problem with mass-masking

[HamsterofDeath]

my problem :
i wanted to draw some 2d-trees, so i did this :

glblend on
draw mask of tree (only a black "shadow" of the tree visible)
draw the tree on the mask

-> everything looks good.

no problem so far. but if i draw a second tree behind the first one, the invisible part of the nearest mask (in this case, the first tree) contained the background i've drawn before - the second tree was ...i dont' know what happened to him. everything inside the first trees rectangle was simply not drawn...

[HamsterofDeath]

the problem occurs every time the trees cross each other...

[HamsterofDeath]

correction : the problem occurs every time one tree is behind another, so their pixel-coordinates are crossing on the screen

[Dodger]

If you're using alpha blending, the depth of completely transparent fragments in your first tree will still be written into the depth buffer, so rendering a second tree behind the first one will fail depth test on those pixels on screen. There are three possible solutions for this problem:

1. Disable depth writes when rendering the trees (render the trees last for this one)

2. Use alpha test, so that fragments below a certain alpha value will be discarded

3. Sort your trees back-to-front

Also, you don't need to render a 'mask' first and then the tree texture on top of that. Using a texture with an alpha channel should be sufficient.

[HamsterofDeath]

thx, but :

@3 : how can i reverse the order ? is there an opengl function or do i have to sort them manually ?
@1 : how do disable depth write ?
@2 : how to add an alpha-channel to a texture with paint shop pro ? i didn't find any button for this...

[Dakeyras]

@3 : how can i reverse the order ? is there an opengl function or do i have to sort them manually ?

manually.

@1 : how do disable depth write ?

glDepthMask( GL_FALSE );
Note that with the depth mask disabled your trees may be incorrectly drawn on top of each other, once again depending on draw order. However, your trees will be depth-tested correctly against the rest of the scene.

@2 : how to add an alpha-channel to a texture with paint shop pro ? i didn't find any button for this...

I don't use PSP so I don't know about this. Since you seem to already have a mask and an RGB texture, you can programatically combine them by using 4 bytes per pixel, and copying the first 3 bytes from the RGB texture and the 4th from the mask/alpha texture.

That will give you RGBA. For texture loading performance, you might want to flip the colours and use BGRA ( GL_BGRA_EXT ).

[HamsterofDeath]

depth writes off solved my problem...
but i'm worried about the manual depth sort..
that would need a lot of calculations, and i'm using java ^^
maybe it's faster to draw a gl_polygon as an "outline" of the 2d-object so the invisible part is not depth-written


the more opengl does, the lesser java slows down the graphics...