[gdal-dev] Space Disk/Memory of Image

[Laurențiu Nicola]

Hello,

The GDAL API actually has a couple of methods that return the size of a file (not dataset) on disk: VSIStatL <https://gdal.org/doxygen/cpl__vsi_8h.html#ac92fbd5e6fc143b026001b32c4c19ed1>, VSIStatExL <https://gdal.org/doxygen/cpl__vsi_8h.html#ad1a691db1ba2f43add25a5c0d6498da6>. Since these work on files, they're not going to return valid results for multi-file formats. On the flip side, they support e.g. reading from ZIP files, which can come in handy.

If you want to compute the compressed size of an image you have in memory, that's not really possible. The compression factor will depend on the image contents, and the only way to determine it is to actually do it and see how large the result is. You can even compress it without writing to disk, using /vsimem/.

To get the uncompressed size, you're basically correct for simple images. You can multiply by the number of bands, which is 3 for an RGB image (like in your case), or 4 for RGBA. But some formats like NetCDF support multiple images in a single file, or even data with 3 or more dimensions. For those, there's no single width and height to use in a formula.

Regards,
Laurentiu

On Tue, May 9, 2023, at 11:47, Elena Ruiz wrote:
> Hello, I need to know the disk size of an image, I calculate it using the following formula:
>  
> Disk size=( sizex*sizey*bits per pixel)/8 =memory size without compression
>  
> But when the image is compressed on disk it occupies less and the size in memory is much larger, is there some method in the C++ GDAL API, to obtain the size on disk, size on memory, or the compression factor?
> Example:
> SizeX: 60416
> SizeY:38656
> Bits per pixel:24
>  
> Size memory= (60416*38656*24)/8=7.006.322.688 bytes
> Size disk=2.137.644.631 bytes
>  
> The format image is *COG – Cloud Optimized GeoTIFF generator, and has JPEG compression*
>  
> Thanks and regards
> 
> 
> 
> *Elena Ruiz *
> Sofware Development & Technical Support
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