1. 概述
DEM（地形文件）天然自帶三維信息，可以將其轉換成gltf模型文件。DEM是柵格數據，可以通過GDAL進行讀取；gltf是一種JSON格式，可以采用nlohmann/json進行讀寫。
2. 詳細
直接把代碼貼出來：
#include <iostream>
#include <fstream>
#include <iomanip>
#include <nlohmann\json.hpp>
#include "fifo_map.hpp"
#include <gdal/gdal_priv.h>
using namespace std;
using namespace nlohmann;
// A workaround to give to use fifo_map as map, we are just ignoring the 'less' compare
template<class K, class V, class dummy_compare, class A>
using my_workaround_fifo_map = fifo_map<K, V, fifo_map_compare<K>, A>;
using my_json = basic_json<my_workaround_fifo_map>;
int main()
{
GDALAllRegister();
CPLSetConfigOption("GDAL_FILENAME_IS_UTF8", "NO"); //支持中文路徑
my_json gltf;
gltf["asset"] = {
{"generator", "CL"},
{"version", "2.0"}
};
gltf["scene"] = 0;
gltf["scenes"] = {
{{"nodes", {0} }}
};
gltf["nodes"] = {
{{"mesh", 0}}
};
my_json positionJson;
positionJson["POSITION"] = 1;
positionJson["TEXCOORD_0"] = 2;
my_json primitivesJson;
primitivesJson = {
{{"attributes", positionJson}, {"indices", 0}, {"material", 0} }
};
gltf["meshes"] = {
{{"primitives", primitivesJson}}
};
my_json pbrJson;
pbrJson["baseColorTexture"]["index"] = 0;
gltf["materials"] = {
{{"pbrMetallicRoughness", pbrJson}}
};
size_t pointNum = 0;
size_t binBufNum = 0;
size_t indicesNum = 0;
{
string binPath = "D:/Work/WebGLTutorial/Data/new.bin";
ofstream binFile(binPath, std::ios::binary);
const char *filePath = "D:/Work/WebGLTutorial/Data/DEM.tif";
GDALDataset* img = (GDALDataset *)GDALOpen(filePath, GA_ReadOnly);
if (!img)
{
printf("Can't Open Image!");
return 0;
}
int bufWidth = img->GetRasterXSize(); //圖像寬度
int bufHeight = img->GetRasterYSize(); //圖像高度
int bandNum = img->GetRasterCount(); //波段數
if (bandNum != 1)
{
printf("DEM波段數不為1");
return 0;
}
int depth = GDALGetDataTypeSize(img->GetRasterBand(1)->GetRasterDataType()) / 8; //圖像深度
//獲取地理坐標信息
double padfTransform[6];
if (img->GetGeoTransform(padfTransform) == CE_Failure)
{
printf("獲取仿射變換參數失敗");
return 0;
}
double startX = padfTransform[0];
double dX = padfTransform[1];
double startY = padfTransform[3];
double dY = padfTransform[5];
//申請buf
size_t imgBufNum = (size_t)bufWidth * bufHeight * bandNum;
float *imgBuf = new float[imgBufNum];
//讀取
img->RasterIO(GF_Read, 0, 0, bufWidth, bufHeight, imgBuf, bufWidth, bufHeight,
GDT_Float32, bandNum, nullptr, bandNum*depth, bufWidth*bandNum*depth, depth);
pointNum = (size_t)bufWidth * bufHeight;
size_t position_texture_num = pointNum * 5;
float *position_texture = new float[position_texture_num];
for (int yi = 0; yi < bufHeight; yi++)
{
for (int xi = 0; xi < bufWidth; xi++)
{
size_t n = (size_t)(bufWidth * 5) * yi + 5 * xi;
position_texture[n] = dX * xi;
position_texture[n+1] = dY * yi;
size_t m = (size_t)(bufWidth * bandNum) * yi + bandNum * xi;
position_texture[n + 2] = imgBuf[m];
position_texture[n + 3] = float(xi) / (bufWidth-1);
position_texture[n + 4] = float(yi) / (bufHeight-1);
}
}
//釋放
delete[] imgBuf;
imgBuf = nullptr;
binFile.write((char*)position_texture, position_texture_num * sizeof(float));
size_t vertexBufNum = position_texture_num * sizeof(float);
binBufNum = binBufNum + vertexBufNum;
int mod = vertexBufNum % sizeof(uint16_t);
if (mod != 0)
{
int spaceNum = sizeof(float) - mod;
char *space = new char[spaceNum];
