Re: [Pysdl-devel] sprites, mouse cursor

[Mark B. <mb...@0x...>]

On Sat, 03 Jun 2000, "David Clark wrote:
> My understanding is that the following minimal test program should
> work. Unfortunately, it doesn't.
> 
> #! /usr/bin/env python
> 
> import sdl
> sdl.init(sdl.INIT_VIDEO)
> video = sdl.video_set_mode(200,200,16,sdl.SWSURFACE)
> video[100][100] = video.map_rgb((255,255,255))
> video.update_rect((0,0,0,0))
> while 1:
>     pass
> 
> This should plot a white pixel at the center of the surface, but no
> white pixel appears. This is strange; I remember testing this back
> when it was introduced in 0.0.3, and it worked fine then. Am I doing
> something wrong?


This is what you would intuitively expect, I imagine, but it's never worked
this way.

video[100] this returns a list of of the pixels at 101
video[100][100] indexes the 101st element of the list returned from video[100]

So when you do video[100][100] = value, what you're doing is assigning value to
this element in the list returned by video[100]. However, this list doesn't
directly map to the memory used by video.

In order to get the behavior you're looking for, you'd need to do something
like:

y_list = video[100]
y_list[100] = video.map_rgb((255, 255, 255))
video[100] = y_list

Cleary for one pixel, it's advantegous to use set_at.

For linear access cases, it's faster to do stuff like:
y = 0
for y_list in video:
	for x in y_list:
		x = get_some_color()
	video[y] = y_list
	y = y + 1

For less linear access cases, but still in loop, it's better
to do something like:

s_copy = video[:]
some_loop_construct:
	s_copy[y][x] = some_color
	x = some_number()
	y = some_number()
	s_copy[y][x] = another_color
video[:] = s_copy

Generally speaking, if you can get your code into a for x in some_range it'll be
faster. If you can't, then it's a good idea to copy the buffer, and then copy
it back when you're done.

If you're not doing a lot of drawing, set_at is probably a good idea.