距离场字体渲染的实现
令人遗憾的是，我从未在我的应用程序中获得过前所未有的纯色字体渲染系统。所以我决定结束这个。
在我搜索字体渲染示例时，我碰巧踏入了valve paper，每个人都赞美。
然而，我缺乏完成此任务的经验和知识。
以下是我所做的总结：
首先，使用Freetype2库，我将一个字符（＆＃39; A＆＃39;）作为位图加载到内存中。
unsigned char u8Character = 'A';
int32_t u32Character = 0;
utf8::utf8to32(&u8Character, &u8Character + 1, &u32Character);
FT_Set_Char_Size(face, 0, 64 * 64, 300, 300);
FT_Load_Char(face, u32Character, FT_LOAD_RENDER);
其次，我将距离场计算为距离位图每个像素的切比雪夫距离，以生成浮点数映射。
注意：老实说，我不知道该怎么做。下面的算法是完全猜测。
template <typename input_type, typename output_type>
output_type chebyshev_distance( input_type from_x
, input_type from_y
, input_type to_x
, input_type to_y )
{
input_type dx = std::abs(to_x - from_x);
input_type dy = std::abs(to_y - from_y);
return static_cast<output_type>(dx > dy ? dx : dy);
}
void GenerateSigendDistanceFieldFrom( const unsigned char* inputBuffer
, int width
, int height
, float* outputBuffer
, bool normalize = false)
{
for (int iy = 0; iy < height; ++iy)
{
for (int ix = 0; ix < width; ++ix)
{
int index = iy*width + ix;
unsigned char value = inputBuffer[index];
int indexMax = width*height;
int indexMin = 0;
int far = width > height ? width : height;
bool found = false;
for (int distance = 1; distance < far; ++distance)
{
int xmin = (ix - distance) >= 0 ? ix - distance : 0;
int ymin = (iy - distance) >= 0 ? iy - distance : 0;
int xmax = (ix + distance) < width ? ix + distance+1 : width;
int ymax = (iy + distance) < height ? iy + distance+1 : height;
int x = xmin;
int y = ymin;
auto fCompareAndFill = [&]() -> bool
{
if (value != inputBuffer[y*width + x])
{
outputBuffer[index] = chebyshev_distance<int, float>(ix, iy, x, y);
if (value < 0xff/2) outputBuffer[index] *= -1;
//outputBuffer[index] = distance;
return true;
}
return false;
};
while (x < xmax)
{
if (fCompareAndFill())
{
found = true;
break;
}
++x;
}
--x;
if (found == true){ break; }
while (y < ymax)
{
if (fCompareAndFill())
{
found = true;
break;
}
++y;
}
--y;
if (found == true){ break; }
while (x >= xmin)
{
if (fCompareAndFill())
{
found = true;
break;
}
--x;
}
++x;
if (found == true){ break; }
while (y >= ymin)
{
if (fCompareAndFill())
{
found = true;
break;
}
--y;
}
if (found == true){ break; }
} // for( int distance = 1; distance < far; ++distance )
} // for( int ix = 0; ix < width; ++ix )
} // for( int iy = 0; iy < height; ++iy )
if( normalize )
{
float min = outputBuffer[0];
float max = outputBuffer[0];
for( int i = 0; i < width*height; ++i )
{
if( outputBuffer[i] < min )
min = outputBuffer[i];
if( outputBuffer[i] > max )
max = outputBuffer[i];
}
float denominator = (max - min);
float newMin = min / denominator;
for( int i = 0; i < width*height; ++i )
{
outputBuffer[i] /= denominator;
outputBuffer[i] -= newMin;
}
}
} // GenerateSigendDistanceFieldFrom
第三，作为测试，我将这个角色的纹理渲染到整个屏幕，大小为800x600，看看它们是如何扩展的。对纹理进行采样＆＃34; GL_LINEAR＆＃34;结果很糟糕。
！1！将Alpha渲染为红色值。
const GLchar fssource[] =
"#version 440 \n"
"out vec4 v_color;"
"in vec2 v_uv;"
"uniform sampler2D u_texture;"
"void main()"
"{"
" float mask = texture(u_texture, v_uv).a;"
" v_color = vec4(mask,0,0,1);"
"}"
;
！2！渲染文字。 alpha的阈值是0.5
const GLchar fssource[] =
"#version 440 \n"
"out vec4 v_color;"
"in vec2 v_uv;"
"uniform sampler2D u_texture;"
"void main()"
"{"
" vec4 result = vec4(1,1,1,1);"
" float mask = texture(u_texture, v_uv).a;"
