Re: On a side note - display resolution changes now handled

[Harold L Hunt II <huntharo at msu dot edu>]

Jehan wrote:
> Harold Hunt wrote:
>
> > You must be tired.  
>
> Well, actually yes :p. I did some canoeing last Friday and I haven't 
> recovered yet. :)
Are you talking about canoeing, or ``canoeing''.  When we went 
``canoeing'' it involved a cooler full of beer and as the day progressed 
the cooler full of beer was spotted more and more often floating down 
the river next to a coule of upside down canoes and a bunch of guys 
trying to get the cooler back into a righted canoe before the precious 
contents were lost.  But I digress... :)

> > If we create a new offscreen surface (which I was not
> > talking about doing, I only said we have to recreate the primary 
> > (onscreen)
> > surface) with a different depth/format/etc than the original offscreen
> > surface, then we have effectively changed the X graphics mode and we 
> > would
> > need to use some sort of X extension to notify clients that all 
> > pixmaps and
> > visuals have been reset.  If there is such an extension, I am not 
> > aware of
> > it.
>
> I'm not familiar with DirectX nor the internal of X so maybe I used the 
> wrong words. I'm not actually talking of changing what the X server 
> think the screen resolution/depth is. But we can already have a 
> different resolution/depth for the visual than for the monitor, which 
> means that there is a conversion at some point when the depth doesn't 
> match. So why can't you just throw away everything that is after the 
> conversion? I would think that, at worst, if the conversion happens when 
> drawing into the offscreen surface, all the buffers would have to be 
> recreated and that X would just have to ask all X windows to redraw 
> their content in the new offscreen buffer.
I think I see where you are confused.

I said previously that we can handle screen resolution changes because 
we essentially just enable scrollbars, if necessary, to allow the extra 
area to be viewed.  With the Shadow GDI engine, that is all that has to 
be done.

However, with the Shadow DirectDraw and Shadow DirectDraw Non-Locking 
engines we must release and recreate the primary surface using the same 
size as it had before.  This is really just a technicality.  You see, 
DirectDraw allows a surface to be larger than the screen size.  But, 
when you change the screen resolution, DirectDraw requires that you 
release the primary surface and create again.  DirectDraw doesn't care 
if you recreate the primary surface using the exact same parameters; 
rather, it just wants you to recreate it.  Yes, this is silly, but that 
is what DirectDraw requires.

====

I also said previously that screen depth changes were much more 
disruptive than screen resolution changes.

First, a little background on surfaces.  We create an offscreen surface 
and we provide the X graphics layers with a pointer into the memory used 
to represent the pixels on that surface.  All X graphics operations (fb, 
shadow, mi, etc.) are done by calculating offsets of various pixels in 
this ``framebuffer'' and applying various transformations to those 
pixels.  Thus, a horizontal blue line would be drawn by offsetting to 
the start of that line, then flipping the value for the next x pixels to 
blue.  The ``shadow'' layer in X allows graphics to be drawn to an 
offscreen framebuffer.  Shadow keeps track of the regions in the 
offscreen fraembuffer that have been updated, and it occasionally calls 
  a ``shadow update'' function that tells us to transfer those regions 
to the screen.  DirectDraw has something called a ``primary surface'' 
that represents what is being displayed on the screen.  When we want to 
display the updated bits of the offscreen framebuffer, we do a ``bit 
block transfer'' from the offscreen surface to the primary surface.

The offscreen surface and the primary surface usually have the same 
format (that is, they have the same pixel format that specifies how many 
bits for red, green, and blue and how many bits are used per pixel value 
in the framebuffer).

If the offscreen surface and the primary surface have the same format, 
then a bit block transfer between them is essentially a memory copy from 
the system memory to the video memory (with lots of fun synching issues 
that Windows takes care of for us).  Imagine for a second that the 
offscreen surface was allowed to have a different format than the 
primary surface.  Then a bit block transfer from the offscreen surface 
to the primary surface now must examine *every single pixel* and 
transform the color values from, say, 16 bits per pixel to 32 bits per 
pixel.  That is a hell of a lot more complex than doing a simple memory 
transfer.

DirectDraw is primarily concerned with enabling high-performance. 
Therefore, I think that allowing the offscreen surface to have a 
different depth than the primary surface would be contradictory to the 
purpose of DirectDraw.

I have not checked the DirectDraw documentation to see whether offscreen 
surfaces must have the same depth as primary surfaces.  I did say that I 
did not think it likely that DirectDraw would allow different depths for 
the two surfaces.  You could verify this, but I am willing to bet that 
different depths are not allowed.

Notice that we never change the format of the offscreen framebuffer. 
Thus, the structure of the framebuffer that X draws to us unchanged.  X 
does not support screen depth changes while running, so changing the 
format of our offscreen framebuffer would causes all graphics operations 
to draw incorrectly and it would possibly cause a segmentation fault if 
the depth of the offscreen framebuffer was decreased (because the total 
memory region would then be smaller than X was expecting).

====

With the Shadow GDI engine, Windows will transform *every single pixel* 
whenever we do a bit block transfer from a DIB to the screen and the 
depth of the two differs.  Thus, we are allowed to have a different 
depth for the X visual than for the Windows screen, but doing so causes 
a huge performance penalty.  However, I think that allowing this and 
providing a popup warning about the performance penalty is better than 
just ceasing to display graphics at all.

====

With the DirectDraw engines I do not think that we can have a differnt 
depth for the two surfaces, so I fear that our only option in that case 
is to cease transferring the updated regions of the offscreen surface 
until such a time as the Windows screen depth has been returned to its 
original value.

====

Does that answer your questions?


Harold