Comments on: Raster map projection with ActionScript 3

By: Andy Woodruff

Andy Woodruff — Wed, 25 Feb 2009 14:00:05 +0000

I don’t know too much about that, I’m afraid. I imagine it’s some equation, not unlike these map projections, but I don’t have experience with that sort of thing in particular.

By: Nico

Nico — Mon, 23 Feb 2009 17:35:10 +0000

Hi,

I’m looking for a source code or a tutorial (as3) in order to transform a cylindrical to a rectilinear (or inverse). if you go to http://www.mapjack.com, you will see 360° panoramic, in the tools (left bottom), ther’s “projection”, that transforms cylindrical to rectilinear…that’s what I want…Do you know how ??

Many Thanks
PS: sorry for my bad english

By: Bookmarks about Actionscript

Bookmarks about Actionscript — Tue, 23 Dec 2008 17:15:27 +0000

[...] – bookmarked by 4 members originally found by sthibault on 2008-11-19 Raster map projection with ActionScript 3 http://www.cartogrammar.com/blog/raster-map-projection-with-actionscript-3/ – bookmarked by 3 [...]

By: Panta

Panta — Sun, 07 Sep 2008 07:38:23 +0000

Great job.

By: Michal Migurski

Michal Migurski — Thu, 03 Jul 2008 17:06:15 +0000

Yeah, that’s true – it may even be an option in GDAL to use the nearest pixel (fast, ugly) vs. an interpolated one (slow, pretty). Actually I think that’s what the drop-down in photoshop that says “nearest neighbor / bilinear / bicubic” has been telling me all these years.

By: Tom Carden

Tom Carden — Thu, 03 Jul 2008 16:33:34 +0000

Weighted average would be the thing!

If you’re sampling from a non-integer coordinate, you can use bilinear interpolation to derive a value from the four neighbouring pixels. Bilinear interpolation is pretty fast, and seems appropriate for Actionscript

GDAL and others probably use bicubic or lanczos interpolation for accuracy, also helpfully linked from this wikipedia article: http://en.wikipedia.org/wiki/Bilinear_interpolation

By: Andy Woodruff

Andy Woodruff — Thu, 03 Jul 2008 15:34:39 +0000

Yeah, I did consider doing it that way, but initially put it aside because (a) I was at first just using those line maps, where spreading out a pixel that is really meant as a one-dimensional representation would probably be ugly, and (b) I was doing the Winkel Tripel projection, where apparently the inverse involves crazy math that's probably beyond me. It's worth a shot for the Mercator, though. Here's the result with the simplest method of finding the nearest pixel from the original map: Compared to my first method... Doing the inverse actually seems to look worse in this case, but I suppose the real solution is not to use the nearest pixel on the unprojected map to the given lat/long, but rather to do some sort of weighted average of the pixels (or pixel centers, I guess) it falls between. Or something.

By: Michal Migurski

Michal Migurski — Thu, 03 Jul 2008 07:21:35 +0000

Hi, I’m so glad you’re doing this, even though it may not be the moment for it yet. =)

For your second example, have you considered doing the inverse projection? For every x, y pixel in the output image, working backwards to the source color in the plate carree image? I generated my gnomonic projection faumaxion tiles (http://teczno.com/faumaxion-II/) in this way, I think it might even be what GDAL does under the hood judging by the output.