realistic feeling.However,too many sampling points 3.Sky model cost much more spending in the computer resource and the improvement of eftects is not so obvious.We This section descnbes a high realisti sky dome choa%the sampling distances as△8=△p=l5, model.To ncrease the realistic feeling of the sky which means only 144 verkex points will be used to scene.the flowing clouds effect is considered.A describe our sky dome model rendering method based on the texture map combined with a light scattering model is proposed to create a 3.2.The simulation of clouds highly realistic sky seene. After building the geometry skeleton,we use sphere 3.1.Sky deme model texture mapping to mask the cloud texture on the sky dome.which will increase the realistic fecling of sky. The sky dome model [14.15]is the skekton of this I's diffcult to simulate clouds in 3D enviroements sky model,which will create a hemi-sphere to cover because the light will be retlected,rettacted and the whole seene (see Figure 1h scamtered when it go through the clouds.We use texture distnbution method [12]to realize the flow of clouds.The core idea of this mcthod is to scroll sky 起xr减a random speed in both u and v directions (see Figure 3).To make it more realistic,we build multi-layer cload textures and different layer mowves at a different speed,then we mix these layer into one 起xure. Figure 1.The sky dome model We define the sky domse model by the polar system with the center of sphere as the origin and r as the adius (see Figur2宝 r=r0osin属仍 '■r属 =r cos(o)cos) Here,is the latitude and is the longitude: 05os/205052.Then we ca get every poimt's coordinales (x y.z)according to its latitude Figure 3.The sketch map for the simulation of and longitude. clouds Now we use the Perlin Noise [10]to generate the cloud texture.Abo Pallister [13]proposed a idea of comhining four noise textures to creale clouds.To build a Pertin Noise,we need a noise fuction and an imerpolation function.The noise fuction i a random namber generator which will gemerate a random namber according to an integer number.The Figure 2.The polar system inerpoltion fiction is alays seleeled from the linear interpolation,cosine interpolation and cubic After construction of the mathemanical model of the ncpo山tic. sky dome,we will sample points on the hemi-sphere, using the discrete sampling on the latitude and longitude coordinates The distance of latitude sampling is Ap and the loegitade one is A6.Then the toeal number of the sample points is 石 “11+1石 (360/A890/△9·Sampling more points will Figure 4.The distribution of noise produce a higher rendering precision and higher 157 3. Sky model This section describes a high realistic sky dome model. To increase the realistic feeling of the sky scene, the flowing clouds effect is considered. A rendering method based on the texture map combined with a light scattering model is proposed to create a highly realistic sky scene. 3.1. Sky dome model The sky dome model [14, 15] is the skeleton of this sky model, which will create a hemi-sphere to cover the whole scene (see Figure 1). Figure 1. The sky dome model We define the sky dome model by the polar system with the center of sphere as the origin and r as the radius (see Figure 2): cos( )sin( ) sin( ) cos( )cos( ) x r y r z r I T I I T Here, I is the latitude and T is the longitude: 0 / 2, 0 2 dd dd I S TS . Then we can get every point’s coordinates (x, y, z) according to its latitude and longitude. Figure 2. The polar system After construction of the mathematical model of the sky dome, we will sample points on the hemi-sphere, using the discrete sampling on the latitude and longitude coordinates. The distance of latitude sampling is 'I and the longitude one is 'T . Then the total number of the sample points is (360 / )(90 / ) ' ' T I . Sampling more points will produce a higher rendering precision and higher realistic feeling. However, too many sampling points cost much more spending in the computer resource, and the improvement of effects is not so obvious. We choose the sampling distances as ' ' T M 15 , which means only 144 vertex points will be used to describe our sky dome model. 3.2. The simulation of clouds After building the geometry skeleton, we use sphere texture mapping to mask the cloud texture on the sky dome, which will increase the realistic feeling of sky. It’s difficult to simulate clouds in 3D environments because the light will be reflected, refracted and scattered when it go through the clouds. We use texture distribution method [12] to realize the flow of clouds. The core idea of this method is to scroll sky texture at a random speed in both u and v directions (see Figure 3). To make it more realistic, we build multi-layer cloud textures and different layer moves at a different speed, then we mix these layer into one texture. Figure 3. The sketch map for the simulation of clouds Now we use the Perlin Noise [10] to generate the cloud texture. Also Pallister [13] proposed an idea of combining four noise textures to create clouds. To build a Perlin Noise, we need a noise function and an interpolation function. The noise function is a random number generator which will generate a random number according to an integer number. The interpolation function is always selected from the linear interpolation, cosine interpolation and cubic interpolation. Figure 4. The distribution of noise