Jeff Molofee - NeHe's OpenGL Tutorials

glEnable(GL_TEXTURE_2D); // Enable 2D Texture Mapping

return TRUE; // Initialization Went OK

}

Now for the fun stuff. Our drawing code! We start off the same as usual. We clear the background color and the depth buffer. Then we reset the modelview matrix, and translate into the screen 2 units so that we can see our scene.

int DrawGLScene(GLvoid) // Here's Where We Do All The Drawing

{

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // Clear The Screen And The Depth Buffer

glLoadIdentity(); // Reset The Modelview Matrix

glTranslatef(0.0f,0.0f,-2.0f); // Move Into The Screen 5 Units

The first line below selects the 'logo' texture. We'll map the texture to the screen using a quad. We specify our four texture coordinates along with our four vertices.

Revised Description by Jonathan Roy: Remember that OpenGL is a vertex-based graphic system. Most of the parameters you set are recorded as attributes of a particular vertex. Texture coordinate is one such attribute. You simply specify appropriate texture coordinates for each vertex of a polygon, and OpenGL automatically fills in the surface between the vertices with the texture, through a process known as interpolation. Interpolation is a standard geometric technique that lets OpenGL determine how a given parameter varies between vertices just by knowing the value that parameter takes at the vertices themselves.

Like in the previous lessons, we pretend we are facing the quad and assign texture coordinates as follows: (0.0, 0.0) to the bottom-left corner, (0.0, 1.0) to the top-left corner, (1.0, 0.0) to the bottom-right, and (1.0, 1.0) to the top-right. Now, given these settings, can you tell what texture coordinates correspond to the quad's middle point? That's right, (0.5, 0.5). But no where in the code did you specify that coordinate, did you? When it draws the quad, OpenGL computes it for you. And the real magic is that it does so whatever the position, size, or orientation of the polygon!

In this lesson we add another interesting twist by assigning texture coordinates with values other than 0.0 and 1.0. Texture coordinates are said to be normalized. Value 0.0 maps to one edge of the texture, while value 1.0 maps to the opposite edge, spanning the full width or height of the texture image in a one unit step, regardless of the polygon's size or the image's size in pixels (which we therefore don't have to worry about when doing texture mapping, and that makes life a whole lot easier). Above 1.0, the mapping simply wraps around at the other edge and the texture repeats. In other words, texture coordinate (0.3, 0.5) for instance, maps to the exact same pixel in the texture image as coordinate (1.3, 0.5), or as (12.3, –2.5). In this lesson, we achieve a tiling effect by specifying value 3.0 instead of 1.0, effectively repeating the texture nine times (3×3 tiling) over the surface of the quad.

Additionally, we use the roll variable to translate (or slide) the texture over the surface of the quad. A value of 0.0 for roll, which is added to the vertical texture coordinate, means that texture mapping on the bottom edge of the quad begins at the bottom edge of the texture image, as shown in the figure on the left. When roll equals 0.5, mapping on the bottom edge of the quad begins halfway up in the image (see figure on the right). Rolling textures can be used to create great effects such as moving clouds, words spinning around an object, etc.

glBindTexture(GL_TEXTURE_2D, texture[0]); // Select Our Logo Texture

glBegin(GL_QUADS); // Start Drawing A Textured Quad

glTexCoord2f(0.0f, –roll+0.0f); glVertex3f(-1.1f, –1.1f, 0.0f); // Bottom Left

glTexCoord2f(3.0f, –roll+0.0f); glVertex3f( 1.1f, –1.1f, 0.0f); // Bottom Right

glTexCoord2f(3.0f, –roll+3.0f); glVertex3f( 1.1f, 1.1f, 0.0f); // Top Right

glTexCoord2f(0.0f, –roll+3.0f); glVertex3f(-1.1f, 1.1f, 0.0f); // Top Left

glEnd(); // Done Drawing The Quad

Anyways… back to reality. Now we enable blending. In order for this effect to work we also have to disable depth testing. It's very important that you do this! If you do not disable depth testing you probably wont see anything. Your entire image will vanish!

glEnable(GL_BLEND); // Enable Blending

glDisable(GL_DEPTH_TEST); // Disable Depth Testing

The first thing we do after we enable blending and disable depth testing is check to see if we're going to mask our image or blend it the old fashioned way. The line of code below checks to see if masking is TRUE. If it is we'll set up blending so that our mask gets drawn to the screen properly.

if (masking) // Is Masking Enabled?

{

If masking is TRUE the line below will set up blending for our mask. A mask is just a copy of the texture we want to draw to the screen but in black and white. Any section of the mask that is white will be transparent. Any sections of the mask that is black will be SOLID.

The blend command below does the following: The Destination color (screen color) will be set to black if the section of our mask that is being copied to the screen is black. This means that sections of the screen that the black portion of our mask covers will turn black. Anything that was on the screen under the mask will be cleared to black. The section of the screen covered by the white mask will not change.

glBlendFunc(GL_DST_COLOR,GL_ZERO); // Blend Screen Color With Zero (Black)

}

Now we check to see what scene to draw. If scene is TRUE we will draw the second scene. If scene is FALSE we will draw the first scene.

if (scene) // Are We Drawing The Second Scene?

{

We don't want things to be too big so we translate one more unit into the screen. This reduces the size of our objects.

After we translate into the screen, we rotate from 0-360 degrees depending on the value of roll. If roll is 0.0 we will be rotating 0 degrees. If roll is 1.0 we will be rotating 360 degrees. Fairly fast rotation, but I didn't feel like creating another variable just to rotate the image in the center of the screen. :)

glTranslatef(0.0f, 0.0f, -1.0f); // Translate Into The Screen One Unit

glRotatef(roll*360, 0.0f, 0.0f, 1.0f); // Rotate On The Z Axis 360 Degrees

We already have the rolling logo on the screen and we've rotated the scene on the Z axis causing any objects we draw to be rotated counter-clockwise, now all we have to do is check to see if masking is on. If it is we'll draw our mask then our object. If masking is off we'll just draw our object.

if (masking) // Is Masking On?

{

If masking is TRUE the code below will draw our mask to the screen. Our blend mode should be set up properly because we had checked for masking once already while setting up the blending. Now all we have to do is draw the mask to the screen. We select mask 2 (because this is the second scene). After we have selected the mask texture we texture map it onto a quad. The quad is 1.1 units to the left and right so that it fills the screen up a little more. We only want one texture to show up so our texture coordinates only go from 0.0 to 1.0.

After drawing our mask to the screen a solid black copy of our final texture will appear on the screen. The final result will look as if someone took a cookie cutter and cut the shape of our final texture out of the screen, leaving an empty black space.

glBindTexture(GL_TEXTURE_2D, texture[3]); // Select The Second Mask Texture

glBegin(GL_QUADS); // Start Drawing A Textured Quad