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…
the problem occurs every time the trees cross each other…
correction : the problem occurs every time one tree is behind another, so their pixel-coordinates are crossing on the screen
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:
Disable depth writes when rendering the trees (render the trees last for this one)
Use alpha test, so that fragments below a certain alpha value will be discarded
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.
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…
@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 ).
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…