Three radio buttons are provided for selecting an option for each texture coordinate. The options Repeat, Clamp to Edge and Clamp to Border establish the method for applying the texture map when the one or more texture coordinate is outside the normal range [0.0, 1.0].
Suppose we set the smallest (left) S coordinate to -1.0 and the Repeat option is selected. As seen in the left display window the texels selected for mapping to the left boundary of the rectangle are those on the left edge of a second instance of the image map. the the horizonal dimension of the rectangle is divided into two parts. If instead the smallest S coordinate is set to -0.5, then we see one and one-half instances of the texture in the S direction. This effect is sometimes called TILING. If one of the clamp options is selected, as seen in the left display window, an empty space appears to the left of the image map. If the option is Clamp to Edge, then the space is filled with the texels along the edge of the texture when applied to the rectangle. If the selected option is Clamp to Border, the space is filled with the border color. The Parameter Input window provides the means for setting both the border color and the texture coordinates.
The tutorial allows you to choose between two of the several options available for this property. The two options are replace (GL_REPLACE) and decal (GL_DECAL). The difference in effect between the two is how the ALPHA value of the texels and/or border color are used. In the case of GL_REPLACE, the alpha value is not used and the texels and border color replace the underlying geometry color. In the case of GL_DECAL, the alpha value is applied. A texel's or color's alpha value sets its transparency. The texel and color is fully transparent when its alpha value is 0.0 and fully opaque when it is 1.0. For example, if we want the underlying geometry color is appear in place of the border color, we set the border color's alpha value to 0.0. Blending of the colors will occur when the alpha is between 0.0 and 1.0.
There are two reset buttons, one to reset the texture coordinates to their initial values and one to reset the border color to its initial value white.