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<p>Hi Arturo,</p>
<p>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.</p>
<p>Cheers, Hannes<br>
</p>
<div class="moz-cite-prefix">Am 30.05.23 um 04:35 schrieb afernandez
via QGIS-Developer:<br>
</div>
<blockquote type="cite"
cite="mid:e4867467c3728.ad83e8afff7c4@odyhpc.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<meta charset="utf-8">
<div id="compose-body-wrapper" dir="auto">
<div dir="auto">
<div dir="auto">Hello Martin,</div>
<div dir="auto">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'). </div>
<div dir="auto">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:</div>
<div dir="auto"># Initial manipulations</div>
<div dir="auto"> dims = var.dimensions</div>
<div dir="auto"> shape = var.shape</div>
<div dir="auto"> driver_name = 'GTIFF'</div>
<div dir="auto"> driver =
gdal.GetDriverByName(driver_name) </div>
<div dir="auto"> np_dtype = var.dtype</div>
<div dir="auto"> type_code =
gdal_array.NumericTypeCodeToGDALTypeCode(np_dtype)</div>
<div dir="auto"> gdal_ds =
driver.Create(_my_path_, cols, rows, 1, gdal.GDT_UInt16) </div>
<div dir="auto">
gdal_ds.SetProjection(_my_projection_)</div>
<div dir="auto">
gdal_ds.SetGeoTransform(_my_transformation_)</div>
<div dir="auto"># Creation of the bands and scaled matrix </div>
<div dir="auto"> band = gdal_ds.GetRasterBand(1) </div>
<div dir="auto"> data = var[_chosen_index_]</div>
<div dir="auto"> data = ma.getdata(data)</div>
<div dir="auto"> data_scaled = np.interp(data,
(data.min(), data.max()), (0, 255))</div>
<div dir="auto"> data_scaled2 =
data_scaled.astype(int) # This is to rescale into integers
so that it can color the layer</div>
<div dir="auto"># *** Lines to set up the color palette ***</div>
<div dir="auto"># Write the array to band once everything
has been rescaled</div>
<div dir="auto"> band.WriteArray(data_scaled2)</div>
<div dir="auto"> gdal_ds.FlushCache() </div>
<div dir="auto"><br>
</div>
<div dir="auto">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.</div>
<div dir="auto">Thanks,</div>
<div dir="auto">Arturo</div>
<div><br>
</div>
</div>
<div dir="auto"><br>
</div>
<div dir="auto" id="tmjah_g_1299"><br>
</div>
<br>
</div>
<div class="replyHeader" dir="auto">Martin Dobias wrote:</div>
<br>
<br>
<div>
<blockquote
cite="mid:CAC2XbFdWLu-0VKRWYK-YF0XSN5FSoFHfX8Ztm6R6HTfcPa4MNw@mail.gmail.com"
type="cite" style="margin:0 0 0 .8ex;border-left:1px #ccc
solid;padding-left:1ex">
<div dir="ltr">
<div>Hi<br>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Fri, May 26, 2023
at 11:30 PM afernandez via QGIS-Developer <<a
href="mailto:qgis-developer@lists.osgeo.org"
moz-do-not-send="true" class="moz-txt-link-freetext">qgis-developer@lists.osgeo.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px
0px 0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
<div>
<div>
<div
id="m_-6129067434502316114compose-body-wrapper"
dir="auto">
<div dir="auto">
<div dir="auto">Hello,</div>
<div dir="auto">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. </div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">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.</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">Regards</div>
<div class="gmail_quote">Martin</div>
<div class="gmail_quote"><br>
</div>
</div>
</blockquote>
</div>
</div>
<br>
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