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resample and resolution

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I am interested how resample works. If I have imagery dimension of 4096x4096 is it possible to change resolution for the target images which are compressed in bgl?

Now a speculation. If there would not be possible to change resolution, then I would need to size down the image. And in this case I have question. If I would need to size down the dimensions of image, what the resample does with the image in real? I could size down to 2048 x 2048. If the original image is 2048 x 2048 used for LOD15, so what will be used for LOD14? Will the resample divide the dimensions by 2? Having 1024 x 1024? Or is there some limit in the program which defines minimum size for LOD14? That would result that the image would nor be changed for LOD14, 13, 12 ... etc. So the effect from saving disk space would be small because only image sized would be that in LOD15.
 
You can undersample or over-sample any image to whatever degree you choose.

If you read the inf file, there is a line that specifies the LOD level:

LOD = Auto

You can alter this to

LOD = Auto , 15
or
LOD = Auto , 14

This will force the upper LOD to the specified level, regardless of source image spatial resolution.

Note, it is metres/pixel rather total image resolution that determines LOD.
 
I need to know how the resample changes the images. I want to calculate how much should I change/lower dimensions of the original image to get optimal file size. I have two sources. One is LOD15 files have in average 8-10MB per one file, the second source has 16MB per file. But both sources was proceeded with LOD15. 80% quality, having 4096x4096 source dimensions! How strange! The filesize is so different. So how is it possible?
 
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If you use any compression ratio other than 100%, the resampled images are compressed and, depending on content, it will result in vastly different filesizes.

Excluded Data (null values) reduce the size, as do "same colour" pixels.

Exactly which algorithm is used for compression ... I don't believe I have seen specified. It appears to be "lossy" .. similar to jpg.

With so many variables, I doubt there is an accurate way to calculate what any given source image will compress down to, other than compiling it for a guide.

An analogy would be to save a 4096 x 4096 picture of yourself as a jpg, then make a 50% black image the same size and save that as a jpg. Are the two images the same size in MB?
 
Yet I don't need use masks. Same scenery, same regional area, same compress ratio. But only difference are source data. Because JPEG source data for LOD15 are not so good in quality, but source JPEG data for LOD16 are very good quality. All JPEG images are then joined together in one BMP image resulting in 4096x4096. So I think the source quality is the only one factor I know. There are not exclusion polygons or masks.
 
I am not quite sure what you are saying.
Are you saying that the lower spatial resolution imagery makes a smaller LOD15 file than the same area (in square metres) compiled to LOD15 from higher resolution imagery? (you must be comparing equal geographic areas at the same LOD if the comparison is meaningful, rather than looking at the resolution of the source imagery)

If so, this will be because it can be more heavily compressed, as there is less colour and image information per square metre.
If you wish to test this, you could compile both with Compression = 100. The filesizes should (in theory) then become roughly similar (filesizes from resample are rarely the "same").
With any degree of compression, more colour variation and spatial information will result in a larger file, due to less ability to compress the imagery, even with the final output bgl being the same spatial resolution.

A mask does not appreciably reduce the size of a file, it simply adds a transparent (mask) layer.
A missing data value, however, does reduce file size.
A solid block of same colour will also reduce the filesize(even if masked out), because it can be compressed heavily.
 
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hcornea said:
Are you saying that the lower spatial resolution imagery makes a smaller LOD15 file than the same area (in square metres) compiled to LOD15 from higher resolution imagery? (you must be comparing equal geographic areas at the same LOD if the comparison is meaningful, rather than looking at the resolution of the source imagery)
Click to expand...

Yeah. The area is exactly the same. I have imagery from maps2bgl. I am not sure with resolution. I know only there are same dimensions for the images. 4096x4096 pixels in one file. Maps2bgl works in two modes. One is LOD15 mode and the second is co called "LOD16". The application downloads jpgs and them builds bitmap 4096x4096 in dimension. So the bitmap is build from the jpgs. The only difference is in the number of jpeg files. LOD15 has less files than "LOD16". But at the result, the bitmaps are exactly the same, and the dimension of xdim and ydim in inf file are the same for both.

I will do some test later, when I will work with the program again.
 
AVSIM library - resample manual - Make photo-real ground textures in Flight Simulator

Before some time I looked for one lost link or manual about resample. It is resample tutorial html page. I found the file now, but it is not public. I recommend this file because it helped me in the beginning. Now I could finish reading the tutorial when I happily found it:

Make photo-real ground textures in Flight Simulator X
http://library.avsim...xsd&DLID=140539
 
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