[ft] Render differences on embedded ARM vs other platforms

[Wessman, Tim (HP Calculators)]

Hello,

Recently, we moved from an RGBT 5551 internal color configuration to an RGBA 
8888 configuration. There is a shared “core” application with a platform 
specific hardware abstraction layer. The code is compiled for windows, mac, 
linux, android, ios, and an embedded ARM hardware system (mix of 32 and 64 bit 
versions). The application is tested using a test application which can perform 
operations and compare to expected values. In the screenshot comparisons, we 
are noticing small differences in occasional pixels. Upon further testing, 
these are a small difference in the alpha channel value on the ARM device 
compared with *all* other platforms which match. Our recent move to the 
increased color depth exposed this change I think which may have been present 
all along.

To the best of my knowledge, all compiler flags and settings are identical on 
all builds. Font is a TTF font (droid sans based), rendered using 
FT_LOAD_FORCE_AUTOHINT and FT_LOAD_TARGET_LIGHT with 
AS_CONFIG_OPTION_USE_WARPER. We are using the built-in sbitmap cache mechanism 
as well.

I’ve attached an output from our test application. The middle output has red 
pixels in the difference spots. Note how the lowercase “s” for example does 
match in the table, but has a difference when used in the sentence. Similar 
happens to the “M” or “K” in the graphic buttons at the bottom. Note that these 
are a different font size then the text in the character table.


  1.  Does anyone have a suggestion as to what build difference or setting 
might account for these differences?
  2.  Could this be some sort of a floating point rounding behavior issue? (hw 
float on the ARM rounding differently than hw/sw on the other systems for 
example)

Basically, I am hoping for a start on where to even begin trying to track this 
down. The goal would be for rendering to be identical across all platforms.

Thank you.

--

Tim Wessman

char_diffs.png
Description: char_diffs.png