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Why does multi-layer map source atlases become huge?

For discussions, questions and problems using MOBAC
SnowFlake
Posts: 2
Joined: 06 Feb 2017, 22:05

Why does multi-layer map source atlases become huge?

Postby SnowFlake » 06 Feb 2017, 22:21

When an atlas is created from a custom multi-layer map source consisting of two maps, I expect the size of the result to be less than the sum of the two individual maps. But the size of the resulting atlas is 10 times that, what could be wrong?

I use MOBAC 2.0.0 beta 3 and are creating MBTiles SQLite atlases.

r_x
Site Admin
Posts: 670
Joined: 25 Jun 2011, 14:00

Re: Why does multi-layer map source atlases become huge?

Postby r_x » 06 Feb 2017, 23:19

Sounds like you habe specified the multi-layer map source to use the PNG file format, but at least one source map source uses JPG. Converting JPG to PNG results in a massive sie increase as both Kompression techniques work totally different.
Please read the MOBAC FAQ and MOBAC README before posting.

SnowFlake
Posts: 2
Joined: 06 Feb 2017, 22:05

Re: Why does multi-layer map source atlases become huge?

Postby SnowFlake » 07 Feb 2017, 22:40

r_x wrote:Sounds like you habe specified the multi-layer map source to use the PNG file format, but at least one source map source uses JPG. Converting JPG to PNG results in a massive sie increase as both Kompression techniques work totally different.

Spot on! I thought both were PNG but one of them is actually JPG. Changing the format of the JPG source and the format of the multi-layer source to JPG drastically reduced the size. The size is however still larger than the total size of the two sources separately. Does anybody know if there is a efficent way to combine a PNG source on top of a JPG source so that the size of the result is approximately the size of the JPG source?

I am trying to create a hybrid map with a layer (roads, city names, etc.) from Eniro on top of Google satellite.

r_x
Site Admin
Posts: 670
Joined: 25 Jun 2011, 14:00

Re: Why does multi-layer map source atlases become huge?

Postby r_x » 08 Feb 2017, 09:33

The JPEG format has different compression level. I assume that by default Java and MOBAC use the compression level 100 (best quality, lowest compression). On a map source level you can not set the compression level, however you can change the compression level when you create an atlas.

Use the "Layer settings: custom tile processing" panel and enable the "Recreate/adjust map tiles". The note regarding that this is CPU intensive does not apply in your case as the images are already processes internally (merging of the different layers). Now you can set the "Tile format" to JPEG and your desired compression level.

Afterwards add the map regions as usually via the "add selection" button. In the tooltip of every layer in the atlas content tree you can see the tile format that you have specified.
Please read the MOBAC FAQ and MOBAC README before posting.


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