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Extrakun

OpenGL Do I need to unbind a texture after drawing a textured primitive?

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Extrakun    150
Hi, I am drawing two primitives; one is bind to a texture, the other is not. However, somehow, the second primitive takes on the texture of the first. I've recorded an animation of what is happening and put it on youtube. As you can see, whenever the textured quad changes the uv coordinate (the numbers are actually from one piece of texture), so does the colour of the cat. Both are enclosed within their own glBegin and glEnd... The code is as such (using Tao.OpenGL)
// For the textured quad
 public void Draw(Texture drawable, float gameTime, float x, float y, float UVStartX, float UVStartY,
                float UVWidth, float UVHeight)
        {
            Gl.glPushMatrix();
            Gl.glTranslatef(x, y, 0.0f);

            // TODO: Calculate world height and world width
            float worldHeight = 1.0f;
            float worldWidth = 1.0f;

            // Bind the texture
            Gl.glBindTexture(Gl.GL_TEXTURE_2D, drawable.TextureID);

            // Begin drawing the quad
            Gl.glBegin(Gl.GL_TRIANGLES);

            // First triangle, first point
            Gl.glTexCoord2f(UVStartX, UVStartY);
            Gl.glVertex3f(0.0f, 0.0f, 0.0f);

            // First triangle, second point
            Gl.glTexCoord2f(UVStartX, UVStartY + UVHeight);
            Gl.glVertex3f(0.0f, worldHeight, 0.0f);

            // First triangle, third point
            //Gl.glTexCoord2f(UVStartX + UVHeight, UVStartY);
            Gl.glTexCoord2f(UVStartX + UVWidth, UVStartY);
            Gl.glVertex3f(worldWidth, 0.0f, 0.0f);

            // Second triangle, first point
            Gl.glTexCoord2f(UVStartX + UVWidth, UVStartY);
            Gl.glVertex3f(worldWidth, 0.0f, 0.0f);

            // Second triangle, second point
            Gl.glTexCoord2f(UVStartX, UVStartY + UVHeight);
            Gl.glVertex3f(0.0f, worldHeight, 0.0f);

            // Second triangle, third point
            Gl.glTexCoord2f(UVStartX + UVWidth, UVStartY + UVHeight);
            Gl.glVertex3f(worldWidth, worldHeight, 0.0f);           
            Gl.glEnd();            

            Gl.glPopMatrix();
           


And the code which draws the 'cat' is actually all triangle primitives
public void Draw(float timeElapsed)
        {
            // Store previous settings
            Gl.glPushMatrix();

            // Translate to location            
            Gl.glTranslatef(posX_, posY_, 0f);            

            // Animation consists of frame; now we just want to first frame
            CacFrame toRender = animation_.Frames[currentFrame_];

            // NOTE: A frame consist of multiple strokes; a stroke consist of multiple vertices and triangles
            // Begin to draw triangles
            Gl.glBegin(Gl.GL_TRIANGLES);

            foreach (CacStroke s in toRender.Strokes)
            {
                for (int strokeIndex = 0; strokeIndex < s.Vertices.Count; strokeIndex++)
                {
                    // Get the triangles that form the stroke
                    List<IndexedTriangle> triangles = s.Triangles;

                    // Get the vertices that form the stroke
                    List<DiffuseVertex> vertices = s.Vertices;

                    // Draw all the triangles
                    foreach (IndexedTriangle eachTriangle in triangles)
                    {
                        for (int i = 0; i < 3; i++)
                        {
                            // extract the diffuse vertex
                            DiffuseVertex currentVertex = vertices[eachTriangle.index[i]];

                            Gl.glColor4f(currentVertex.R, currentVertex.G, currentVertex.B, currentVertex.A);
                            Gl.glVertex3f(currentVertex.X, currentVertex.Y, currentVertex.Z);
                        }
                    }
                }
            }

            Gl.glEnd();

            // Restore OpenGL setting
            Gl.glPopMatrix();


I call Gl.glEnd() after drawing each of the primitive. Do I need to 'unbind' the texture so that the cat will not change colour? (It's supposed to be white).

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AndyEsser    394
OpenGL is a state machine, that is, if you set something, it stays that way until you unset it.

In the case above, you are binding a texture to one object, but it is continuing to stay bound, even though the model you are drawing is untextured. Therefore it is taking the previous state (the bound texture) and using that on the model.

There are two ways to get around this.

Either gall glBindTexture with the 2nd arguement being NULL, this will unbind any bound texture. Or simply call glDisable(GL_TEXTURE2D), which will disable all texturing, however you will need to remember to call glEnable(GL_TEXTURE2D) before drawing the object with the texture.

Hope this helps.

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Brother Bob    10344
Quote:
Original post by AndyEsser
There are two ways to get around this.

Either gall glBindTexture with the 2nd arguement being NULL, this will unbind any bound texture. Or simply call glDisable(GL_TEXTURE2D), which will disable all texturing, however you will need to remember to call glEnable(GL_TEXTURE2D) before drawing the object with the texture.

Hope this helps.


Binding texture ID zero will bind texture ID zero. Texture ID zero is the default texture and is a perfectly valid texture object to use for texturing (you can upload an image to it and use it). That is a very bad way to "disable" texturing, becuase texturing is still enabled and in use.

If you want to disable texturing, then my advice is to... disable texturing.

edit: Just a minor adjustmet on 0 being a texture object. It's not, actually. It is the way to disable texture objects and fall back to old style texture management, but for the purpose of explaining, it behaves as a default object that cannot be deleted.

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Extrakun    150
Thanks for the answer!

I tried disabling texture before the drawing the primitives which I only want to have the diffuse colors and it works; I just have to remember re-enabling the texturing before the drawing the quad.

On a separate note, will multiple glEnable(GL_TEXTURE_2D) cause lost in performance? I am thinking of moving the texture enabling and disabling into the Draw() method of the sprite itself.

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