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OpenGL strangeness w / transparency and Z buffer (2d map editor)

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hi, ive been working on my map editor lately for my tile based 2d RPG. it's made w /C++ and SDL. anyway, on to the point. ive recently added a "show solidity" check-box to my editor. basically, this will show if a tile is Solid or Nonsolid by drawing an S or an N over the tile. anyway, to do this was simple. each frame, when i draw the visible tiles of the map, i check if "show solidity" is checked off, and if so, i check what type of tile this is. if its SOLID, i draw a quad here with the SOLID texture. if its NONSOLID, i draw the quad here with the nonsolid texture. anyway,im getting a very weird problem which bothers me and makes me think i still dont fully understand the very basics of OpenGL. first i have to give more info. Each tile can contain up to 5 different layers, each layer having a varying Z value. i have znear = -10 and zfar = 10, so i can use any Z value bettweeen -10 and 10. i stick to values between -2 and 2 for tiles and objects, though, since 5 layers is more then enough. for the buttons and the S and N tiles, and everything else, i draw with a Z value of 5 so that they are automatically drawn on top of the other stuff. besides having tiles, im able to place a "free floating object". this free floating map object is not a tile, but a regular quad with a texture, drawn each frame. this free floating object also can have a Z value ranging from -2 to 2, which represents its layer. here comes the problem: when "show solidity" is checked off, all of a suddent my free floating map objects disapear!!! it doesnt make any sence! ive ran through my code a dozen times, and i know that all the variables are what i think they are. ive tried disabling different states to see if that effected anything. basically, for some reason, if "show solidity" is checked off, my free floating objects just vanish. first, heres a pic of the S and N textures. these are what i bind to a quad and render over a quad if "show solidity" is marked off. now, heres a screenshot of the editor, with "show solidity" not checked off: everything is perfect. the building you see there is a free floating map object. its Z value is 0. also note that all the tiles in the scene are also 0, too. so everything here's Z value is 0, EXCEPT the blue quad you see drawn in the center of the building. this is a blue quad, whos Z value is 5, and i draw him in the center of the building so that i can know where the objects "center" is (for sorting purposes). now, look what happends when i check off "show solidity" see what happends? all of a sudden, my building disapears! it doesnt make sence... i mean, if the building disapeared, why didnt the tiles disapear too? they all share the same Z value. i dont get. i should also note that the 'S' and 'N' textures are ALWAYS drawn with the Z value of 5. this means that it *should* be drawn over everything. apparently, it is, but where is my building? since the texture is transparent, the S and N images should simply just be drawn over the building, like they are drawn over the rest of the tiles... i dont get it. in psuedo code, heres what my rendering loop looks like:

for(int y = 0; y < visable_y_tiles; y++)
{
     for(int x = 0; x < visable_x_tiles; x++)
     {
        
        //DRAWN WITH A Z VALUE OF 0
        draw_tile(map[y][x]);
  
        if(show_solidity)
        {
           //THESE ARE DRAWN WITH A Z VALUE OF 5
            if(map[y][x] IS SOLID)
               draw_quad(solid_texture,x*32,y*32);
           else draw_quad(nonsolid_texture,x*32,y*32);
         }
      }
}

for(each free floating map object)
{
    obj.Render();  <--------- DRAWS WITH A Z VALUE OF 0
}



 ---------- NOW SOMEWHERE ELSE IN THE CODE ---------------


Free_Floating_Map_Object::Render()
{

   //DRAWN WITH A Z VALUE OF 0
    draw_quad(my_texture,x,y);
   
     //DRAWN WITH A Z VALUE OF 5
    draw_blue_quad(my_center_position);
}




sorry for such a long post. this is one of those weird bugs that take a long time to explain. i hope im missing something obvious. in the psuedo code i show the order in which i drawn things and the Z value each thing has. the way i have it set up is closer to 10 means closer to the screen, and further from 10 means further into the screen. thanks alot for any help!!

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Guest Anonymous Poster
What do you have your near/far values set in gluPerspective()?

This could be a z-buffer resolution issue. If your far plane is set to something really large, or near is something really small, then the precision of your z-values near your image plane might be much worse than 1. If your quad at 5 was getting mapped to the same depth as a quad at 0, that could explain it.

This seems a little weird, but that was my first thought.

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Guest Anonymous Poster
When you said you have znear = -10, zfar = 10, are those the near/far plane values for gluPerspective?

If they are, then you are misunderstanding what they are. znear can NEVER be zero or negative. You can set it to something like .001.

What this means is the distance from the camera to the image plane. It doesn't make sense to have an image plane behind the camera. If you want a range from -10 to 10, set your camera at -11, near at 1, and far at 21.

If this is what you have done, I appologize. If not, try this and see if it helps.

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hey anonymous, thanks for the reply. actually, im using those values in the call to glOrtho() and set my zNear to -10 and zFar to 10,and i make 0,0 the top left corner and w/h of screen res the bottom right corner.... I dont use gluPerspective at all.. i probably should have mentioned that in the post [smile]. thanks for anymore help.