" if( mask >= 0.5 ) { result.a = 1; }\n"
" else { result.a = 0; }\n"
" v_color = result;"
"}"
！3！渲染文字。 alpha的阈值是0.7
const GLchar fssource[] =
"#version 440 \n"
"out vec4 v_color;"
"in vec2 v_uv;"
"uniform sampler2D u_texture;"
"void main()"
"{"
" vec4 result = vec4(1,1,1,1);"
" float mask = texture(u_texture, v_uv).a;"
" if( mask >= 0.7 ) { result.a = 1; }\n"
" else { result.a = 0; }\n"
" v_color = result;"
"}"
字体看起来很崎岖，显然距离场太亮了。该算法应该使用0.5阈值。 不仅结果不正确，距离场的产生也需要太多时间。因此，我无法使用高分辨率图像作为输入。
我在这里做了一些明显错误的事情，但似乎我自己想弄清楚如何产生正确的结果。
但是请你帮助我，如果你知道我做错了什么。
以下是整个源文件：
#include <iostream>
#include <iomanip>
#include <algorithm>
#include <fstream>
#include <vector>
#include <cstdint>
#include <climits>
#include "ft2build.h"
#include FT_FREETYPE_H
#include "../utf8_v2_3_4/Source/utf8.h"
#include <SDL.h>
#include "../Glew/glew.h"
#include <gl/GL.h>
#undef main
template <typename input_type, typename output_type>
output_type chebyshev_distance( input_type from_x
, input_type from_y
, input_type to_x
, input_type to_y )
{
input_type dx = std::abs(to_x - from_x);
input_type dy = std::abs(to_y - from_y);
return static_cast<output_type>(dx > dy ? dx : dy);
}
void GenerateSigendDistanceFieldFrom( const unsigned char* inputBuffer
, int width
, int height
, float* outputBuffer
, bool normalize = false)
{
for (int iy = 0; iy < height; ++iy)
{
for (int ix = 0; ix < width; ++ix)
{
int index = iy*width + ix;
unsigned char value = inputBuffer[index];
int indexMax = width*height;
int indexMin = 0;
int far = width > height ? width : height;
bool found = false;
for (int distance = 1; distance < far; ++distance)
{
int xmin = (ix - distance) >= 0 ? ix - distance : 0;
int ymin = (iy - distance) >= 0 ? iy - distance : 0;
int xmax = (ix + distance) < width ? ix + distance+1 : width;
int ymax = (iy + distance) < height ? iy + distance+1 : height;
int x = xmin;
int y = ymin;
auto fCompareAndFill = [&]() -> bool
{
if (value != inputBuffer[y*width + x])
{
outputBuffer[index] = chebyshev_distance<int, float>(ix, iy, x, y);
if (value < 0xff/2) outputBuffer[index] *= -1;
//outputBuffer[index] = distance;
return true;
}
return false;
};
while (x < xmax)
{
if (fCompareAndFill())
{
found = true;
break;
}
++x;
}
--x;
if (found == true){ break; }
while (y < ymax)
{
if (fCompareAndFill())
{
found = true;
break;
}
++y;
}
--y;
if (found == true){ break; }
while (x >= xmin)
{
if (fCompareAndFill())
{
found = true;
break;
}
--x;
}
++x;
if (found == true){ break; }
while (y >= ymin)
{
if (fCompareAndFill())
{
found = true;
break;
}
--y;
}
if (found == true){ break; }
} // for( int distance = 1; distance < far; ++distance )
} // for( int ix = 0; ix < width; ++ix )
} // for( int iy = 0; iy < height; ++iy )
if( normalize )
{
float min = outputBuffer[0];
float max = outputBuffer[0];
for( int i = 0; i < width*height; ++i )
{
if( outputBuffer[i] < min )
min = outputBuffer[i];
if( outputBuffer[i] > max )
max = outputBuffer[i];
}
float denominator = (max - min);
float newMin = min / denominator;
for( int i = 0; i < width*height; ++i )
{
outputBuffer[i] /= denominator;
outputBuffer[i] -= newMin;
}
}
} // GenerateSigendDistanceFieldFrom
namespace
{
SDL_Window* window = NULL;
SDL_Surface* screenSurface = NULL;
FT_Library freetype;
FT_Face face;
SDL_GLContext glContext;
GLuint glProgram = 0;
GLuint vbo = 0;
