[gdal-dev] RE: progressive rendering

[Adam Nowacki]

Norman Barker wrote:
> I have created RFC 24 on 
> 
> http://trac.osgeo.org/gdal/wiki/rfc24_progressive_data_support

I'd suggest creating completely new interface for this.
Why is this better (imo):
- application decides when and how ofter the updates occur, minimal 
threading issues
- clearly defined synchronization points: NextAsyncRasterIOMessage(), 
LockBuffer(), UnlockBuffer()
- requests can be aborted with EndAsyncRasterIO();


class GDALAsyncRasterIOMessage {
   GDALAsyncRasterIO *asyncrasterio;

   void *userptr;

  // GARM_UPDATE, GARM_COMPLETE, GARM_ERROR, ...
   int what;

   int xoff, yoff;
   int xsize, ysize;
   // ...
};


class GDALAsyncRasterIO {
// lock a whole buffer
   void LockBuffer();

// lock only a block
   void LockBuffer(int xbufoff, int ybufoff, int xbufsize, int ybufsize);

   void UnlockBuffer();
};


class GDALAsyncDataset : public GDALDataset {
   GDALAsyncRasterIO *AsyncRasterIO( /* same as RasterIO */ , void 
*userptr);

   void EndAsyncRasterIO(GDALAsyncRasterIO *);

// if there are no new messages return NULL if wait is false or wait for 
new message if wait is true
   GDALAsyncRasterIOMessage *NextAsyncRasterIOMessage(bool wait);

   void ReleaseAsyncRasterIOMessage(GDALAsyncRasterIOMessage *m);
};




// ###############
//  How to use it
// ###############

GDALDataset *ds = GDALOpen( /* ... */, GA_ReadOnly);

// start asynchronous raster io, can have multiple running at same time
GDALAsyncRasterIO *r = ds->AsyncRasterIO(GF_Read, xoff, yoff, xsize, 
ysize, bufptr, bufxsize, bufysize, bufdatatype, 3, NULL, 0, 0, 0, userptr);

while (...) {
   GDALAsyncRasterIOMessage *m = ds->NextAsyncRasterIOMessage(true);

   if (m) {
     if (m->what == GARM_UPDATE) {
// lock the buffer so there will be no updates while we read from it
       m->asyncrasterio->LockBuffer( /* ... */ );

// display updated region

       m->asyncrasterio->UnlockBuffer();
     } else {
// handle completion, display error message, ...

     }

     ds->ReleaseAsyncRasterIOMessage(m);
   }

}

ds->EndAsyncRasterIO(r);