Expose transparent background colors for displayModel (PR #12658)
Resolves concerns about contributed PR: #9197
Workaround for #6467
# Summary of the issue:
From #6467 (comment)
> When text is written with the GDI background mode set to transparent, the display model doesn't necessary report the
> correct background color for that text. In this case, the GDI hook should look at the actual background color before the
> text is drawn, rather than using the result of GetBkColor.
# Description of fix:
There have been two prior attempts to fix this, both causing severe performance regressions.
- #6844: "Display model: Try to provide the correct background color for transparent text drawing (issue #6467)"
- #7440: "displayModel: second try at recording the actual background color for text when a transparant background color is set."
This implementation merely exposes information about the potential transparency of the text background.
Using the samples in https://github.com/nvaccess/testDisplayModel
to test with various GDI dependent GUI APIs shows that the GetBkColor result can be:
- Opaque and matches visual presentation(rawGDI)
- Transparent and matches visual presentation: (mfcTest, GDItest)
- Transparent and does not match visual presentation: (anecdotal)
- The exposure of the transparency (via the alpha value on the colors.RGB class) will allow appModules
or add-ons to customize NVDA's presentation as necessary.
For users, there is no change in behavior by default. An option is introduced to the advanced settings panel to allow reporting of transparency in colors. The intention here is that developers will use this to identify where
color reporting could be improved.
The prior approach used the colorref high-level byte of a colorref struct. However, MSDN requires this byte be zero.
This prior approach did not pass the colorref into any Windows API's, so it was safe. But to prevent this from being accidentally introduced in the future, a compatible alternative type is introduced and used instead.
In order to stay compatible with python code interpreting this color value, a single bit is set if the color is considered transparent.
# Testing:
- Built applications from https://github.com/nvaccess/testDisplayModel
- Tested with/without transparency reporting enabled (advanced panel)
- Ensure color reporting is enabled in document formatting panel of NVDA settings
- Use NVDA+Numpad 7 to switch to screen review
- Use Numpad 7, Numpad 8, Numpad 9 to read the dialog
Co-authored-by: Leonard de Ruijter <leonard@babbage.com>
