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Grokking The Gimp
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8.3 The Emboss and Bump map Filters

There are two filters that are particularly useful for rendering. These are Emboss and Bump map.

The Emboss filter, found in Image:Filters/Distorts, creates the effect that an image is embossed into a thin metal plate. Figure  8.9

  
Figure 8.9: The Emboss Filter Dialog
Figure 8.9

illustrates the use of the filter for a very simple case. Figure  8.9(a) shows an image that was created using a radial gradient, and Figure  8.9(b) shows the corresponding dialog for the Emboss filter. The dialog consists of a preview, two radio buttons labeled Emboss and Bumpmap, and three sliders called Azimuth, Elevation, and Depth.

The three sliders in the filter dialog control the embossing effect, and Figure  8.10

  
Figure 8.10: Geometric View of the Emboss Filter Options
Figure 8.10

shows a 3D scene that defines the effect of each one. In the scene there is a pyramid object illuminated by a light  source. Two angles, the Azimuth and Elevation, define the position of the light with respect to the pyramid. The Azimuth describes the light source's angular position in the horizontal plane, similar to a compass bearing. The Elevation is the angular measure of the source above the plane.

When the light source is on one side of the object, the other side must be in shadow, as shown in Figure  8.10. When the Elevation is 90o, the light source is positioned vertically over the object, and all faces are equally well illuminated, which eliminates the shadow. There is, however, more reflected light coming from surfaces that are perpendicular to the rays of light than from those that are oblique. Thus, Azimuth and Elevation control the direction of light and shadow.

``Wait a minute! That's fine for a 3D object,'' you might say. ``But we are working with flat images...how does Emboss determine the relative angles for something that, in reality, is completely flat?'' I was hoping you would ask. Figure  8.11

  
Figure 8.11: How Emboss is Applied to a 2D Image: Magnitudes of the Gradient Vectors
Figure 8.11

shows the radial gradient from Figure  8.9(a) in a 3D context. That is, the image is 2D, but an imaginary light source is positioned over it at some azimuth and elevation. The little red arrows shown in the image plane explain how the filter embosses the image.

The red arrows, called gradient vectors,  are computed for each pixel in the image (only three are shown in Figure  8.11). For each pixel, the gradient is drawn in the direction that pixel values decrease the fastest, and the length of the arrow is determined by the amplitude of that change. Thus, in Figure  8.11, there are two arrows each pointing from their pixels in the direction of greatest change, and one is longer than the other because its slope of change is greater. The third arrow is just a dot because it is located in a region where there is no change in pixel value. Its length is zero.

The Emboss filter works by computing the amount each pixel's gradient vector projects along the vector from the pixel to the light source. You can see that the gradient vector labeled a in Figure  8.12

  
Figure 8.12: How Emboss is Applied to a 2D Image: Directions of the Gradient Vectors
Figure 8.12

has a positive projection onto the vector pointing towards the light source. Alternatively, the gradient vector labeled b has a negative projection because it points away from the light source. Finally, the gradient vector labeled c has no component along the direction to the light source. Its projection is zero.

Gradient arrows that have a large positive projection in the light source direction are mapped to lighter grays up to the maximum projection, which is mapped to white. Gradient arrows that have negative projections are mapped to darker grays up to the maximum negative projection, which is mapped to black. Pixels whose gradient arrows have no component in the light source direction are mapped to the midtone gray, 127R 127G 127B. Exactly how much light or dark is used in the map depends on the third slider control, labeled Depth in the Emboss dialog (see Figures  8.9(b) and 8.10). A Depth value of zero produces no shadow, and a large value produces a deep, well-defined one.

Referring back to Figure  8.9(a), the Azimuth slider is set to zero. This means that the light source is positioned to the right of the image. Furthermore, the Elevation slider is set to 45o, meaning that the left side of the image is in the shadow zone. This agrees with the vector analysis given earlier. The pixels on the left side of Figure  8.9(a) have gradient vectors that point away from the light source, and so these pixels are mapped to darker values. The pixel values on the right side have gradient vectors pointing towards the light source, so their pixels are mapped to lighter values. The result is the embossed image looks like a cone, which can be seen in the image preview area in Figure  8.9(b).

