From: Alper Akçan (distch@hotmail.com)
Date: Sat 28 Dec 2002 - 22:46:15 IST
Hi all,
> > how can I use rgba colors in svgalib?
>
>What does it mean?
>
>--
>Matan Ziv-Av. matan@svgalib.org
I `do not know` much about rgba colors, but I as I read from web sites A
stands for transparecny (alpha chanel I think ?!). But it defines the levels
of trancparency (I think ;)).
R stands for red, G, green, B blue, as U already know. A is for
transparency.
for example; 255,0,0 is pure red in rgb.
255, 0, 0, 0 is also pure red with no transparency in rgba.
but if A was 255 like;
255, 0, 0, 255 it would be transparent so red color would not seen.
A can get values of 0 <= A <= 255, trancparency of that pixel is defined
with the value of A.
So;
When we want to draw a color with a trancparency level, we first must know
the old color of the pixel, that we are drawing on to. second, the color
that we want to draw, final (the most important) "a" trancparency level.
thist function takes the old color of the pixel, new colorof the pixel, and
trans. level.
func (int c1, int c2, int a)
{
int ret;
// here we should get a color that is transparent according to the trans.
level (a). so, return value is a function of three (old col, new col, trans.
level). But how?
return ret.
}
this functions takes the coordinates of the pixel(x, y), the color that we
want to draw (r, g, b), and the trancparencylevel (a).
simple_rgba_color_drawing_func (int x, int y, int r, int g, int b, int a)
{
int r_, g_, b_;
int final_color;
// Fist get the old color of pixel
gl_getpixelrgb (x, y, &r_, &g_, &b_);
// gets the final color
final_color = func (gl_rgbcolor(r_, g_, b_), gl_rgbcolor(r, g, b), a);
// draws the final color
gl_setpixel (x, y, final_color);
}
Again, I have no idea how a level of rgba colors is used, but I think the
idea should be like this for a trans. level. I am asking that what would be
the function for (old_col, new_col, trans_lev) to get a new color?
All I know is if A == 0 then that is not transparent, if A == 255 it is
transparent like a glass (not seen).
Alper.
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