Sign in to follow this  
slabix

OpenGL Alpha blending and z-ordering.

Recommended Posts

Hi.

There's a problem I've been working on for a couple of days. I'm using OpenGL with SDL in C++.

I have three objects: Player, Map (based on tileset) and Background. Player's depth is equal to 100 (which is the foreground), Map's is about 50, and Background's is equal to 25. For now, background is just filled rectangle, and tilemap is quite random. Here's what i get:
1: http://img846.imageshack.us/img846/9745/79045196.png , 2: http://img801.imageshack.us/img801/3808/38667626.png and 3: http://img823.imageshack.us/img823/1944/45278783.png

Transparent pixels of player has alpha channel set to 0, but still - he's making a black box, that's covers the Map, but doesn't cover the Background.

Here are my OpenGL settings:
[spoiler]
[code]
SDL_GL_SetAttribute( SDL_GL_RED_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_GREEN_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_BLUE_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_ALPHA_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_DEPTH_SIZE, 16 );
SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );
// Projekcja - otworzenie planszy opengl.
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, win_w, win_h, 0, -100, 0);
// Utworzenie, hmm... drugiej planszy
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
// Ustawienia:
glEnable(GL_TEXTURE_2D); // W??czenie teksturowania 2D.
glShadeModel(GL_SMOOTH); // W??czenie smootha.
glEnable(GL_BLEND); // W??czenie blendowania.
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // Ustawienia blendowania.
glClearColor(0, 0, 0, 1); // Ustawienie czystego koloru.
glClearDepth(100.0); // Ustawienie g??boko?ci.
glEnable(GL_DEPTH_TEST); // W??czenie testowania g??boko?ci.
glDepthFunc(GL_LEQUAL); // Ustawienie testowania g??boko?ci.
glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST); // Ustawienie ?adniejszej perspektywy.[/code][/spoiler]

Here's code that sets the alpha transparency: (also - preconfigured alpha pixels (for example in Gimp) works the same):

[spoiler][code]// Utworzenie koloru przezroczysto?ci, ustawiaj?c kana? alpha dla ka?dego zielonego piksela na 255.
SDL_Color color;
for (int y = 0; y < optimized->h; y++)
{
for (int x = 0; x < optimized->w; x++)
{
unsigned int pix = ((unsigned int*)optimized->pixels)[y*(optimized->pitch/sizeof(unsigned int)) + x];
#if SDL_BYTEORDER == SDL_BIG_ENDIAN
color = {(pix & 0x00ff0000)/0x10000,(pix & 0x0000ff00)/0x100,(pix & 0x000000ff),0};
#else
color = {(pix & 0x000000ff),(pix &0x0000ff00)/0x100,(pix & 0x00ff0000)/0x10000,0};
#endif
if (color.r == 0 and color.g == 255 and color.b == 0)
{
((unsigned int*)optimized->pixels)[y*(optimized->pitch/sizeof(unsigned int)) + x] = SDL_MapRGBA(optimized->format, 0, 0, 0,0);
}
}
}[/code][/spoiler]

Here's my code for drawing a quad (like the Background):
[spoiler][code]void draw_rect(GLfloat x, GLfloat y, GLfloat w, GLfloat h, GLfloat z, GLfloat r)
{
glDisable(GL_TEXTURE_2D);
glRotatef(r,0.0,0.0,1.0);
glTranslatef(x, y, z); // Ustawienie g?ównego punktu..
glBegin(GL_QUADS); // Rozpoczecie rysowanie zape?nionego kwadratu.
// Cztery rogi.
glVertex3f(0, 0, 0);
glVertex3f(w, 0, 0);
glVertex3f(w, h, 0);
glVertex3f(0, h, 0);
//Zako?czenie rysowania.
glEnd();
glLoadIdentity();
glEnable(GL_TEXTURE_2D);
}[/code][/spoiler]

Texture loading function:
[spoiler][code]void generate_texture(Sprite * s)
{
glGenTextures(1, &(s->texture_id));
// Format tekstury.
GLenum texture_format;
// Sprawdzenie formatu tekstury.
if (s->sprite_surface->format->BytesPerPixel == 4) // Je?li jest kana? alpha.
if (s->sprite_surface->format->Rmask == 0x000000ff) texture_format = GL_RGBA;
else texture_format = GL_BGRA;
else
if (s->sprite_surface->format->Rmask == 0x000000ff) texture_format = GL_RGB;
else texture_format = GL_BGR;
// Ustawienia tekstury.
glBindTexture(GL_TEXTURE_2D, s->texture_id); // Wybranie.
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); // Ust. rozci?gania.
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

