BUG with 32bpp depth visual on 16bpp device

[Petr Cvek]

Hello,

I've observed a weird/possible bug behavior on 16bpp display.

I'm trying to modify flaxpdf (FLTK based) PDF viewer to work on 16bpp. 
It uses xrender for bitmap resizing, which works only for 24+bpp color 
modes. However the X screen on 16bpp machine seems to support some 
virtual visual with 32 depth.

Problem is when I try to set 32bpp in the fltk by calling:

Fl::visual(FL_RGB8);

the application starts, but all widgets are black and white.

Debugging this I wrote my own simple X window application, which uses 
32bpp visual and it behaves in a same way as FLTK. This means a 
definition of blue pixels with min to max intensity makes an incomplete 
green gradient on LCD, definition of green min to max makes green 
striped with red gradients and definition of blue min to max makes 
constant full white (there is no way to make blue color in depth == 32).

The relevant visual definition from xdpyinfo:
   default visual id:  0x21
   visual:
     visual id:    0x21
     class:    TrueColor
     depth:    16 planes
     available colormap entries:    64 per subfield
     red, green, blue masks:    0xf800, 0x7e0, 0x1f
     significant bits in color specification:    8 bits
   visual:
     visual id:    0x41
     class:    TrueColor
     depth:    32 planes
     available colormap entries:    256 per subfield
     red, green, blue masks:    0xff0000, 0xff00, 0xff
     significant bits in color specification:    8 bits

Maybe I'm making some mistakes in the colormap allocation? But the same 
code (along with flaxpdf) works without a problem on my Radeon RX460 GPU 
(non default visual):
   visual:
     visual id:    0x62
     class:    TrueColor
     depth:    32 planes
     available colormap entries:    256 per subfield
     red, green, blue masks:    0xff0000, 0xff00, 0xff
     significant bits in color specification:    8 bits

The 16bpp device is just dumb fbdev, so any 32bpp to 16bpp 
transformations must be done somewhere in the X server. Do you have some 
hints where to begin? (i'm using X.Org X Server 1.19.5 from development 
tree of buildroot)

best regards,
Petr