Compression time and file size

[Martin Over]

Hi,
I am writing a community module for the geoserver.
Unfortunately the advantages of the smaller download size are cancelled out by a longer processing time.
Other map servers also have this problem: https://help.supermap.com/iDesktop/en/tutorial/MapTiles/WEBPTileType/

The larger the images, the greater this effect.
Why is this so?

best regards,

Martin

[Maryla Ustarroz]

Hi,

That's a very good question actually! The short answer is that WebP achieves better compression rates than jpeg/png by being more complex, and the cost of this complexity is that it's slower, particularly at encoding.

The way WebP and other modern image formats work, is that they have many ways of representing the pixels of an image. One way might work well on one area of the image, while another way might work better in another area. To find the best way of encoding the image, the encoding software basically has to try several methods until it finds the best one. This is what takes time. Therefore, it's not usually advisable to convert images to WebP on the fly. WebP files should be generated in advance.

You can actually tweak the encoding time, by telling the encoder to try more or less combinations. Of course, if you tell it to try fewer options, it will be faster but you won't have as much compression gains.

Using cwebp, use the -m flag if using lossy compression (0=fast to 6=slowest) or -z flag for lossless (0=fast to 9=slowest])

Using the C api, you can change the "method" parameter on WebPConfig, or call WebPConfigLosslessPreset() for lossless.

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[James Zern]

On Thu, Feb 16, 2023 at 9:51 AM 'Maryla Ustarroz' via WebP Discussion <webp-d...@webmproject.org> wrote:

Hi,

That's a very good question actually! The short answer is that WebP achieves better compression rates than jpeg/png by being more complex, and the cost of this complexity is that it's slower, particularly at encoding.

The way WebP and other modern image formats work, is that they have many ways of representing the pixels of an image. One way might work well on one area of the image, while another way might work better in another area. To find the best way of encoding the image, the encoding software basically has to try several methods until it finds the best one. This is what takes time. Therefore, it's not usually advisable to convert images to WebP on the fly. WebP files should be generated in advance.

There are more details about some of the compression techniques here if you're interested:

https://developers.google.com/speed/webp/docs/compression

https://web.dev/learn/images/webp/

 

You can actually tweak the encoding time, by telling the encoder to try more or less combinations. Of course, if you tell it to try fewer options, it will be faster but you won't have as much compression gains.

Using cwebp, use the -m flag if using lossy compression (0=fast to 6=slowest) or -z flag for lossless (0=fast to 9=slowest])

Using the C api, you can change the "method" parameter on WebPConfig, or call WebPConfigLosslessPreset() for lossless.

On Thu, Feb 16, 2023 at 5:38 PM Martin Over <overma...@gmail.com> wrote:

Hi,
I am writing a community module for the geoserver.
Unfortunately the advantages of the smaller download size are cancelled out by a longer processing time.
Other map servers also have this problem: https://help.supermap.com/iDesktop/en/tutorial/MapTiles/WEBPTileType/

The larger the images, the greater this effect.
Why is this so?

best regards,

Martin

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[Martin Over]

Thanks a lot for the quick answers!