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A single message from topic:  maps - feedback wanted   in forum:  Templot talk
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Single message by Martin Wynne       Topic: maps - feedback wanted
Posted: 24 Nov 2020 20:31
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Martin Wynne
West Of The Severn, United Kingdom

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I mentioned earlier that it is now possible to load the NLS historic 25"/mile maps as tiled maps. Which is great news -- it means maps of any size can be easily captured at maximum zoom without any restriction caused by your screen size, or need to align multiple captures. It will be in the next program update.

I have since discovered that the highest zoom level available as original tiles is level 18 for the 25" maps. The higher zoom levels available on the NLS web site are actually implemented by upscaling the image in the browser. This means of course that capturing higher levels than 18 doesn't get you any more detail, the same level of detail is simply being zoomed larger.

Most browsers use a resampling image upscaler to provide a smoother result than a simple stretch of each pixel.

Such resampling isn't available in Templot when zooming background shapes on the trackpad, because at the high zoom levels we normally need it would take forever and reduce the screen response to a slow crawl through treacle.

But the result isn't very pretty or comfortable to work over:

1. map tiles as loaded:

It can also be confusing -- see for example how the platform edge has merged with the rail above.

A few updates back I added a function to reduce the contrast of map images when working over them, which makes it a lot easier to see what you are doing -- here on the left:

2. low-contrast converted original:

But it still doesn't match the screenshot captures from smooth browser resampling.

So I have found some old resampling code from Anders Melander which could be used in Templot, as a one-time conversion on the background shapes. It's very slow, several seconds per tile, but only needs to be done once.

The difficulty is that there are 7 different resampling algorithms available, with varying degrees of smoothing and speed, and each one can have an adjustable edge-effect setting. Mostly they are optimised for colour photographs, rather than scans of historic maps, so the default settings are not necessarily the best for our purpose. And then in each case there is the degree of zooming to apply. An utter minefield of different settings which would take me forever to evaluate.

From which I need to select 1 or maybe 2 options to offer in the background shapes. Here are some examples. In each case the original as loaded tile is on the right:

3. "Triangle" interpolation only. This is similar to the browser resampling for zoom level 19:

4. "Triangle" interpolation with low-contrast conversion:

5. "B-Spline" interpolation only:

6. "B-Spline" interpolation with low-contrast conversion:

Answers on a postcard. Which if any of these image effects would you find the most comfortable to work over? In each case the original loaded tile is on the right.

I think my preference is for image 5. above -- notice the improved separation between the platform edge and the rail.

There are of course several other settings you might use at the same time -- Templot can also make a negative image, and there are other colour schemes for the templates.

The use of these NLS maps is getting more popular for track planning, so it would be worthwhile finding the optimum screen display for them. This would be optional of course.

It's a pity the 50" maps aren't available for the whole country, but it's great that we now have almost full coverage for the 25" maps.



A single message from topic:  maps - feedback wanted   in forum:  Templot talk
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