binBufNum = binBufNum + sizeof(char) * spaceNum;
memset(space, 0, sizeof(char) * spaceNum);
binFile.write(space, sizeof(char) * spaceNum);
delete[] space;
space = nullptr;
}
indicesNum = (size_t)(bufWidth - 1) * (bufHeight - 1) * 2 * 3;
uint16_t *indices = new uint16_t[indicesNum];
for (int yi = 0; yi < bufHeight-1; yi++)
{
for (int xi = 0; xi < bufWidth-1; xi++)
{
uint16_t m00 = (uint16_t)(bufWidth * yi + xi) ;
uint16_t m01 = (uint16_t)(bufWidth * (yi+1) + xi);
uint16_t m11 = (uint16_t)(bufWidth * (yi + 1) + xi + 1);
uint16_t m10 = (uint16_t)(bufWidth * yi + xi + 1);
size_t n = (size_t)(bufWidth - 1) * yi + xi;
indices[n * 6] = m00;
indices[n * 6 + 1] = m01;
indices[n * 6 + 2] = m11;
indices[n * 6 + 3] = m11;
indices[n * 6 + 4] = m10;
indices[n * 6 + 5] = m00;
}
}
binFile.write((char*)indices, sizeof(uint16_t) * indicesNum);
binBufNum = binBufNum + sizeof(uint16_t) * indicesNum;
delete[] position_texture;
position_texture = nullptr;
delete[] indices;
indices = nullptr;
}
gltf["textures"] = {
{{"sampler", 0}, {"source", 0}}
};
gltf["images"] = {
{{"uri", "tex.jpg"}}
};
gltf["samplers"] = {
{{"magFilter", 9729}, {"minFilter", 9987}, {"wrapS", 33648}, {"wrapT", 33648}}
};
gltf["buffers"] = {
{{"uri", "new.bin"}, {"byteLength", binBufNum}}
};
my_json indicesBufferJson;
indicesBufferJson["buffer"] = 0;
indicesBufferJson["byteOffset"] = pointNum * 5 * 4;
indicesBufferJson["byteLength"] = indicesNum * 2;
indicesBufferJson["target"] = 34963;
my_json positionBufferJson;
positionBufferJson["buffer"] = 0;
positionBufferJson["byteStride"] = sizeof(float) * 5;
positionBufferJson["byteOffset"] = 0;
positionBufferJson["byteLength"] = pointNum * 5 * 4;
positionBufferJson["target"] = 34962;
gltf["bufferViews"] = {
indicesBufferJson, positionBufferJson
};
my_json indicesAccessors;
indicesAccessors["bufferView"] = 0;
indicesAccessors["byteOffset"] = 0;
indicesAccessors["componentType"] = 5123;
indicesAccessors["count"] = indicesNum;
indicesAccessors["type"] = "SCALAR";
indicesAccessors["max"] = { 18719 };
indicesAccessors["min"] = { 0 };
my_json positionAccessors;
positionAccessors["bufferView"] = 1;
positionAccessors["byteOffset"] = 0;
positionAccessors["componentType"] = 5126;
positionAccessors["count"] = pointNum;
positionAccessors["type"] = "VEC3";
positionAccessors["max"] = { 770, 0.0, 1261.151611328125 };
positionAccessors["min"] = { 0.0, -2390, 733.5555419921875 };
my_json textureAccessors;
textureAccessors["bufferView"] = 1;
textureAccessors["byteOffset"] = sizeof(float) * 3;
textureAccessors["componentType"] = 5126;
textureAccessors["count"] = pointNum;
textureAccessors["type"] = "VEC2";
textureAccessors["max"] = { 1, 1 };
textureAccessors["min"] = { 0, 0 };
gltf["accessors"] = {
indicesAccessors, positionAccessors, textureAccessors
};
string jsonFile = "D:/Work/WebGLTutorial/Data/new.gltf";
std::ofstream outFile(jsonFile);
outFile << std::setw(4) << gltf << std::endl;
}
1.這里使用的DEM是tif格式的圖像，使用GDAL讀取。由於顯示模型文件不需要大坐標，所以沒有把DEM的起始XY坐標值算進去。同時附帶了一張紋理貼圖，正好覆蓋整個DEM的范圍。
2.轉換的的原理非常簡單，就是將DEM的每個網格繪制成兩個三角形，通過頂點索引進行繪制。gltf具體的規范可以參看github上的教程，網上還有相關的中文翻譯。
3.原生的nlohmann/json組件寫出來的JSON格式是根據字符串順序排序不是根據插入順序排序的，查閱的時候不方便。所以這里使用了nlohmann::fifo_map容器專門化對象類型。
3. 結果
轉換出來的結果用OSG顯示如下：
4. 參考
[1] github上的gltf教程
[2] gltf教程中文翻譯
[3] nlohmann/json關於保留插入順序的討論