CN112017284B - Three-dimensional digital earth real-time terrain shadow simulation method based on light cone diagram - Google Patents
Three-dimensional digital earth real-time terrain shadow simulation method based on light cone diagram Download PDFInfo
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Abstract
The invention discloses a three-dimensional digital earth real-time terrain shadow simulation method based on a light cone diagram, which comprises the following steps: scheduling terrain tiles of a quad-tree structure according to different heights of cameras or visual angles, and loading images and DEM data to generate terrain grids; for each visible tile, applying DEM data of a plurality of adjacent sibling tiles with a certain level of father node tile as a center to generate a terrain light cone map; when each pixel of the terrain is rendered, comparing the sunlight direction with light cone information sampled from the light cone image, and depicting a shadow color for each pixel to generate a shadow image; and fusing shadow images and other layer data such as images and the like, thereby displaying real-time dynamic shadows on the three-dimensional digital terrestrial globe terrain and outputting the shadows to display equipment. The attached drawing of the abstract is a flow chart for realizing the invention.
Description
Technical Field
The invention relates to the technical field of digital earth simulation of a three-dimensional geographic information system, in particular to a three-dimensional digital earth real-time terrain shadow simulation method.
Background
The three-dimensional geographic information system digital earth simulation technology combines a plurality of disciplines such as a computer technology, a graphic image technology, a control technology and the like to carry out three-dimensional modeling on the real world and drive and display in a three-dimensional scene in real time, thereby extending human sense organs. The method has the advantages that the earth surface model is built on the digital earth by using elevation data, satellite images and the like, geographical information such as geographical positions, landforms, soil hydrology, vegetation buildings and the like can be visually displayed, and users can freely control and roam in three-dimensional scenes. In order to enhance the stereoscopic impression of the landform, the prior art uses a computer graphics illumination model to color the earth model according to the sun direction and the normal direction of the earth model, which is generally reflected in that the side of the hillside facing the sun is brighter and the side facing away from the sun is darker, thereby increasing the visual stereoscopic impression. However, such methods cannot reflect the occlusion of the terrain to the self, for example, a small hill facing the sun and the sun are occluded by a large mountain, and the small hill should be darker rather than brighter in the shadow of the large mountain, so that a shadow simulation technology considering the occlusion relationship needs to be applied. The existing three-dimensional model shadow simulation technology comprises a template shadow technology, a texture shadow technology and the like, but when the methods are applied to digital earth terrain simulation, the problems that shadow casting distance is insufficient, shadow images are not clear, the edges of the shadow images are seriously jagged, the rendering frame rate is greatly reduced and the like when the shadows of huge mountains are processed are faced.
Disclosure of Invention
The invention aims to solve the defects in the prior art, and provides a three-dimensional digital earth real-time terrain shadow simulation method based on a light cone diagram, which can process characteristics of a huge scale model, high operation efficiency, vivid image effect and the like.
In order to achieve the purpose, the invention adopts the following technical scheme:
a three-dimensional digital earth real-time terrain shadow simulation method based on a light cone diagram comprises the following steps:
1) scheduling terrain tiles of a quadtree structure according to different heights of cameras or visual angles, and loading images and DEM data to generate a terrain grid;
2) for each visible tile, applying DEM data of 9 adjacent sibling tiles with a certain level parent node tile as a center to generate a terrain light cone map;
3) when each terrain pixel is rendered, the sunlight direction is compared with the light cone information sampled from the light cone image, and a shadow image is generated by depicting a shadow color for each pixel;
4) and fusing shadow images and other layer data such as images and the like, thereby displaying real-time dynamic shadows on the three-dimensional digital terrestrial globe terrain and outputting the shadows to display equipment.
