Re: R: xfbdev alpha channel

Marc Aurele La France <tsi@xxxxxxxxxxx> · Mon, 27 Aug 2007 17:17:56 -0600 (Mountain Daylight Time)

On Mon, 27 Aug 2007, Tabaro Toni wrote:

On Mon, 27 Aug 2007, Marc Aurele La France wrote:

On Mon, 27 Aug 2007, Tabaro Toni wrote:

Hi, i have succesfully cross-compiled the xfree 4.7.0 on a embedded
board with mipsel processor and framebuffer on a 2.6.12 linux
environment, with the framebuffer running at 960x1080x32 ARGB8888 or
1280x720x16 ARGB1555.
The Xfbdev program start and work (if i export a display on a i386
machine and start xclock the xclock program work).
The problem is: to see the image on the board i must run a program in
background which set the alpha bit to 1 (on the 1280x720x16 ARGB 1555
frame buffer) or byte to 0xff (in the case of 960x1080x32 ARGB8888),
otherwise the screen remains black.

That is an oddball alpha implementation.  What adapter is this?  Does the
(text) screen appear correctly before starting Xfbdev?

A im not using the text mode, on that embedded system there is only
console /dev/ttyS0 and framebuffer is on graphics hardware.  This is an
implementation on one stb, the graphic plane is over a video plane and the
alpha is needed for seeing the video under graphics

That's not the point.  Xfbdev will not function at all without a kernel 

driver for your adapter.  Therefore, the kernel must be displaying something 

before the X server starts, unless that too is being blacked out.  So, again, 

what does the screen show before starting Xfbdev?

I think the Xfbdev can't handle the alpha channel correctly, leaving
it to 0.
I dont need the translucency or other features, only see the image on
screen, maybe i can patch the framebuffer code to leave the alpha at
fixed value, is someone able to help me by suggest the part of code to
patch?
I searched on the programs/Xserver/hw/tinyx/fbdev/ or
programs/Xserver/fb/ path, but i am not able to understand how the
alpha is handled :-(

The X server does not touch the alpha channel, but uses whatever
applications provide in that field.

Maybe i have not explained well the problem, i try on another way:
When normally i start "Xfbdef -ac" on another board with different
processor (powerpc) the screen show the X tiled background with the
xcursor, on board with mips the "Xfbdev -ac" show black screen but is
running fine.
If i change the alpha channel bit directly to the frambuffer with a c
program and with the Xfbdev program running i see the X bacground and the
mouse, if this c program repeat that function every second the x is running
refreshing itself every second (every time my prog touch the alpha bit).
This without an X application running, if the X application is running i
see the app on screen only when my c program do the alpha bit set.

I think the Xfbdev on framebuffer is tested on a framebuffer without real
alpha channel, on PC, my boards are embedded board with real alpha used to
create holes on graphics needed for seeing underlining video plane over it.

... which means this is a problem with your adapter, not with Xfbdev.  Xfbdev 

is designed to use a dumb framebuffer through a kernel interface.  No alpha 

channel support whatsoever.  Xfbdev is intentionally too generic to have 

knowledge of the chipset specifics necessary to control an alpha channel.

As far as I can tell, your options are:

1) There might be a jumper or firmware/BIOS setting that reverses the meaning 

of the alpha channel, or disables it.

2) There might be an ioctl you can use (in a modified Xfbdev) to do so.

3) There might be another X server more specific to your adapter, with an 

option to control the alpha channel.  What does `lspci` say?

4) You say the adapter supports depths 15/16 and 24/32, and imply you have 

some means of switching between them.  Does the adapter support depth 24/24 

(i.e. eliminate the alpha channel alltogether)?

5) You might be stuck with your program to invert the alpha channel.

Marc.

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