[QGIS-Developer] How to speed up rendering

[Johannes Kröger (WhereGroup)]

Hi Arturo,

make sure you create overviews on your big rasters or QGIS will re-read 
the whole file all the time to render an appropriate representation in 
the map.

Cheers, Hannes

Am 30.05.23 um 04:35 schrieb afernandez via QGIS-Developer:
> Hello Martin,
> I'm trying to understand what you're saying but my background is in 
> numerical methods where tasks are split into threads for acceleration 
> purposes. If a single layer is rendered by a single core (as you write 
> and I was observing), how would a 1,000 x 1,000 (or a 5,000 x 5,000) 
> layer be handled? Wouldn't it take too long? (However, you want to 
> define 'too long').
> As far as more details about the process, the tensor 'var' (n x m x 
> index where I have tried different combinations but let's say n x m is 
> 100 x 200) of floating point numbers is manipulated according to the 
> following pseudo-code:
> # Initial manipulations
>             dims = var.dimensions
>             shape = var.shape
>             driver_name = 'GTIFF'
>             driver = gdal.GetDriverByName(driver_name)
>             np_dtype = var.dtype
>             type_code = gdal_array.NumericTypeCodeToGDALTypeCode(np_dtype)
>             gdal_ds = driver.Create(_my_path_, cols, rows, 1, 
> gdal.GDT_UInt16)
> gdal_ds.SetProjection(_my_projection_)
> gdal_ds.SetGeoTransform(_my_transformation_)
> # Creation of the bands and scaled matrix
>             band = gdal_ds.GetRasterBand(1)
>             data = var[_chosen_index_]
>             data = ma.getdata(data)
>             data_scaled =  np.interp(data, (data.min(), data.max()), 
> (0, 255))
>             data_scaled2 = data_scaled.astype(int) # This is to 
> rescale into integers so that it can color the layer
> # *** Lines to set up the color palette ***
> # Write the array to band once everything has been rescaled
>             band.WriteArray(data_scaled2)
>             gdal_ds.FlushCache()
>
> This procedure seems to work well and takes a reasonable amount of 
> time. Then, the time is consumed when doing the rendering itself. I'm 
> not entirely sure what you mean by 'sub-optimal raster format' or 
> 'thre are overviews missing' so I cannot address them specifically.
> Thanks,
> Arturo
>
>
>
>
> Martin Dobias wrote:
>
>
>> Hi
>>
>> On Fri, May 26, 2023 at 11:30 PM afernandez via QGIS-Developer 
>> <qgis-developer at lists.osgeo.org> wrote:
>>
>>     Hello,
>>     One of my project would greatly benefit from a much faster
>>     rendering. After some testing, the bulk (>99%) of the time is
>>     spent at rendering the layer with QgsRasterLayer(...,...). What I
>>     have also noticed is that rendering time does not change with the
>>     number of cores, which suggests that the renderer might be using
>>     a single thread. I checked the API documentation but couldn't
>>     find anything about multithreading or some other procedure to
>>     accelerate rendering. I was wondering if anyone has any
>>     suggestion or has faced a similar difficulty.
>>
>>
>> Multi-threaded rendering is done at the granularity of whole map 
>> layers, so a single layer is rendered by a single CPU core. Therefore 
>> if you have just one layer that is slow to render, the 
>> multi-threading is not going to help. It is best to do some testing 
>> how you can improve the layer's rendering - for example, it could be 
>> that you are using some sub-optimal raster format, or there are 
>> overviews missing, or something else... one would need more details.
>>
>> Regards
>> Martin
>>
>
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-- 
Johannes Kröger / GIS-Entwickler/-Berater

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