Further, the step 1) includes that according to the difference of the heights of the cameras or the visual angles, the terrain tile quadtrees constructed according to the ink card tray or WGS84 projection slice rule are scheduled, and terrain tiles of different blocks within a certain level range are selected for display. Through calculating the projected area of tiling projection to three-dimensional rendering window, when projected area is greater than certain numerical value, then switch to next level tiling to make the place that is close to the camera show high level tiling, show low level tiling in the place far away from the camera, reach the balance of display effect and display efficiency. Reading DEM raster data according to the grade and the geographic range of the terrain tiles to generate height pattern texture and transmitting the height pattern texture into a GPU (graphics processing unit), wherein the DEM raster data can be DEM data in formats such as tif/. egc/. egx and the like, and also can be pre-cut DEM tile pyramid data; reading DOM image data to generate image texture and transmitting the image texture into a GPU, wherein the DOM raster data can be image data in a format of tif/img and the like, and can also be pre-cut image tile pyramid data.
Further, the step 2) includes that for each visible tile, DEM data of m adjacent sibling tiles taking a parent node tile of an upper n-level of the tile as a center are obtained, the value range of n is different from 1 to 3, the default value is 2, the larger the number is, the wider the projection range of the terrain shadow is, the value of m is generally 9, namely, DEM data of a complete Sudoku is obtained. Processing each grid of the current terrain tile block by applying a GPU parallel processing technology according to the acquired DEM data, sampling the DEM data in 8 peripheral directions by a certain step length by taking the coordinate position of the grid as the center, calculating the maximum slope in each direction, generating a maximum slope ray, fitting the maximum slope ray in the 8 directions as an envelope to form a cone, namely a light cone light _ cone, storing the light _ cone.direction and the light _ cone.angle of the horn mouth of the light cone into an R32G32B32A32 pixel, and outputting all the light _ cones to a terrain light cone map which can be cached to accelerate the next rendering speed.
Further, step 3) includes rendering each terrain pixel, comparing the line connecting the pixel and the sun as the sunlight, the sunlight and the light cone information sampled from the light cone map, when the sunlight is within the light cone, it indicates that the pixel can be irradiated by the sunlight, otherwise, the pixel is in the shadow, and generating a shadow image by describing the shadow color for each terrain pixel in the shadow according to the method.
Further, the step 4) comprises fusing the shadow image and other layer data such as images in a modulated mode, so that the image in the shadow area is darkened, and real-time dynamic shadow is displayed on the three-dimensional digital terrestrial geography and is output to a display device.
The invention has the following beneficial effects:
1. the terrain shadow simulation effect reflects the shielding of the terrain itself on sunlight;
2. the huge mountain can also cast a shadow far enough, and the shadow contour is clear without sawteeth;
3. the algorithm for generating the terrain shadow in real time is low in complexity, and the rendering efficiency is greatly improved compared with that of the traditional shadow technology;
4. the method is applied to a three-dimensional geographic information system platform software EV-Globe interface of EARTH View Image, and the terrain shadow of the digital EARTH is efficiently and high-quality displayed in a real-time simulation manner;
drawings
FIG. 1 is a flow chart of an implementation of the present invention;
FIG. 2 is a quad-tree of terrain tiles of the present invention;
FIG. 3 is a schematic diagram of the DEM sampled in 8 directions around the center of the grid according to the present invention;
FIG. 4 is a two-dimensional diagram of a light cone enveloped by a ray with the maximum slope according to the present invention;
FIG. 5 is a two-dimensional schematic diagram of the present invention for determining whether a shadow region is present based on the inclusion relationship of solar rays and light cones;
FIG. 6 is a comparison graph of three-dimensional digital terrestrial terrain simulation effects before and after opening a terrain shadow according to the present invention;
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments.
Referring to fig. 1-6, a seamless splicing display simulation method for digital earth skirt-free terrain tiles comprises the following steps: 1) scheduling terrain tiles of a quadtree structure according to different heights of cameras or visual angles, and loading images and DEM data to generate a terrain grid; 2) for each visible tiling, applying DEM data of a plurality of adjacent sibling tiling taking a certain level parent tiling as a center to generate a terrain light cone map; 3) when each terrain pixel is rendered, the sunlight direction is compared with light cone information sampled from the light cone image, and a shadow image is generated by depicting a shadow color for each pixel; 4) and fusing shadow images and other layer data such as images and the like, thereby displaying real-time dynamic shadows on the three-dimensional digital terrestrial globe terrain and outputting the shadows to display equipment.
