From: Nicolas FRANCOIS (AKA El Bofo) <nicolas.francois@free.fr>
To: "Jun P.FURUSE" <Jun.Furuse@inria.fr>
Cc: Caml List <caml-list@inria.fr>
Subject: [Caml-list] Re: Problem with Graph module
Date: Tue, 18 Jun 2002 19:01:39 +0200 [thread overview]
Message-ID: <20020618190139.69122974.nicolas.francois@free.fr> (raw)
In-Reply-To: <20020618.114831.1025221351.Jun.Furuse@inria.fr>
Le Tue, 18 Jun 2002 11:48:31 +0200 (CEST) "Jun P.FURUSE"
<Jun.Furuse@inria.fr> a écrit :
> I verified that this code prints out "BUG" when X display has less
> than 24bits color depth. In such environment, the colour mapping from
> Graphics to X is not one-to-one. This means that point_color may not
> always return the exact colour which you drew on the screen.
> For example, in an X display of 15bpp, the Graphics colour 0xffffff
> and 0xf8f8f8 are translated to the same X white colour.
> This is also true for the default white background color of Graphics.
> point_color returns 0xf8f8f8, not 0xffffff, this is why the above
> code prints out "BUG".
OK. Now I have this simple problem : I can use Direct Rendering from X
with my Voodoo card in 15 bits, and correct color translation in 24 bits.
is that right ?
> In Camllight, the translation between Caml colour and X colour
> performs communication between programs and the X server. Since it is
> highly expensive operation, the colour translation of O'Caml Graphics
> is rewritten, so that it would never ask the X server about
> colours. It improves the speed of colour query impressively.
> Unfortunately, the both colour translation code do not return always
> the same result. The different behavior of the Nicolas' O'Caml and
> Camllight programs can be explained by this difference of colour
> translation.
Who said computer science is not an exact one ?
> I think this does not mean the new colour translation of O'Caml is
> buggy. Even in Camllight, the same things can happen, unless the colour
> translation is one-to-one. (It happens less, but not never.)
>
> The important things (which should be noted in graphics.mli) are that
>
> * you may get different colour index from one you draw,
>
> * the colour indices predefined in Graphics for the basic colours
> such as white, blue, red, etc can be just canonical candidate
> indices for them.
>
> You should not compare these colour indices against the colours
> got from the screen. For example, in Nicolas' program, instead of
> comparing Graphics.white, we should use the colour index obtained
> from the screen at the begining of the program:
>
> let screen_white = point_color 0 0
> (* this can be different from Graphics.white *)
Great. Thanks for the tip. Euh, and what if I have to test several colors
? Oh, yes, here it is :
let screen_from_color c =
set_color x;
plot 0 0;
point_color 0 0;;
Right ? I'll try it now.
\bye, and thanks again.
--
Nicolas FRANCOIS
http://nicolas.francois.free.fr
A TRUE Klingon programmer does NOT comment his code
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next prev parent reply other threads:[~2002-06-18 16:53 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-06-17 1:12 [Caml-list] " Nicolas FRANCOIS
2002-06-17 13:23 ` Jun P.FURUSE
2002-06-17 17:47 ` [Caml-list] Nicolas FRANCOIS
2002-06-18 9:48 ` [Caml-list] Re: Problem with Graph module Jun P.FURUSE
2002-06-18 17:01 ` Nicolas FRANCOIS [this message]
2002-06-18 23:21 ` Nicolas FRANCOIS
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