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13.2.1 计算顶点
在图13.4中，计算三角形网格上的顶点，我们只是在开始产生顶点的地方，一行一行的生成顶点数据，直到结束为止。单元格的顶点与顶点之间有一块空白区域，这会让我们取得x、z坐标，但y坐标是什么呢？得到y坐标很容易，当读取高度图数据结构时会找到对应的入口。
注意：这个操作使用一个巨大的顶点缓存去保存所有地形上的所有顶点。这可能会引起硬件局限性的问题。例如：3D设备都预设了最大图元数和最大顶点索引值。检查D3DCAPS9结构的MaxPrimitiveCount和MaxVertexlndex成员来获得你的设备的限定值。
计算纹理坐标，看图13.5，给我们一个简单的设定，允许我们用(u, v)纹理坐标去对应地形顶点坐标。
最后，用代码生成顶点:
bool cTerrain::generate_vertices()
{
if(FAILED(m_device->CreateVertexBuffer(m_num_vertices * sizeof(cTerrainVertex), D3DUSAGE_WRITEONLY,
TERRAIN_VERTEX_FVF, D3DPOOL_MANAGED, &m_vertex_buffer, NULL)))
{
return false;
}
// coordinates to start generating vertices at
int start_x = -m_width / 2;
int start_z = m_depth / 2;
// coordinates to end generating vertices at
int end_x = m_width / 2;
int end_z = -m_depth / 2;
// compute the increment size of the texture coordinates from one vertex to the next
float u_coord_increment_size = 1.0f / m_num_cells_per_row;
float v_coord_increment_size = 1.0f / m_num_cells_per_col;
cTerrainVertex* v;
m_vertex_buffer->Lock(0, 0, (void**)&v, 0);
int row = 0;
for(int z = start_z; z >= end_z; z -= m_cell_spacing)
{
int column = 0;
for(int x = start_x; x <= end_x; x += m_cell_spacing)
{
int index = row * m_num_verts_per_row + column;
v[index] = cTerrainVertex(x, m_height_map[index], z,
column * u_coord_increment_size, row * v_coord_increment_size);
column++;
}
row++;
}
m_vertex_buffer->Unlock();
return true;
}
13.2.2 计算索引-定义三角形
计算三角形网格的索引，只需要循环访问每一个格子，从左上到右下，如图13.4，并且计算组成格子的2个三角形。
代码生成索引：
bool cTerrain::generate_indices()
{
if(FAILED(m_device->CreateIndexBuffer(
m_num_triangles * 3 * sizeof(WORD), // 3 indices per triangle
D3DUSAGE_WRITEONLY, D3DFMT_INDEX16, D3DPOOL_MANAGED, &m_index_buffer, NULL)))
{
return false;
}
WORD* indices;
m_index_buffer->Lock(0, 0, (void**)&indices, 0);
// index to start of a group of 6 indices that describe the two triangles that make up a quad
int base_index = 0;
// loop through and compute the triangles of each quad
for(int row = 0; row < m_num_cells_per_col; row++)
{
for(int col = 0; col < m_num_cells_per_row; col++)
{
indices[base_index] = row * m_num_verts_per_row + col;
indices[base_index + 1] = row * m_num_verts_per_row + (col+1);
indices[base_index + 2] = (row+1) * m_num_verts_per_row + col;
indices[base_index + 3] = (row+1) * m_num_verts_per_row + col;
indices[base_index + 4] = row * m_num_verts_per_row + (col+1);
indices[base_index + 5] = (row+1) * m_num_verts_per_row + (col+1);
base_index += 6; // next quad
}
}
m_index_buffer->Unlock();
return true;
}
cTerrain类提供2个方法去处理地形的纹理。最简单的方法是读取一个已经制作好的纹理文件并使用它:
bool cTerrain::load_texture(const string& filename)
{
if(FAILED(D3DXCreateTextureFromFile(m_device, filename.c_str(), &m_texture)))
return false;
return true;
}
13.3.1 程序上的处理方法
一个可选择的方法是用程序计算地形的纹理，就是说，我们创建一个空纹理，根据定义的参数用代码计算每一个部分的颜色，在例子中，参数是地形的高度。
我们用cTerrain::generate_texture方法去生成纹理，首先用D3DXCreateTexture方法创建一个空的纹理，锁定高度级别（top level，纹理图的一个成员，有多个级别），不断的循环每一个texel（图素）并给它上色，texel的颜色取决于与方格对应的高度（近似高度）。我们的想法是：地形中较低的地方是沙滩色，中间的地方像是绿色的小山丘，较高的地方颜色好像雪山。我们定义的高度是方格中左上角的近似高度。
一旦每个texel都有了颜色，我们想让每一个texel变暗或是变亮，这基于光打在格子中对应的texel上的角度，由Terrain::lightTerrain方法实现。
bool cTerrain::generate_texture(D3DXVECTOR3* dir_to_light)
{
// Method fills the top surface of a texture procedurally, then lights the top surface.
// Finally, it fills the other mipmap surfaces based on the top surface data using D3DXFilterTexture.
// texel for each quad cell
int texture_width = m_num_cells_per_row;
int texture_height = m_num_cells_per_col;
// create an empty texture with a complete mipmap chain
if(FAILED(D3DXCreateTexture(m_device, texture_width, texture_height, 0, 0, D3DFMT_X8R8G8B8,
D3DPOOL_MANAGED, &m_texture)))
{
return false;
}
D3DSURFACE_DESC texture_desc;
m_texture->GetLevelDesc(0, &texture_desc);
// make sure we got the requested format because our code that fills the texture is
// hard coded to a 32 bit pixel depth.
if(texture_desc.Format != D3DFMT_X8R8G8B8)
return false;
// lock top entire texture surface
D3DLOCKED_RECT locked_rect;
m_texture->LockRect(0, &locked_rect, NULL, 0);
DWORD* image_data = (DWORD*) locked_rect.pBits;
for(int row = 0; row < texture_height; row++)
{
for(int col = 0; col < texture_width; col++)
{
D3DXCOLOR color;
// get height of upper left vertex of quad
float height = get_height_map_entry(row, col) / m_height_scale;
if(height < 42.5f) color = BEACH_SAND;
else if(height < 85.0f) color = LIGHT_YELLOW_GREEN;
else if(height < 127.5f) color = PUREGREEN;
else if(height < 170.0f) color = DARK_YELLOW_GREEN;
else if(height < 212.5f) color = DARKBROWN;
else color = WHITE;
// fill locked data, note we divide the pitch by four because the pitch is given in bytes
// and there are 4 bytes per DWORD.
image_data[row * (locked_rect.Pitch / 4) + col] = (D3DCOLOR) color;
}
}
m_texture->UnlockRect(0);
if(! light_terrain(dir_to_light))
{
MessageBox(NULL, "light_terrain() - FAILED", "ERROR", MB_OK);
return false;
}
if(FAILED(D3DXFilterTexture(m_texture, NULL, 0, D3DX_DEFAULT)))
{
MessageBox(NULL, "D3DXFilterTexture() - FAILED", "ERROR", MB_OK);
return false;
}
return true;
}