TNG from a GROMACS simulation is to keep 100 frames of the most
>> frequently written data in each frame set. When generating a TNG from
>> trjconv (like the file you provided) it is not as easy to predict what
>> data to expect and the output frequency of it. Therefore a frame set is
>> created every time data is written - this will probably be changed in
>> the future to facilitate more efficient compression from trjconv too.
>>> Problem #2: Reading past the end of file.
>>>
>>> stat = tng_util_particle_data_next_frame_read(trj, TNG_TRAJ_POSITIONS, &values,
>>>                                                      &datatype, &frame, &physical_time);
>>>
>>> This function should return false when no more frames are available, right? It doesn't do this and reads forever returning garbage in values when called in a loop. stat is always true. How should I use it correctly to finish reading at the end of file?
>>> Also concerning the strange thing with the number of frames and frame sets - what it reads actually each time?
>>>
>>> Problem #3: tng_util_pos_read_range() behaves insane
>>>
>>> int n=0;
>>> bool stat = true;
>>> while(stat){
>>>        stat = tng_util_pos_read_range(trj,n,n, &ptr, &len);
>>>        n++;
>>> }
>>>
>>> This always stops at n=10 (why?!), while the number of frames is 1086! What I'm doing wrong here?
>> I will have a look at this, and will try to give more details next week
>> when I've got more time.
>>> I've also tried VMD plugin from pre-release VMD 1.9.2. It is said that this plugin is contributed by Gromacs developers, so it should work in principle. It doesn't work because of the bug #2 - it reads forever beyond the end of file.
>> This bug was discovered a few days after the release and is fixed in the
>> TNG library in the TNG repository. The fix is included in the
>> release-5-0 branch of GROMACS, but I don't think there has been any bug
>> fix release. I also think it will be fixed in the next VMD pre-release.
>>> The test tng file is here: https://drive.google.com/file/d/0Bx_ng_72VH8BQlFsTGJOVC1oMjQ/edit?usp=sharing
>>>
>>>
>>> Any help is appreciated!
>> I hope the answers help you at least a bit. Please tell me if anything
>> is unclear and I will try to explain it better.
>>
>> Kind Regards,
>>
>> Magnus
>>
>>> Sincerely,
>>> Semen
>>>
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