So much for the theory. How about an example? Figure  8.13(a)

  
Figure 8.13: Example of Using the Emboss Filter
Figure 8.13

illustrates the photo of a daisy, and Figure  8.13(c) shows the result of applying the Emboss filter to it. The filter options are shown in Figure  8.13(b). The result is impressive. Figure  8.13(c) shows a version of the daisy that appears to be embossed into a thin sheet of metal.

You should note that, although the result of the Emboss filter is a grayscale, it is still an RGB image. Furthermore, it is important to know that Emboss does not work on images with an alpha channel. The alpha channel must be removed by choosing Flatten Image from the Layers menu before the image can be embossed. Flatten Image is described in more detail in Section  2.7.2.

A filter that works in a manner similar to Emboss is Bump map,  which is located in the Image:Filters/Map menu. The Bump map filter is more versatile than Emboss, and it is very useful for rendering (see Sections  8.6 and 8.7 for two interesting applications of bumpmapping).

The Bump map filter works by embossing one image and then multiplying the result with another image. This creates the effect of embedding the texture of the first image into the second. The filter's dialog has many options that control the final effect.

Figures  8.14

  
Figure 8.14: Cape Cod from Space and a Pattern Texture
Figure 8.14

and 8.15
  
Figure 8.15: Applying the Bump map Filter to Cape Cod
Figure 8.15

illustrate an example of using Bump map. Figure  8.14(a) shows a photo from space of Cape Cod in the great state of Massachusetts, and Figure  8.14(b) shows a small image created by filling with a pattern selected from the Pattern Selection dialog. These two images are the raw materials used by Bump map in what follows.

Figure  8.15(a) shows the Bump map dialog, and Figure  8.15(b) shows the result of applying the filter to Figure  8.14(a) using the image in Figure  8.14(b) as the embossing map. As you can see in Figure  8.15(a), the Bump map filter dialog has a few more options than the Emboss filter. The most important is the menu labeled Bump map (not to be confused with the filter name). Clicking on this menu provides a list of all the layers in all open images, and the image layer chosen in this menu is the one that is used as the embossing map. However, unlike the restriction of the Emboss filter, Bump map allows any layer, with or without an alpha channel, to be used. The Bump map dialog also has all the usual options for embossing: Azimuth, Elevation, and Depth.

Bump map also has a number of additional options. There are X Offset and Y Offset, which are useful for positioning the embossing image with respect to the embossed one. There is also the Waterlevel option which is only useful if the embossing image has an alpha channel. In this case, the embossing is applied as a function of the alpha and Waterlevel values. Low values of alpha diminish the embossing effect, and high values increase it. This is modulated by the value of Waterlevel. High values of Waterlevel accentuate the alpha channel effect, and low values proportionally diminish it.

The Ambient  option in the Bump map dialog controls the relative strength of the directional light source (refer back to Figure  8.10). An ambient  light source is one that shines equally from all directions. This is the kind of illumination that is useful in a photography studio, where all shadows on a subject must be eliminated. A strong ambient light source cancels the effect of a directional one. A directional light source creates shadows because it illuminates a scene from a single direction, but the more ambient light there is, the weaker the shadow making effect of the directional light becomes. Thus, you can use the Ambient option in the Bump map dialog to control the relative amount of directional light and, in so doing, control the strength of the shadows.

Multiplying an embossing map with an image produces a result that is darker than either of the first two. This is a natural result of image multiplication (see the description of the Multiply blending mode in Section  5.6). The Compensate for Darkening checkbox reduces this effect. The Invert Bumpmap checkbox simply inverts the relationship of shadow and light in the embossing map.

Finally, there are three radio buttons that control the multiplication process. These are labeled Linear Map, Spherical Map, and Sinusoidal Map. The Linear Map option does a straight multiply of the embossing map with the image. The Sinusoidal and Spherical Map options, however, act somewhat like the Curves tool (see Section  6.2.1). That is, the effect of the embossing map is warped according to a curve. The sinusoidal curve provides more contrast to the midtone embossing values and the spherical curve more to the shadow and highlight values. This may sound a little complicated, but it isn't. Just play around with them, and you'll see right away what they do.




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