Re: fb_imageblit semantic

[Antonino Daplas <adaplas@pol.net>]

On Mon, 2003-03-17 at 20:25, Petr Vandrovec wrote:
> On 17 Mar 03 at 13:18, Geert Uytterhoeven wrote:
> > 
> > That depends... How do we draw the monochrome penguin? Using image->depth is 1
> > or 8? The latter (current method) is slower, since we need to expand the
> > monochrome logo to 8-bit first, and (usually) compress it to 1-bit in the fbdev
> > driver afterwards.
> 
> As far as I can see, it gets monochromatic logo and converts it to
> 8bpp format ;-) (fb_set_logo, needs_logo = 1 or ~1) And then imageblit
> converts it back to 1bpp.
>  

Yes.  That's the sacrifice for simplicity and consistency, low color
depths sacrifice on speed, high color depth sacrifice on color range. 
The real beneficiary is 8-bit pseudo-pixel.

Of course, there's always a choice:

1.  allow support for non-monochrome expansion/reduction in
cfb_imageblit(), ie input data is 4bpp and framebuffer format is 16 bpp,
directcolor.

2.  prepare all logo so image->depth matches framebuffer depth.  This
will require  that fb_prepare_logo() has additional code to pack
image->data properly

3.  Just add conditionals like:

if depth == 1
	logo->data == logo_bw
else if depth == 4
	logo->data == logo_4
else if depth == 8
	logo->data == logo_8
else
	prepare_logo_to_match_fb_depth

For #2 and #3, what do we do when var->bits_per_pixel > 8?  Do we still
use image->depth=8? Do we still use the LUT?  

4.  Go back to the 2.4 way of drawing the logo, let the upper layer do
everything, and just directly memcopy the data to the framebuffer.

> > And perhaps we may want to draw 32-bit ARGB images later?
> > 
> > So I see the following possible valid values for image->depth:
> >   - 8 (logo with up to 256 colors and LUT)
> >   - optional 1 (monochrome logo, if we don't want to expand?)
> >   - optional 32 (ARGB image, dithering left to the driver?)
> 
> I still do not understand 'if we don't want to expand'. This forces too
> much knowledge on upper layer, as far as I can tell.

I agree.

Tony




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