The specific implementation steps are as follows:
step 1), constructing a terrain tile quadtree, wherein if the tile is sliced according to the mercator projection rule, the number of the first-level tiles can be 1, 4 (2 x 2), and the like, and the sum of the spans of all the first-level tiles is a longitude range (-180, 180) and a latitude range (-85, 85); if the slicing rule is WGS84 projection, the number of the first-level tiles may be 2 (1 × 2), 8 (2 × 4), etc., the sum of all the first-level tile spans is longitude range (-180, 180), latitude range (-90, 90), and the next-level quad-tree tile range is obtained by using the bisection method based on the first-level tile range, so as to continuously and dynamically construct the multi-level terrain tile quad-tree.
And scheduling the quad-tree of the terrain tiles according to different heights of the cameras or visual angles, and selecting the terrain tiles of different blocks within a certain level range for display. By calculating the projection area of the tile projected to the three-dimensional rendering window, when the projection area is larger than a certain value, the next-level tile is switched to, so that the high-level tile is displayed at a place close to the camera, the low-level tile is displayed at a place far away from the camera, and the balance between the display effect and the display efficiency is achieved. Reading DEM raster data according to the grade and the geographic range of the terrain tiles to generate height map texture and transmitting the height map texture into a GPU (graphics processing Unit), wherein the DEM raster data can be DEM data in formats of tif/. egc/. egx and the like, and also can be pre-cut DEM tile pyramid data; reading DOM image data to generate image texture and transmitting the image texture into a GPU, wherein the DOM raster data can be image data in a format of tif/img and the like, or image tile pyramid data which is cut in advance
And 2) for each visible tile, acquiring DEM data of m adjacent sibling tiles taking the parent tile of the upper n levels of the tile as the center, wherein the value range of n is different from 1 to 3, the default value is 2, the larger the number is, the wider the projection range of the terrain shadow is, and the value of m is generally 9, namely, the DEM data of a complete nine-square grid is acquired. And processing each grid of the current terrain tiling by applying a GPU parallel processing technology according to the acquired DEM data, and sampling the DEM data in 8 peripheral directions D0 to D7 by a certain step length by taking the coordinate position of the grid as a center. The maximum slope is calculated in each direction, a maximum slope ray is generated, a cone, namely a light cone light _ cone, is fitted according to the maximum slope rays L0 to L7 in 8 directions with the grid center P as an origin as an envelope, the direction of a light cone horn mouth and the opening angle light _ cone are stored in an R32G32B32A32 pixel, all the light _ cones are output to a terrain light cone map, and the terrain light cone map can be buffered to accelerate the next rendering speed.
And step 3), transmitting the terrain data and the light cone map into a GPU for rendering, in a fragment shading stage of a three-dimensional rendering pipeline, comparing a connecting line of a three-dimensional space coordinate and a sun space coordinate of each terrain pixel as sun light, sun light and light cone information light _ cone sampled from the light cone map in the fragment shader, namely a fragment shader, when the sun light is within the light _ cone, the pixel can be irradiated by the sun light, otherwise, the pixel is in the terrain shadow, and according to the method, drawing shadow colors, such as dark grey, for each terrain pixel in the shadow to generate a shadow image. The sun is at different positions relative to the earth and the sun _ light direction is different with different time, so that different shadow images can be generated in real time at different moments. The method for determining whether the sun _ light is within the light _ cone includes the following steps:
Alpha＝arccos(sun_light.dotProduct(light_cone.direction))
Beta＝light_cone.angle/2
alpha < Beta then sun _ light is inside light _ cone, Alpha > Beta then sun _ light is outside light _ cone, where:
angle between Alpha, sun _ light and light _ cone flare orientations
Beta: half flare angle of light _ cone horn mouth
The calculation method is extremely low in time complexity and very efficient.
