iipmooviewer2-client rounding errors

  • njfranck

    njfranck - 2012-01-05

    Hi there,

    I tried the new iipmooviewer2 beta as found on http://merovingio.c2rmf.cnrs.f

    The method "calculateSizes" contains a rouding error, that only emerges when
    dealing with small tiles:

    w: 2561, h:2563, tile-width:256,tile-height:256

    in the method "calculateSizes" this is written:

    tx = Math.floor(tx/2);ty = Math.floor(ty/2);

    but it should be this:

    tx = Math.ceil(tx/2); ty = Math.ceil(ty/2);

    that way at 2561/2 does not become 1280 (and therefore thinking of 5 tiles in
    but 1281 (so actually 6 tiles), because a tile of width 1px needs to be there

  • Ruven

    Ruven - 2012-01-05

    Are you getting wrong calculations with your images? The calculation in
    calculateSizes() is to determine the sizes of each resolution, which it seems
    is always floor(tx/2). However, the calculation of the number of tiles in fact
    occurs in the loadGrid() function where there is indeed a ceil():

    var xtiles = Math.ceil( this.wid / this.tileSize.h );
    var ytiles = Math.ceil( this.hei / this.tileSize.h );

    Do you have an example where this does not work correctly?

  • njfranck

    njfranck - 2012-01-06

    consider this image:

    width: 2561
    height: 2563

    this is wat calculateSizes does for one resolution:
    2561/2 = 1280.5;

    this resolution should have 6 tiles in width: 1280.5 / 256 = 5.001953125
    yes it's 5.001953125, but that means that it contains a very small tile in
    but that is crucial for the correct working of the client. Ceiling the value
    to 6 corrects this values

    but 1280 is stored due to floor

    zoomIn then calculates this.wid from this resolution-hash

    // Get the image size for this resolution
    this.wid = this.resolutions[this.view.res].w;
    this.hei = this.resolutions[this.view.res].h;

    loadGrid() then simple uses this.wid to determine the number of tiles, but the
    error already occurred in a previous state.

    Maybe better to avoid rounding the numbers in the hash this.resolutions?

  • njfranck

    njfranck - 2012-01-06

    I mean: 1280/256 just gives you 5 tiles, instead of 6 actually in width

  • Ruven

    Ruven - 2012-01-06

    this resolution should have 6 tiles in width: 1280.5 / 256 = 5.001953125

    Not really, because you cannot have a real resolution with a fraction of a
    pixel. The resolutions in the TIFF or JEG2000 are physically generated at
    sizes that are rounded down.

  • njfranck

    njfranck - 2012-01-06

    Strange, but the example-jpeg2000 only works correctly when I make this
    "corrections". Just try it out ;-)

  • Ruven

    Ruven - 2012-01-06

    You're right. Your image contains the following resolutions:
    2561 x 2543
    1281 x 1272
    641 x 636
    321 x 318
    161 x 159
    81 x 80
    41 x 40
    21 x 20

    so it's in fact rounding up every time. I tried recompressing with Kakadu and
    it gives the same result. With TIFF, however, these resolutions are rounded
    down. Perhaps the solution would be for the server to send a list of available
    resolutions, rather than trying to calculate this?

  • njfranck

    njfranck - 2012-01-09

    I think so ;-). Either way, the server knows more about the file than the
    client ever will. And it makes the client more simple.



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