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video buffers not initialising properly for paging
Neil Walker
Member #210
April 2000
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Hello,
I'm sure this'll become obvious after some sleep but I've noticed a problem with video paging/triple buffering in my code.

Basically, I draw to my active video back buffer then invoke the video paging code (BitmapPages is an array of two video bitmaps), as below:

draw(); //draws to the 'DrawingSurface' which is BitmapPages[0] first run
show_video_bitmap(BitmapPages[BitmapPagesActive]);
BitmapPagesActive=1-BitmapPagesActive;
DrawingSurface=BitmapPages[BitmapPagesActive];

But the first time I call draw() and draw to DrawingSurface (which is bitmappages[0] first time) it is drawing to the screen and not the video buffer but then subsequently is ok.

I guess the daft thing to rule out is will my BitmapPages[0] ever be the screen, and if so how do I avoid this.

Neil.
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Simon Parzer
Member #3,330
March 2003
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AFAIK the first video bitmap allocated is in the same memory area as the screen memory.

BITMAP* video = create_video_bitmap(SCREEN_W,SCREEN_H,...); // This is gonna be screen memory
BITMAP* real_video01 = create_video_bitmap(...);
.....
destroy_bitmap(video);

Thomas Fjellstrom
Member #476
June 2000
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Quote:

BITMAP* video = create_video_bitmap(SCREEN_W,SCREEN_H,...); // This is gonna be screen memory
BITMAP* real_video01 = create_video_bitmap(...);
.....
destroy_bitmap(video);

Why the hell would you do that?

Just use the video and video1 pointers after and forget about "screen".

--
Thomas Fjellstrom - [website] - [email] - [Allegro Wiki] - [Allegro TODO]
"If you can't think of a better solution, don't try to make a better solution." -- weapon_S
"The less evidence we have for what we believe is certain, the more violently we defend beliefs against those who don't agree" -- https://twitter.com/neiltyson/status/592870205409353730

Simon Parzer
Member #3,330
March 2003
avatar

Quote:

Why the hell would you do that?

Just use the video and video1 pointers after and forget about "screen".

Memory leak? dunno..

Thomas Fjellstrom
Member #476
June 2000
avatar

Not a leak. screen is taken care of by allegro, and you free video and video1 when you're done with them.

--
Thomas Fjellstrom - [website] - [email] - [Allegro Wiki] - [Allegro TODO]
"If you can't think of a better solution, don't try to make a better solution." -- weapon_S
"The less evidence we have for what we believe is certain, the more violently we defend beliefs against those who don't agree" -- https://twitter.com/neiltyson/status/592870205409353730

Simon Parzer
Member #3,330
March 2003
avatar

ok then

Neil Walker
Member #210
April 2000
avatar

Are you sure? If so, are you re saying when I create page1, page2 video bitmaps I should initially set my drawing buffer pointer to page2? perhaps someone could take a note to update the documentation as below from the manual is using video page 0 first:

BITMAP *video_page[2];
      video_page[0] = create_video_bitmap(SCREEN_W, SCREEN_H);
      video_page[1] = create_video_bitmap(SCREEN_W, SCREEN_H);
      current_page = 0;
      ...
      draw_screen(video_page[current_page]);
      show_video_bitmap(video_page[current_page]);
      current_page = (current_page+1)%2;

Following that, the same is happening with triple buffering, should I carry on the same principle and start using page 2?

Neil.
MAME Cabinet Blog / AXL LIBRARY (a games framework) / AXL Documentation and Tutorial

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Thomas Fjellstrom
Member #476
June 2000
avatar

Pretty much I think. the first fullscreen sized page takes the space the visible screen did. and I believe its in the docs somewhere.

--
Thomas Fjellstrom - [website] - [email] - [Allegro Wiki] - [Allegro TODO]
"If you can't think of a better solution, don't try to make a better solution." -- weapon_S
"The less evidence we have for what we believe is certain, the more violently we defend beliefs against those who don't agree" -- https://twitter.com/neiltyson/status/592870205409353730

Neil Walker
Member #210
April 2000
avatar

[edit]That did the trick, I vote the manual should be updated :)

Thanks both of you.

[OLD]
Well my code snippet was from here
http://allegro.cc/manual/api/graphics-modes/show_video_bitmap

and

http://allegro.cc/manual/api/graphics-modes/request_video_bitmap

I'll give my code a try later as it's just a config change to swap between double/paging/triple. I'm also getting a bizarre problem with the allegro trans blender doing the wrong thing at certain alpha levels with paging/triple, but that's another thing for me check once I try out the above :)

Neil.
MAME Cabinet Blog / AXL LIBRARY (a games framework) / AXL Documentation and Tutorial

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Dustin Dettmer
Member #3,935
October 2003
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The docs mention that the first video bitmap is actually the screen. Maybe it should be said in more places so it is easily found?

IIRC its in the create_video_bitmap docs. Maybe show_video_bitmap and request_video_bitmap should also mention this fact.

[edit]

Quote:

I'm also getting a bizarre problem with the allegro trans blender doing the wrong thing at certain alpha levels with paging/triple

While I don't have an answer for that effect, generally doing transparency or alpha blending is a bad idea for anything but a double buffer setup. Reading from the video memory is generally considered a bad idea (which alpha / trans must do).

Neil Walker
Member #210
April 2000
avatar

So that's why my fades seem better in double buffer mode :) I need to re-test my code using fblend, maybe that'll look better given it's slightly faster.

As for the manual, I know it's mentioned, in places, but if people are copying the example code like I did they should change the starting buffer from the 0 index to the 1 index buffer created.

Neil.
MAME Cabinet Blog / AXL LIBRARY (a games framework) / AXL Documentation and Tutorial

wii:0356-1384-6687-2022, kart:3308-4806-6002. XBOX:chucklepie

Dustin Dettmer
Member #3,935
October 2003
avatar

Ah yes, that example should definitely be changed, that makes sense. I'll make a post on AD

Neil Walker
Member #210
April 2000
avatar

According to another thread I already started, to branch this finding, evert mentioned you can't guarantee this either. The only way is to draw to a buffer so you know which one is active. I changed my code to this, which now works a treat.

video paging:

        clear(BitmapPages[1]);
        show_video_bitmap(BitmapPages[1]);
  activePage=0;

triple buffers:

        clear(BitmapPages[1]);
  do{}while(poll_scroll());
  request_video_bitmap(BitmapPages[1]);
  activePage=0;

Neil.
MAME Cabinet Blog / AXL LIBRARY (a games framework) / AXL Documentation and Tutorial

wii:0356-1384-6687-2022, kart:3308-4806-6002. XBOX:chucklepie

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