I am making a line using the following algorithm
The line starts from the point where I click the mouse and draws to wherever I point to.

while(!key[KEY_ESC])
{
if (mouse_b&1) {
int x = mouse_x;
int y = mouse_y;

while (mouse_b&1)
{

line(bmp, x, y, mouse_x,mouse_y,makecol(255,255, 255));
rest (10);

draw_sprite(buffer, bmp, 0, 0);
blit(buffer, screen, 0,0,0,0,1400,1050);
clear_bitmap(buffer);
clear_bitmap(bmp);
}
}
}

destroy_bitmap(buffer);
return 0;
}

The problem I have is that all lines get erased every time I draw a new one. I also want to be able to draw this line on various backgrounds. Any help would be　greatly appreciated.

I got rid of some of the things that were unnecessary

while(!key[KEY_ESC])
{
if (mouse_b&1) {
int x = mouse_x;
int y = mouse_y;

while (mouse_b&1)
{
line(bmp, x, y, mouse_x,mouse_y,makecol(255,255, 255));
rest (10);
blit(bmp,buffer,0,0,0,0,1400,1050);
masked_blit(bmp, screen, 0,0,0,0,1400,1050);
clear_bitmap(bmp);
}
}
blit(buffer,screen,0,0,0,0,1400,1050);
}
I do not really get how to copy the line from bmp to screen. I also tried to create an intermediate bitmap that holds all the lines that I draw. I think that the problem with my code is that it stops copying when I release the mouse button.]

EDIT
blit(buffer,screen,0,0,0,0,1400,1050); - I think that this is supposed to make the line permanent on the screen. But for some reason it does not work

Please, use the code tags, they make your posts much easier to read. You can see how by clicking on "Help" where you write the post.

Here's one method that might work. Haven't tested it, only spent five minutes writing it...

I don't like that code, but it should do exactly what you want it to do. The reason I don't like it is because the method I use in my drawing program is a lot better, but about 20 times more complicated.

--- Kris Asick (Gemini)
--- http://www.pixelships.com

Black is subjective.

In 8-bit mode, palette entry #0 is almost always black, unless you change it. Thus if you change it, that becomes your new transparent colour.

In 15/16/24/32-bit mode, magic pink is the transparent colour. You can make this colour by calling makecol(255,0,255). If you want transparent areas on a bitmap in higher colour depths, it needs to be that colour.

To draw things onto the screen with transparent areas, you need to use masked_blit(), draw_sprite(), draw_character_ex(), or any of the other sprite-based drawing commands.

I checked the code I provided by the way. It's bugged, but very easy to fix. If you move the "int initx = -999, inity = -999;" line to just before the while loop begins, it works perfectly.

--- Kris Asick (Gemini)
--- http://www.pixelships.com

Unfortunately, the mask colours are built from #define directives, which means the only way to make the transparency colour different is to create your own routine for blitting which accepts different transparency colours.

Though another method is to use 32-bit, alpha blended sprites, but Allegro's routines for doing that are not the fastest.

An old method for doing transparent blitting is to have two sprites, one which represents the sprite, another which represents the transparent areas called the mask. You then draw the mask to the screen using an AND binary operation and draw the sprite using an OR binary operation. Again though, Allegro doesn't have built-in features to accommodate this method of drawing.

--- Kris Asick (Gemini)
--- http://www.pixelships.com