FBO render to texture results in white texture

Question

I'm trying to use FBO's for performance improvements and for the gained knowledge by trying out something new, but I have run in following problem: When I render some texture to the back buffer it works perfect (just a bit slow because of multiple layers of textures on top of each other) But when I try to first draw the textures onto another texture using an FBO the new texture always keeps white.

Now to the Code This is my FBOsetup function (called at the beginning (after creating the Opengl context)):

int FBOId;
int FBOTexId;
void setupFBO(){
        IntBuffer buffer = ByteBuffer.allocateDirect(1 * 4).order(ByteOrder.nativeOrder()).asIntBuffer();
    glGenFramebuffersEXT(buffer);
    FBOId= buffer.get();

    FBOTexId=glGenTextures();
    glBindTexture(GL_TEXTURE_2D, FBOTexId);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, tileSize, tileSize, 0, GL_RGB, GL_UNSIGNED_BYTE, ByteBuffer.allocateDirect(4*tileSize*tileSize));

    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, FBOId);
    glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, FBOTexId, 0);
}

Now the render function:

void redraw(){
    glPushMatrix();
    glPushAttrib(GL_VIEWPORT_BIT | GL_COLOR_BUFFER_BIT);

    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, FBOId);

    glLoadIdentity();
    glOrtho(0, tileSize, tileSize, 0, -1, 1);
    glMatrixMode(GL_MODELVIEW);
    glEnable(GL_TEXTURE_2D);

    Main.drawTexture(0, 0, tileSize, tileSize, textureRotation, textureId);

    glBindFramebufferEXT( EXTFramebufferObject.GL_FRAMEBUFFER_EXT, 0);
    glPopAttrib();
    glPopMatrix();
}

And the final draw function:

void draw(){
    Main.drawTexture(x*tileSize, y*tileSize, tileSize, tileSize, FBOTexId);
}

And for all who want it my drawTexture function:

public static void drawTexture(float x, float y, float xSize, float ySize, float rot, int textureId){
    GL11.glBindTexture(GL11.GL_TEXTURE_2D, textureId);
    GL11.glMatrixMode(GL11.GL_TEXTURE);
        GL11.glLoadIdentity();
        GL11.glTranslatef(0.5f,0.5f,0.0f);
        GL11.glRotatef(rot,0.0f,0.0f,1.0f);
        GL11.glTranslatef(-0.5f,-0.5f,0.0f);
    GL11.glMatrixMode(GL11.GL_MODELVIEW);
    GL11.glBegin(GL11.GL_QUADS);
        GL11.glTexCoord2f(0,0);
        GL11.glVertex2f(x,y);
        GL11.glTexCoord2f(1,0);
        GL11.glVertex2f(x+xSize,y);
        GL11.glTexCoord2f(1,1);
        GL11.glVertex2f(x+xSize,y+ySize);
        GL11.glTexCoord2f(0,1);
        GL11.glVertex2f(x,y+ySize);
    GL11.glEnd();
    GL11.glMatrixMode(GL11.GL_MODELVIEW);
}

If I replace the Main.draw line in the draw() function with this Main.drawTexture(x*tileSize, y*tileSize, tileSize, tileSize, textureRotation, textureId); it directly draws the texture to the correct position on the screen, but the problem is that I would always have to redraw all layers of textures and with the FBO's I draw all the layers everytime something changes and then only draw the texture from the FBO.

in your first code block your assigning to FBOTexId, is this a custom function? it should be glGenTextures(1, &FBOTexId). Same with glGenFramebufferEXT? — tamato
There is no reason to pass in info into glTexImage2d last argument, you will be filling that in by drawing to the FBO — tamato
What is your clear color? Can you show the code for when you are drawing to the framebuffer — tamato
You may want to use GL_RGBA as the 7th argument to glTexImage2D just to things consistent with the 3rd argument. — tamato

Ranking Ranking · Accepted Answer · 2013-08-08T10:32:16

Well first thanks for all the comments. I havent realy found the error nor do I understand why this is necessary but this is how it works:

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); 
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

By adding these two lines of code after the glBindTexture(GL_TEXTURE_2D, FBOTextId) in the setupFBO() function everything works fine. If u replace the GL_NEAREST in above lines with GL_LINEAR works too, but it has some strange artifacts when scaling the image.

By the way just a short question which has nothing to do with this thread: Does someone know a way of making parts of a texture (by drawing to it with an fbo) transparant by evaluating the colors from another texture?

FBO render to texture results in white texture

1 Answers