Comparison and Editing Effects

4.2 Comparison and Editing Effects

4.2.1 BTF Comparison

Given an image, for comparison we show images of the material designed by our editing result, texture mapping and the original BTF data (rendered with Local PCA [MMK03a]) in Figure 4.1 and Figure 4.2. Note that, since the BTF data did not capture the appearance of the material when theta angle is higher than certain degrees (75 degree for the wallpaper data in Figure 4.1 and 80 degree for the WL cool data in Figure 4.2), the region that is not lighted is bounded by white dotted lines. For the material design shown in Figure 4.1, we use a single image of the wallpaper that contains less specular and shadowing effects as the input texture. Through a series of editing processes as described in Chapter 3, we obtain the final result. Without the meso-geometry information, the result of the texture mapping seems flat. Also, since the texture mapping method does not separate the material into different regions, the specular phenomena are all the same over the surface. Consequently, the rendering result is quite different from the ideal one. Comparing the edited result to the rendered BTF image (using local PCA), we reproduce the similar appearance of the BTF. The main difference lies in the dark brown region, because the image we chose for the diffuse color map may contain specular effect that should be excluded. However, our editing result captures the main features of the lighting phenomena of the material. Since the human visual system generally is not very sensitive to the detail of the material when the main features (such as overall color and the width of specular highlight) are reasonably reproduced, our result looks quite realistic.

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Figure 4.1: The comparison of different rendering methods for wallpaper. Top Left: texture mapping. Top Right: BTF rendered using local PCA. Bottom: our editing result.

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In Figure 4.2, we compare another BTF data which has a great variation in height. The creation flow of the result is shown in Figure 4.3. Since the lower part of the material has a brighter color, the height map using grayscale is not correct. To solve the problem, as Figure 4.3 shows, we inverse the color texture and use the “Bucket Fill Tool” to fill the holes that should appear higher when rendering. But still the lower region should have a wider width, so we applied the “Erode Filter” to extend the lower region. Finally, some simple drawing is applied to remove the noise. For the diffuse color map, the editing processes is quite simple, except that we blur the diffuse color map in the end to make the rendering result better approximate the ideal one (The resolution of the WL cool BTF data is 64 × 64 that cause the blurry effect when rendering using local PCA). Although texture mapping result is quite different from the ideal one, the lighting phenomena is quite similar. The main difference lies in the highlights near the bottom right and the middle top of the model which are well produced in our editing result.

Our editing result is much similar to the ideal one. However, the shadow is not reproduced quite well by the parallax occlusion mapping. Although it would be hard to create a material that has exactly the same appearance of the material in reality, our material design system has a great strength in material editing. In the following paragraphs we show how the material’s appearance changes when the edit is applied.

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Figure 4.2: The comparison of different rendering methods for WL cool. Top Left: texture mapping. Top Right: BTF rendered using local PCA. Bottom: our editing result.

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Figure 4.3: The material creation flow of WL cool.

4.2.2 Material Editing

In Figure 4.4, we change the brightness and contrast of the height map to make the mate-rial smoother globally. If artists wish to have smoother or rougher shape in a certain region,

“Blur/Sharpen” brush can be used to change the desired region very easily (Figure 4.5). Also, artist can use warping tools to create artistic effect (Figure 4.6).

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Figure 4.4: Changing the height map of wood.

Figure 4.5: Using “Blur/Sharpen” brush tool to smooth the higher region of the material.

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Figure 4.6: Using “Interactive Warp” to warp the height map counterclockwisely.

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Since all the data are stored in the texture maps and our design system is based on a drawing system, changing the color of the material would be really a piece of cake. In Fiugre 4.7, we use “Adjust Hue/Lightness/Saturation” to change the materials’ color.

Figure 4.7: Changing the color of the material.

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The difference between the effects of the specular mask map and the Fresnel map can be seen in Figure 4.8. We exclude the specular mask term by setting it to 1.0 when editing the Fresnel map and vice versa. As described in Chapter 3, the editing result of the Fresnel map is brighter than the editing result of the specular mask map when it is illuminated at the grazing angle and viewed from the opposite side. Thus for a glossy material, we usually give a brighter Fresnel map.

Figure 4.8: Comparison between the effect of specular mask map and Fresnel map. The dark regions of the specular mask map and the Fresnel map are all set to 0.25; the brighter regions are all set to 1.0. Top Right: Fresnel map is set to 0.25; specular mask map is set to 1.0. Bottom Right: Fresnel map is set to 1.0; specular mask map is set to 0.25.

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The tilted reflection map is used to enhance the tilted reflection. For a real material, the default value is usually used (via the “default button”) as it acts the same as the original BRDF.

But still, artists would wish to control this tilted reflection enhancement to match their expec-tation. As shown in 4.9, we can let the enhancement of the tilted reflection start at a higher angle or a lower angle as compared to the default setting. We can even let the enhancement of tilted reflection appear in a certain range. In Figure 4.10, the enhancement is increased to the maximum till the 60 degree and decrease to zero at 90 degree. In this case, we observe that the specular intensity is decreased near the silhouette of the ball.

Figure 4.9: Changing tilted reflection map. Left: the tilted reflection starts at a higher angle.

Middle: default setting. Right: the tilted reflection starts at a lower angle.

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Figure 4.10: Enhancement of the tilted reflection appears in a certain range. Left: the tilted reflection starts at a lower angle. Right: the restricted tilted reflection.

Specular color map stores the specular color under different lighting and viewing directions.

It can be specially designed to hide some marks in the material which can only be observed in some specific directions (Figure 4.11). This kind of appearance mostly appears on sports coat or aluminum paper with embossed patterns. In addition to that, we can also give the rim light effect to an object easily by drawing the periphery of the specular color map (Figure 4.12).

Also, we can combine another specular color map with the original one to mimic translucent effect on some objects (e.g., the lacquer of a vase) as shown in Figure 4.13.

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Figure 4.11: Specially designed specular color maps. Top Left: the appearances of the material under different lighting and viewing directions. Bottom Left: the corresponding index map and the specular color maps. Right: rendered image.

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Figure 4.12: Using specular color maps to produce rim light effects. Top: the original material.

Bottom: the material with blue rim light effect.

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Figure 4.13: Using another specular color map to produce the lacquer appearance.

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Rotation map can be utilized to produce the specular effect which changes continuously over the phi angle (Figure 4.14 and Figure 4.18). Such continuously changing effect can not be produced by using a few specular color maps.

Figure 4.14: Using rotation map to create brushed metal effect.

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We can also change the rotation angle of a region to produce special effect. In Figure 4.15, we inverse the rotation angle of the “CGGM” text, thus the text would have the opposite specular phenomena of the other region which makes the text darker when the other region is brighter and vice versa.

Figure 4.15: Using rotation map to make special effect.

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Rotation map can also be utilized to reduce the number of specular color maps. If the specular color maps are almost identical with only a difference over the rotation (In Figure 4.16, the specular color 2 is the specular color 1 with 180 degree of rotation). We can encode the rotation information into the rotation map, and changed the corresponding index to the index of the referenced specular color map to reduce the number of the specular color maps (Figure 4.16).

Figure 4.16: Using rotation map to reduce the number of specular color maps. Left: the appear-ance of the original material and its corresponding texture maps. Right: the appearappear-ance of the material and its corresponding texture maps after the reduction.