GLuint vao = 0;
GLuint glTexture = 0;
}
GLuint MakeShader( GLenum shaderType, const char* source, int slen )
{
auto shader = glCreateShader(shaderType);
glShaderSource(shader, 1, (const GLchar**)&source, &slen);
glCompileShader(shader);
GLint success;
glGetShaderiv(shader, GL_COMPILE_STATUS, &success);
if (success == GL_FALSE)
{
std::vector<GLchar> glInfoLogBuffer;
int len;
glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &len);
glInfoLogBuffer.resize(len + 1);
GLsizei outlen;
glGetShaderInfoLog(shader, glInfoLogBuffer.size(), &outlen, glInfoLogBuffer.data());
glInfoLogBuffer.back() = 0;
std::cout << glInfoLogBuffer.data() << std::endl;
return 0;
}
return shader;
}
GLuint MakeProgram( GLuint vshader, GLuint fshader )
{
auto program = glCreateProgram();
glAttachShader(program, vshader);
glAttachShader(program, fshader);
glLinkProgram(program);
GLint success;
glGetProgramiv(program, GL_LINK_STATUS, &success);
if( success == GL_FALSE )
{
int len;
glGetProgramiv(program, GL_INFO_LOG_LENGTH, &len);
std::vector<GLchar> buffer;
buffer.resize(len+1);
buffer.back() = 0;
glGetProgramInfoLog(program, buffer.size(), &len, buffer.data());
std::cout << buffer.data() << std::endl;
return 0;
}
return program;
}
int Initialize()
{
if (SDL_Init(SDL_INIT_VIDEO) < 0)
{
std::cout << "SDL could not initialize!";
return -1;
}
window = SDL_CreateWindow("My Window", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 800, 600, SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN);
if (!window)
{
std::cout << "Window could not be created!";
return -1;
}
screenSurface = SDL_GetWindowSurface(window);
SDL_FillRect(screenSurface, 0, SDL_MapRGB(screenSurface->format, 0xFF, 0xFF, 0xFF));
SDL_UpdateWindowSurface(window);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
glContext = SDL_GL_CreateContext(window);
SDL_GL_SetSwapInterval(1);
GLenum glError = glewInit();
if( glError != GLEW_OK )
{
std::cout << "Failed to initialize glew" << std::endl;
return -1;
}
// ----------------------
// Vertex Shader
const GLchar vssource[] =
"#version 440 \n"
"layout(location=0) in vec3 a_position;"
"layout(location=1) in vec2 a_uv;"
"out vec2 v_uv;"
"void main()"
"{"
" gl_Position = vec4(a_position,1);"
" v_uv = a_uv;"
"}\n"
;
auto vshader = MakeShader(GL_VERTEX_SHADER, vssource, _countof(vssource));
// --------------------
// Fragment Shader
//const GLchar fssource[] =
// "#version 440 \n"
// "out vec4 v_color;"
// "in vec2 v_uv;"
// "uniform sampler2D u_texture;"
// "void main()"
// "{"
// " float mask = texture(u_texture, v_uv).a;"
// " v_color = vec4(mask,0,0,1);"
// "}"
//;
const GLchar fssource[] =
"#version 440 \n"
"out vec4 v_color;"
"in vec2 v_uv;"
"uniform sampler2D u_texture;"
"void main()"
"{"
" vec4 result = vec4(1,1,1,1);"
" float mask = texture(u_texture, v_uv).a;"
" if( mask >= 0.7 ) { result.a = 1; }\n"
" else { result.a = 0; }\n"
" v_color = result;"
"}"
;
auto fshader = MakeShader(GL_FRAGMENT_SHADER, fssource, _countof(fssource));
// --------------------
// Shader Program
glProgram = MakeProgram( vshader, fshader );
// --------------------
// Vertex Buffer Object
float vb[] =
{
-1, -1, 0,
1, -1, 0,
-1, 1, 0,
1, -1, 0,
1, 1, 0,
-1, 1, 0,
0, 0,
1, 0,
0, 1,
1, 0,
1, 1,
0, 1,
};
