Raster exports sharpness depends weirdly on export size

johannesjohannes Global Mapper UserPosts: 10
edited January 2012 in Raster Data
Hi gm team. GM13: My source is a IGN Spain 1:25k raster tile (8bit tif&tfw, UTM30, ED50, about 5000x4000 pixel). I load a tile, set interpolation to bicubic, reproject to Transverse Mercator WGS84. It looks perfectly sharp on screen. I export to 8bit geotiff image optimized palette using current screen pixel size. When I export just a very tiny "all data visible on screen", the result is perfectly sharp and fine and virtually indistinguishable from what I see on screen. So far so good. However, when I export a bigger "all loaded data" (all other settings equal!), the resulting image is unsharp and fuzzy. I tried plenty of different formats, 8bit/24bit, geotiff/ecw/etc, the results are always the same: All is fine when I export "all data visible on screen". But as soon as I go for bigger exports (either draw a box or all loaded data), things get unsharp and fuzzy. I dont really understand whats going on here. How does the dimension of the exported data set influence an exports image quality? PS: The forum kills all line breaks?!

Comments

  • johannesjohannes Global Mapper User Posts: 10
    edited January 2012
    Correction: I checked the sharpness by loading the result into GM on top of the original tile and turning it on and off in control center. It seems the fuzzyness is only in GMs display routines, not in the exports itself: When I load the small export, it looks sharp in GM. When I load the large export, it looks unsharp in GM. However, when I load the exports into Photoshop, they are both equally sharp.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited January 2012
    If you go to the Options dialog for the layers in Global Mapper what do you have the resampling set to? They will come into Global Mapper with nearest neighbor (i.e. no resampling) by default, but perhaps if you set it back to bicubic they will have the sharpness you want?

    Thanks,

    Mike
  • johannesjohannes Global Mapper User Posts: 10
    edited January 2012
    I always set interpolation to bicubic for all layers. Best results for my kind of data. Anyway, the raster data of the exports is actually fine. I was just confused by how GM displays them when I re-import them and compare them with original data. Apparently, the size of a layer makes a difference as to how it's displayed, even though the actual pixel data is more or less equal. Load a 500x500 pixel export on top original data, zoom to actual pixel size, crisp and clear. Load a 10000x10000 pixel export on top of original data, zoom to actual pixel size, the same region looks fuzzy. But if you compare the very same region of the two exports in photoshop, they're both crisp and clear and almost identical.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited January 2012
    Is the pixel resolution (i.e. the ground size of each pixel) the same between the two? When resampling kicks in should depend on the size of the pixels compared to a screen pixel. Which map projection are you using and how large of an area of the earth does your data cover?

    Thanks,

    Mike
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