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Hmm....is it possible that you still have the S or N texture bound when you draw your tiles?

Have you tried reversing the order in which you draw the tiles that dissapear and the tiles that remain visible?

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hi renderer, i think your a little confused on whats happening. i think its due to the shrunken screenshot. anyway, no tiles are disapearing. the building is not a bunch of tiles, it is a single quad that represents a "free floating map object". this FFMO is disapearing when i want to "view solidity". checking off "view solidity" just means to draw an S or an N quad over each tile based on if they were solid or non solid. it just allows me to see if a tile is solid or not.

anyway, when i im showing solidity, for some reason my building disapears. im not sure why, since, the S and N quads are on a much higher Z level (meaning they should be drawn over the building), so if they are above the building, how come i cant see the building given that the S and N textures are transparent? in fact, i can even see the tiles that are underneath the building. the building and the tiles both have a Z of 0 and the S and N have a Z of 5. also, the tiles are drawn first, so the building *should* be on top of the tiles. it just doesnt really make sence... where did my building go?

to give you a better idea, heres a non shrunked screeny. first, this is what the map looks like in normal view, without "showing solidity"



next, heres what it looks like when i "view solidity"



do you see what im talking about? the building just vanishes! given the fact the building and tiles both have the same Z value, and the building is drawn after the tiles, i should be seeing a building right now!!

anyway, heres what it should look like, although in the real version there would be no tiles drawn over the building since the building and tiles share the same Z and the building is drawn last. i made this in Paint Shop Pro:



thanks for any help

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Hmm...can't tell if it's OpenGL or possibly just a bug somewhere in your code. I'd try rearanging the order you draw...draw the building before the S/N quads and see what happens.

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ok, wow, im so confused...

i took your advise and tried rendering the S/N quad AFTER i render the building. this worked! i can see my building now, the S/N is drawn over the building, and i can still see the building since the S/N has a transparent background.

this doesnt make any sence! why would i have to draw the S/N AFTER the building? why cant i draw it before? i have verified the Z values, so i dont get it... to sum it up:

-draw my tiles at 0 Z value
-draw my S/N at 5 Z value
-draw my building at 0 Z value

the building disapears, even through the S/N has a transparent background!

-draw my tiles at 0 Z value
-draw my building at 0 Z value
-draw my S/N at 5 Z value

this works! the S/N is drawn over the building, and since it has a transparent background, i can still see the building.

why does my building disapear using the first method? why dont my tiles disapear, too, since they share the same Z value? well, i guess they dont disapear because they are drawn before the S/N, but like i said, the S/N is transparent, so anything under it, i should see... it just doesnt make any sence.

thanks a lot for any further help. i'd really like to figure out why this is happening. i dont like not understanding something as important as the basics of rendering. does it have to do with transparency or something? the other thing is, i dont want to have to draw my S/N after i draw the buildings. I'd like to draw the S/N immediately after i draw the tiles, since they share the same rendering loop. i dont want to have to draw in 2 passes. thanks again.

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Quote:
this doesnt make any sence! why would i have to draw the S/N AFTER the building? why cant i draw it before? i have verified the Z values, so i dont get it... to sum it up:


You do have depth checking enabled, right?

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you mean depth testing, right? then yes, i do have that enabled, otherwise i dont think all the other stuff that messes with the Z axis would be working (and it is...). heres all the code i use to set-up OpenGL.


void System::Init_OpenGL()
{

/* Enable Texture Mapping ( NEW ) */
glEnable(GL_TEXTURE_2D);

/* Enable smooth shading */
glShadeModel(GL_SMOOTH);

/* Set the background black */
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

/* Depth buffer setup */
glClearDepth(1.0f);

/* Enables Depth Testing */
glEnable(GL_DEPTH_TEST);

/* The Type Of Depth Test To Do */
glDepthFunc(GL_LEQUAL);

/* Really Nice Perspective Calculations */
glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);
glHint(GL_POINT_SMOOTH_HINT,GL_NICEST);

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

/* Setup our viewport. */
glViewport(0,0,SCREEN_RW,SCREEN_RH);

/* change to the projection matrix and set our viewing volume. */
glMatrixMode(GL_PROJECTION);

glLoadIdentity();


glOrtho(0,SCREEN_RW,SCREEN_RH,0,-10.0f,10.0f);


glMatrixMode(GL_MODELVIEW);

/* Reset The View */
glLoadIdentity();


// DISABLE V SYNC !!!
typedef void (APIENTRY * WGLSWAPINTERVALEXT) (int);
WGLSWAPINTERVALEXT wglSwapIntervalEXT = (WGLSWAPINTERVALEXT) wglGetProcAddress("wglSwapIntervalEXT");

if (wglSwapIntervalEXT)
wglSwapIntervalEXT(0); // disable vertical synchronisation


}

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