[pdal] Migrating from libLAS

[Andrew Bell]

On Tue, Sep 1, 2015 at 9:53 PM, Vaclav Petras <wenzeslaus at gmail.com> wrote:
> Hi all,
>
> I'm porting GRASS GIS's r.in.lidar module [1] (which is using libLAS) to
> PDAL and I think I can write a migration guide. Do you think it should go to
> the docs? If yes, where? You can view the current state in my fork [2].
> Unfortunately, it is transitioning from libLAS's C API to PDAL C++ API since
> r.in.lidar is using the C API.
>
> I have several questions/TODOs in file [2] (feel free to review them).
> However, now I would like to ask about iterating over the points in the
> dataset. I do it in the way that I call execute on the reader and then I
> iterate over points in PointView.  It seems to me that I'm going over the
> dataset two times, one reading and then my writing instead of just reading
> and writing at the same time as in case of libLAS API.

The short answer is yes, this is what you're doing.  There is a
callback in some readers that handle point-at-a-time semantics, but
it's much easier to do what you're doing.

>  It also seems that all points are loaded in the memory, should I then check some limitations or
> add some error handling?

We only handle 2^32 - 1 points at this time.  We assume that your
system will throw an error if you run out of memory, though unless you
have a small system with limited virtual memory and a very large
dataset, it seems unlikely.

Note that PDAL's design is different than libLAS because it's not just
a LAS reader.  You can use the callback mechanism (setReadCb) along
with a custom PointTable (derived from BasePointTable) to do your own
thing if that's what you want.  We built this to allow people to read
the data without actually storing it in PDAL-allocated memory.

On your transition guide:

Point validity: there is no such concept

hasColor: determines whether the LAS dataformat_id supports RGB
hasDim(Id::Red): determines whether the dimension Red was registered
with PDAL.  If all you're doing is reading LAS, hasColor() should be
true when hasDim(Id::Red) is true.

Memory management: PDAL generally takes care of itself.  In your
example you allocate a reader on the heap, but it can be stuck on the
stack just as well.  There are no hard requirements on this.

-- 
Andrew Bell
andrew.bell.ia at gmail.com