====== Image File Support in Rhino 5======

Rhino 5 supports the attachment of image files in the [[http://docs.mcneel.com/rhino/5/help/en-us/commands/backgroundbitmap.htm#Place|BackgroundBitmap]] and [[http://docs.mcneel.com/rhino/5/help/en-us/commands/pictureframe.htm|PictureFrame]] commands.
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**Rhino works best with image files that are "power of 2" in size.** 
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//This is because Rhino displays image files as textures - always.//
===== Images stored as textures =====
To display a file as a texture, Rhino uses OpenGL to //texture map// a polygon or polygon mesh. This is important information for many reasons:

  - OpenGL stores textures in GPU memory.
  - Because of #1, OpenGL implementations have hardware requirements (limitations) on how big a texture can physically be.
  - Because of #2, there is no //standard maximum// defined size for a texture. It's based on your video card and drivers.
  - Because of #3, Rhino can't just use any file at any resolution and assume it's going to work.
  - Because of #4, if Rhino encounters an image file that does not meet your video hardware capabilities, it must resize the image to a size that can be used by your hardware.

**//It is not a requirement for Rhino 5 that image files be exactly sized to a power of 2.//**
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But, the internal representation in Rhino (and thus for OpenGL) __is__ a power of 2. 
In other words, you can throw any file or texture at Rhino 5 and it will make sure it works on most video hardware.

===== Adjusting for the power of 2 =====
So how does it make it work if the image is not a power of 2?

//**If Rhino needs to resize an image, it will resize it to the next highest power of 2 that is greater than the current dimensions but less than or equal to the maximum dimensions supported by the current hardware.**//

Here are a few examples to illustrate this.
//Assume the current hardware's texture size limitation is 8192 x 8192.//
If Rhino sees a file with dimensions:

  * 256 x 256 - Rhino will do nothing, perfect powers of 2.
  * 1024 x 1024  - Rhino will do nothing, perfect powers of 2.
  * 1024 x 256 - Again, Rhino will do nothing, perfect powers of 2.
  * 127 x 127 - Rhino will resize the image to 128 x 128, next highest value that is a power of 2 and greater than the current dimensions.
  * 129 x 129 - Rhino will resize the image to 256 x 256, again, the next highest value that is a power of 2 and greater than the current dimensions.

Rhino will never //downsize// an image because that's a //lossy// operation. So it will always increase dimensions and never decrease them, except in the following cases:
  * 9000 x 6000 - Rhino will resize (downsize) the image to 8192 x 8192. 9000 exceeds the 8192 maximum limitation and 6000 isn't a power of 2. The next available power of 2 is 8192.
  * 9000 x 9000 - Rhino will resize (downsize) the image to 8192 x 8192. Both dimensions exceed hardware limitations and are adjusted to the maximum limits.

===== Pixelization =====
When Rhino does resize the image, it does not use any kind of sampling or bi-linear filtering. It is a //straight linear stretch or shrink//. 
So, if the difference between original dimensions and adjusted dimensions is greater, the results look worse.
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Take the 129 x 129 example above. Rhino basically has to double the size of the image resulting in every pixel now being a 2 x 2 block in the final result.
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**This is the major cause of pixelization in the display of the attached image.** 
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But, the results can even look worse if it tried resizing to 128 x 128. Banding can often occur because entire rows or columns of pixels have been removed. 
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//Rhino will not downsize anything unless it absolutely has to due to hardware limitations.//

===== Basic recommendation=====

//**Always keep images as close to values that are just less than powers of 2 as possible. And if you can, try to make them powers of 2**.//

Given today's video hardware, the two limits we see most of are 4096 x 4096  and 8192 x 8192. So if you're generating large textures, those are the dimensions to which you should limit your image generator. 
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//Making them any larger is a waste of time, space, and will cause Rhino to throw out a lot of good pixel information.//