[gdal-dev] How to decrease file size of colored raster bands?

[Laurențiu Nicola]

Hi,

There are a couple of things you can try:
 • you seem to be scaling the data to 8-bit, but saving it as UInt16; GDT_Byte should work, yielding a 50% reduction. I didn't quite understand the comment about the color, does the palette not work with 8-bit images?
 • you can enable and tune compression: check out COMPRESS, PREDICTOR and the level options in https://gdal.org/drivers/raster/gtiff.html. ZSTD and DEFLATE are pretty good.
 • you can enable the tiled mode (TILED on the same page as above), which sometimes yields smaller files
 • if you find large files difficult to use, you can enable overview creation or use the COG driver (it's a subset of GTiff)
 • finally, you can use a lossy compression format like JPEG or LERC, but it's probably not what you want
Regards,
Laurentiu

On Thu, Jun 1, 2023, at 17:35, afernandez wrote:
> Hello,
> I'm generating a raster file with GDAL. The pseudo-code (where the raster is loaded as 'var') for the colored version reads:
> # 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 works well but the problem is that the generated file becomes too large and difficult to work with. If I change to a black and white representation by simply changing the 7th line to: 
>             gdal_ds = driver.Create(out_path, cols, rows, 1, type_code) 
> The new file has a size smaller than 1% of the colored one. I was wondering if there is anything intermediate (with colors but generating a smaller size) or if I would need a more radical approach such as using a driver different from GTIFF or something else.
> Thanks.
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