c++ - 距离场字体渲染的实现

Question

真遗憾，我从来没有在我的应用程序上获得可靠的字体渲染系统。所以我决定结束这一切。

当我在搜索字体渲染示例时，我碰巧遇到了这个 valve paper得到大家的称赞。

但我缺乏经验和知识来完成这个。

这是我所做的总结:

首先，使用 Freetype2 库，我将一个字符 ('A') 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位图每个像素的切比雪夫距离，以生成 float 图。

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，我将这个角色纹理渲染到尺寸为 800x600 的整个屏幕上，以查看它们如何扩展。纹理采样“GL_LINEAR”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!渲染文本。 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!渲染文本。 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;"
    "}"

字体看起来凹凸不平，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，距离场的生成也太费时了。因此，我无法使用高分辨率图像作为输入。

这里我做的事情显然是错误的，但似乎我要靠自己弄清楚如何生成正确的结果。

但是，如果你知道我做错了什么，请帮助我。

下面是完整的源文件:

#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;
}

Answer 1

如果我理解正确，您可以将位图字符大小设置为 64(在 Freetype 26.6 定点类型中为 64.64)，但是您需要将位图拉伸(stretch)到更大的大小，从而进行缩放。

我建议您将字符大小(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不变，或者最终缩小它的大小。这并不意味着质量下降，而放大(将大小为 x 的光栅图像变为更大的大小)会将您带到观察到的问题。然后缩小任何插值方案都会给你不错的结果。