An engineer who worked on these 10.1 improvements gave me sone additional information that is worth sharing:

2-10 bytes per character is the permanent data structure that's needed to render lines. We still consume 30-120 bytes of temporary data during the creation of this structure (30 in the common case, 120 in other cases with non-default styling or circumstances). Numbers are approximative.

Gordon Smith

Adobe Flex SDK Team

Since TextLines are created one-by-one, I believe Player 10.1 needs to use 30-120 bytes of temporary memory per character in that line. So if there are 100 characters in the line, that would be 3-12K of temporary memory. Once the TextLine is created, that memory can be released. Then the next TextLine needs 3-12K of temporary memory. I'll check this with the Player engineer and get back if this is incorrect.

Gordon Smith

Adobe Flex SDK Team

There are two data structures that contribute to the memory picture.

One is a "temporary" data structure that is used for the creation of the TextLines of a TextBlock. This data structure has ~one entry for each character in the TextBlock. In 10.0, the per entry cost is more or less what you have measured (150 bytes). In 10.1, we have arranged to make that structure ~30 bytes for many common cases, but it remains ~100+ bytes in others.

The other is the data structure that describes the content of a TextLine.  In 10.1, that data structure contains all the data necessary to display the line, determine atom boundaries, etc It is ~ 2-10 bytes/character, with 2 being a common case for English text, no kerning.

In 10.1, we also added a method on TextBlock that lets you release the "temporary" data structure. It will be recreated when needed (e.g. you change the content of the TextBlock and/or you create new lines from it).