glGenBuffers(1, &vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, sizeof(vb), vb, GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, 0);
// --------------------
// Vertex Array Object
glGenVertexArrays(1, &vao);
glBindVertexArray(vao);
glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 0, (const GLvoid*)(sizeof(float)*18));
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);
//
// Freetype
//
FT_Error error = FT_Init_FreeType(&freetype);
if (error)
{
std::cout << "FreeType: error occured with error code: " << error << std::endl;
}
error = FT_New_Face(freetype, "C:/Windows/Fonts/Arial.ttf", 0, &face);
if (error)
{
std::cout << "FreeType: error occured with error code: " << error << std::endl;
}
error = FT_Set_Char_Size(face, 0, 64 * 64, 300, 300);
if (error)
{
std::cout << "FreeType: error occured with error code: " << error << std::endl;
}
unsigned char u8Character = 'A';
int32_t u32Character = 0;
utf8::utf8to32(&u8Character, &u8Character + 1, &u32Character);
error = FT_Load_Char(face, u32Character, FT_LOAD_RENDER);
if (error)
{
std::cout << "FreeType: error occured with error code: " << error << std::endl;
}
auto bitmap = face->glyph->bitmap;
const int width = bitmap.width;
const int height = bitmap.rows;
const int size = width*height;
std::vector<float> outputBuffer;
outputBuffer.resize(size);
GenerateSigendDistanceFieldFrom(face->glyph->bitmap.buffer, width, height, outputBuffer.data(), true);
std::ofstream ofs("testout.txt");
for (int i = 0; i < height; ++i)
{
for (int j = 0; j < width; ++j)
{
ofs << bitmap.buffer[i*width + j] << ' ';
}
ofs << std::endl;
}
ofs << std::endl;
for (int i = 0; i < height; ++i)
{
for (int j = 0; j < width; ++j)
{
ofs << std::setw(6) << std::setprecision(2) << std::fixed << outputBuffer[i*width + j];
}
ofs << std::endl;
}
// ----
// --------------------
// Texture
std::vector<float> invertY;
invertY.reserve(size);
for( int i = height-1; i >= 0; --i )
{
for( int j = 0; j < width; ++j )
{
invertY.push_back(outputBuffer[i*width+j]);
}
}
glGenTextures(1, &glTexture);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, glTexture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, width, height, 0, GL_ALPHA, GL_FLOAT, invertY.data());
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glGenerateMipmap(GL_TEXTURE_2D);
glClearColor(0.5f,0.5f,1.0f,1.0f);
return 0;
}
void Release()
{
SDL_DestroyWindow(window);
SDL_Quit();
}
int main(int argc, char* argv[])
{
Initialize();
bool quit = false;
while( !quit )
{
SDL_Event e;
if( SDL_PollEvent(&e) != 0 )
{
if (e.type == SDL_QUIT)
{
quit = true;
}
} // if( SDL_PollEvent(&e) != 0 )
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glUseProgram(glProgram);
GLint loc = glGetUniformLocation(glProgram, "u_texture");
glUniform1i(loc, 0);
glBindVertexArray(vao);
glDrawArrays(GL_TRIANGLES, 0, 6);
glBindVertexArray(0);
glUseProgram(0);
SDL_GL_SwapWindow(window);
} // while( !quit )
Release();
return 0;
}
2 个答案:
答案 0 :(得分：1)
如果我理解正确，你将位图字符大小设置为64（在Freetype 26.6定点类型中= 64.64），但是你需要将位图拉伸到更大的尺寸，因此缩放。
我建议你将字符大小（使用FT_Set_Char_Size）设置为等于或大于最终大小的维度。然后SW的其余部分应该保持位图原样，或者最终缩小它。这并不意味着质量损失，而升级（将大小为x的光栅图像转换为更大的尺寸）会将您带到观察到的问题。然后，为了降尺度，任何插值方案都会给你不错的结果。
答案 1 :(得分：0)
要获得平滑的边缘，您需要在边缘周围使用smoothstep，可能从0.5 alpha到0.6。 至于带符号的距离场需要花费太多时间，你不应该在运行时生成纹理，而应该事先生成它然后加载它，并存储所有必要的数据。
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