And 4) fusing the shadow image and other layer data such as the image in a model mode, namely multiplying the shadow image in the shadow image by the colors of other layers such as the image:
OutColor ═ ShadowColor @ ImageColor, where:
outcolor: color output to a display device
ShadowColor: shadow color
Imagecolor: color of other layers such as image
Since the shadow color is dark gray, the image in the shadow area is darkened after multiplication, so that the real-time dynamic shadow is displayed on the three-dimensional digital terrestrial terrain and is output to a display device.
The three-dimensional digital terrain simulation images before and after the shadow is opened are compared, so that the illumination effect of the terrain is obviously more real after the shadow is opened, and the shielding of mountains on sunlight is perfectly reflected.
In conclusion, tests show that the method achieves the effect of high-efficiency and high-quality simulation of the digital earth terrain shadow in the three-dimensional geographic information system; the three-dimensional digital earth real-time terrain shadow simulation method based on the light cone diagram is innovative in the field of three-dimensional simulation of Chinese geographic information, can be well applied to a large three-dimensional geographic information digital earth three-dimensional interface, can perfectly reflect the shielding of the digital earth terrain to sunlight, enables a huge mountain range to project shadows far enough, has clear shadow contour without sawteeth, is low in algorithm complexity of real-time generation of terrain shadows, and is greatly improved in rendering efficiency compared with the traditional shadow technology. The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered as the technical solutions and the inventive concepts of the present invention within the technical scope of the present invention.
Claims (2)
1. A three-dimensional digital earth real-time terrain shadow simulation method based on a light cone diagram comprises the following steps:
step 1, scheduling terrain tiles of a quad-tree structure according to different heights of cameras or visual angles, and loading images and DEM data to generate a terrain grid; according to the difference of the heights of the cameras or the visual angles, scheduling a terrain tile quadtree constructed according to the Mocha or WGS84 projection slice rule, and selecting terrain tiles of different blocks within a certain level range for display; reading DEM raster data according to the level and the geographical range of the new terrain tiles needing to be loaded and displayed, wherein the DEM raster data can be DEM data in a tif/egc/egx format or a pre-cut DEM tile pyramid; reading DOM image data to generate image texture and transmitting the image texture into a GPU, wherein the DOM raster data can be image data in a tif/img format or a pre-cut image tile pyramid; for each visible tiling, applying DEM data of 9 adjacent sibling tiling taking a certain level parent node tiling as a center to generate a terrain light cone map;
step 2, when each terrain pixel is rendered, comparing the sunlight direction with light cone information sampled from the light cone image, depicting a shadow color for each pixel, and generating a shadow image; when each terrain pixel is rendered, the connecting line of the pixel and the sun is used as the sunlight, the sunlight is compared with the light cone information sampled from the light cone image, when the sunlight is in the light cone, the pixel can be irradiated by the sunlight, otherwise, the pixel is in the shadow, and according to the method, shadow colors are drawn for each terrain pixel in the shadow to generate a shadow image;
step 3, fusing the shadow image and the image map layer data, thereby displaying a real-time dynamic shadow on the three-dimensional digital terrestrial terrain and outputting the real-time dynamic shadow to display equipment; and fusing the shadow image and the image map layer data in a modulated mode to darken the image in the shadow area, so that the real-time dynamic shadow is displayed on the three-dimensional digital earth terrain and is output to a display device.
2. The method for simulating the three-dimensional digital earth real-time terrain shadow based on the light cone diagram in the claim 1 is characterized in that the light cone diagram is generated by using DEM data of m adjacent sibling tiles taking a parent node tile block of an upper n-level of a rendered terrain tile block as a center, the value range of n is 1 to 3, the larger the number is, the wider the projection range of the terrain shadow is, the value of m is 9, when generating a terrain light cone map, applying GPU to accelerate parallel calculation, sampling the DEM data to 8 directions around the current terrain tile by taking the coordinate position of each grid as the center according to a certain step length, calculating the maximum slope, fitting into a cone by taking the maximum slope ray of the 8 directions as an envelope line, namely, the light cone stores the direction and the opening angle of the light cone bell mouth into one float4 pixel, and outputs all light cone information as a terrain light cone map.
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