// Wygenerowanie tekstury.
glTexImage2D(GL_TEXTURE_2D, 0, s->sprite_surface->format->BytesPerPixel, s->sprite_surface->w, s->sprite_surface->h, 0,
texture_format, GL_UNSIGNED_BYTE, s->sprite_surface->pixels);
glLoadIdentity();
}[/code][/spoiler]

And, at last, my texture blitting function: (I already have all my images 256x256, so I don't have to resize the pixeldatas)
[spoiler][code]// Wy?wietlenie tekstury.
void draw_texture(Sprite *s)
{
// Wybór tekstury.
glBindTexture(GL_TEXTURE_2D, s->texture_id);
// Ustawienia.
glRotatef(s->angle, 0.0,0.0,1.0);
glTranslatef(s->x, s->y, s->z);
glColor4f(static_cast<GLfloat>(s->color.r)/255, static_cast<GLfloat>(s->color.g)/255,
static_cast<GLfloat>(s->color.b)/255, 1.0);//static_cast<GLfloat>(s->alpha)/255);
// Rozpocz?cie rysowania prostok?ta.
float mx = (s->clip_x/static_cast<float>(s->width));
float my = (s->clip_y/static_cast<float>(s->height));
float mw = (s->clip_width/static_cast<float>(s->width));
float mh = (s->clip_height/static_cast<float>(s->height));
glBegin(GL_QUADS);
// Górny lewy róg.
glTexCoord2f(mx, my); glVertex3d(s->zoom*(-s->center_x), s->zoom*(-s->center_y), 0);
// Górny prawy róg.
glTexCoord2f(mx+mw, my); glVertex3d(s->zoom*(s->clip_width-s->center_x), s->zoom*(-s->center_y), 0);
// Dolny prawy róg.
glTexCoord2f(mx+mw, my+mh); glVertex3d(s->zoom*(s->clip_width-s->center_x), s->zoom*(s->clip_height-s->center_y), 0);
// Dolny lewy róg.
glTexCoord2f(mx, my+mh); glVertex3d(s->zoom*(-s->center_x), s->zoom*(s->clip_height-s->center_y), 0);
glEnd();
glLoadIdentity();
}[/code][/spoiler]


I already tried to change depth of my objects, I tried to change the color of transparency (and did it manually throught Gimp) and tried almost all of the DepthFunc and BlendFunct settings.

Anybody have any ideas, why my texture still gives that strange, black box which covers the map?

Share this post


Link to post
Share on other sites
Where's the code that shows the order in which you are drawing the background, the map and the player?

EDIT: I ask, because drawing order can affect things when alpha-blending. If you draw the player before you draw the map, then the depth fragments output by the transparent portions of the player can keep the map from being drawn. If you draw the player after you draw the map, this shouldn't be a problem.

Share this post


Link to post
Share on other sites
Ouch.

I thought it doesn't matter, because I have set their z value.

Hmm... So if I want some tiles to be below the player, I have to draw them first, and if I want them to be on top of him, I have to draw them later? Too bad, I was sure that opengl has kind of automatic z-ordering, that allows me to draw'em in any order. That makes me to reorganize whole Map class (I have some tiles below and some above the player).

Ech, anyway, thanks, It's working now...

[spoiler][code] GL::set_color(1.0,0.0,0.0,0.5);
GL::draw_rect(0,0,400,400,25);
player.draw(); // <--
map.draw(); // <-- I switch'd them[/code][/spoiler]

Share this post


Link to post
Share on other sites
Z-buffering works great for solid geometry, but transparency has always played hell with it, since transparent pixels still output depth. You can use [url="http://www.opengl.org/sdk/docs/man/xhtml/glAlphaFunc.xml"]glAlphaFunc[/url] (and call glEnable(GL_ALPHA_TEST) )to actually snip out pixels whose alpha is 0, in which case they won't output a depth fragment. However, if the edges of your sprites are anti-aliased for a smoother blend, then this can result in a halo being drawn around your characters, since those partially-transparent pixels will still output depth and will still need to be properly blended with their background. Generally speaking, whenever alpha transparency or alpha blending is involved, the objects need to be sorted and drawn back to front.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Forum Statistics

