Bicubic resampling

[global_mapper]

For a sample that large you would need to FTP the data to ftp.globalmapper.com using a username of 'upload' and a password of 'upload'.

The bicubic resampling method is very well defined in terms of how to compute the color at a given location given the surrounding 16 samples/pixels, so I don't think that Photoshop is using bicubic if they are getting significantly better results. For significant sub-sampling vs. just resampling at a similar resolution there are much better algorithms than bicubic, namely boxing algorithms that consider larger numbers of pixels or just average out the values within a box rather than doing a weighted average heavily skewed towards the value at the actual sample location. Those are things we may be able to easily add now that an interface is in place for using multiple sampling methods.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

[tjhb]

For all I know Photoshop's implementation of "bicubic" does have something extra added to it. They're such a centre of gravity that they might feel they can use whatever names they like.

For what it's worth, the available resampling methods in Photoshop CS4 are listed as follows (with their usage hints):

• Nearest Neighbour (preserve hard edges)
• Bilinear
• Bicubic (best for smooth gradients)
• Bicubic Smoother (best for enlargement)
• Bicubic Sharper (best for reduction)

(In this case I used the method listed as plain bicubic.)

I've uploaded the file Resampling.zip to your ftp site and I hope you can take a look.

You'll immediately see what we mean about the difference in quality, and why we have hesitated to use Global Mapper for resampling down.

Your point about the new architecture opening further possibilities seems really encouraging. Thanks.

[global_mapper]

I went ahead and added 3 new resampling methods, all box average resampling with sample sizes of 3x3, 4x4, and 5x5. These are better when downsampling than the base bicubic resampling method. I tried the 4x4 (i.e. average of 16 closest pixels) on your data and got results very close to what Photoshop was producing for you. The more you are downsampling the larger average you should use. For going 10m to 40m, the 4x4 size would be the best fit.

I have placed a new build at http://www.globalmapper.com/global_mapper11.zip with the change for you to try. Simply download that file and extract the contents into your existing v11.xx installation folder to give it a try. If you are using the 64-bit version, there is a new build available at http://www.globalmapper.com/global_mapper11_64bit.zip .

Let me know if I can be of further assistance.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

[global_mapper]

I went ahead and added 3 new resampling methods, all box average resampling with sample sizes of 3x3, 4x4, and 5x5. These are better when downsampling than the base bicubic resampling method. I tried the 4x4 (i.e. average of 16 closest pixels) on your data and got results very close to what Photoshop was producing for you. The more you are downsampling the larger average you should use. For going 10m to 40m, the 4x4 size would be the best fit.

I have placed a new build at http://www.globalmapper.com/global_mapper11.zip with the change for you to try. Simply download that file and extract the contents into your existing v11.xx installation folder to give it a try. If you are using the 64-bit version, there is a new build available at http://www.globalmapper.com/global_mapper11_64bit.zip .

Let me know if I can be of further assistance.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

[tjhb]

Mike what can we do to thank you? You were dead right in your earlier comments, and now you've made a superior resampling tool. This is fantastic. Thank you.

I agree that, on this data and for this subsampling ratio, a 4×4 box average is about ideal. The result is marginally better than by using Photoshop's "bicubic". For what it's worth, I agree that subsampling in Photoshop must being do something quite different from the "bicubic" resampliing it claims.

Nailed. Thank you again Mike.

A new level of usefulness.

[global_mapper]

Regarding bicubic and Photoshop, I think Photoshop is using the base bicubic resampling, then applying an additional filter. There seem to be a lot of different enhancements to bicubic with different filters that are used for different purposes. In my opinion, the bicubic resampling is better when upsampling or just slightly downsampling, and the box averagers are better when downsampling significantly.

Let me know if I can be of further assistance.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com