    • Total Topics
      627762
    • Total Posts
      2978970
  • Similar Content

    • By DelicateTreeFrog
      Hello! As an exercise for delving into modern OpenGL, I'm creating a simple .obj renderer. I want to support things like varying degrees of specularity, geometry opacity, things like that, on a per-material basis. Different materials can also have different textures. Basic .obj necessities. I've done this in old school OpenGL, but modern OpenGL has its own thing going on, and I'd like to conform as closely to the standards as possible so as to keep the program running correctly, and I'm hoping to avoid picking up bad habits this early on.
      Reading around on the OpenGL Wiki, one tip in particular really stands out to me on this page:
      For something like a renderer for .obj files, this sort of thing seems almost ideal, but according to the wiki, it's a bad idea. Interesting to note!
      So, here's what the plan is so far as far as loading goes:
      Set up a type for materials so that materials can be created and destroyed. They will contain things like diffuse color, diffuse texture, geometry opacity, and so on, for each material in the .mtl file. Since .obj files are conveniently split up by material, I can load different groups of vertices/normals/UVs and triangles into different blocks of data for different models. When it comes to the rendering, I get a bit lost. I can either:
      Between drawing triangle groups, call glUseProgram to use a different shader for that particular geometry (so a unique shader just for the material that is shared by this triangle group). or
      Between drawing triangle groups, call glUniform a few times to adjust different parameters within the "master shader", such as specularity, diffuse color, and geometry opacity. In both cases, I still have to call glBindTexture between drawing triangle groups in order to bind the diffuse texture used by the material, so there doesn't seem to be a way around having the CPU do *something* during the rendering process instead of letting the GPU do everything all at once.
      The second option here seems less cluttered, however. There are less shaders to keep up with while one "master shader" handles it all. I don't have to duplicate any code or compile multiple shaders. Arguably, I could always have the shader program for each material be embedded in the material itself, and be auto-generated upon loading the material from the .mtl file. But this still leads to constantly calling glUseProgram, much more than is probably necessary in order to properly render the .obj. There seem to be a number of differing opinions on if it's okay to use hundreds of shaders or if it's best to just use tens of shaders.
      So, ultimately, what is the "right" way to do this? Does using a "master shader" (or a few variants of one) bog down the system compared to using hundreds of shader programs each dedicated to their own corresponding materials? Keeping in mind that the "master shaders" would have to track these additional uniforms and potentially have numerous branches of ifs, it may be possible that the ifs will lead to additional and unnecessary processing. But would that more expensive than constantly calling glUseProgram to switch shaders, or storing the shaders to begin with?
      With all these angles to consider, it's difficult to come to a conclusion. Both possible methods work, and both seem rather convenient for their own reasons, but which is the most performant? Please help this beginner/dummy understand. Thank you!
    • By JJCDeveloper
      I want to make professional java 3d game with server program and database,packet handling for multiplayer and client-server communicating,maps rendering,models,and stuffs Which aspect of java can I learn and where can I learn java Lwjgl OpenGL rendering Like minecraft and world of tanks
    • By AyeRonTarpas
      A friend of mine and I are making a 2D game engine as a learning experience and to hopefully build upon the experience in the long run.

      -What I'm using:
          C++;. Since im learning this language while in college and its one of the popular language to make games with why not.     Visual Studios; Im using a windows so yea.     SDL or GLFW; was thinking about SDL since i do some research on it where it is catching my interest but i hear SDL is a huge package compared to GLFW, so i may do GLFW to start with as learning since i may get overwhelmed with SDL.  
      -Questions
      Knowing what we want in the engine what should our main focus be in terms of learning. File managements, with headers, functions ect. How can i properly manage files with out confusing myself and my friend when sharing code. Alternative to Visual studios: My friend has a mac and cant properly use Vis studios, is there another alternative to it?  
    • By ferreiradaselva
      Both functions are available since 3.0, and I'm currently using `glMapBuffer()`, which works fine.
      But, I was wondering if anyone has experienced advantage in using `glMapBufferRange()`, which allows to specify the range of the mapped buffer. Could this be only a safety measure or does it improve performance?
      Note: I'm not asking about glBufferSubData()/glBufferData. Those two are irrelevant in this case.
    • By xhcao
      Before using void glBindImageTexture(    GLuint unit, GLuint texture, GLint level, GLboolean layered, GLint layer, GLenum access, GLenum format), does need to make sure that texture is completeness. 
  • Popular Now