目录
一、前言
libpng、FreeImage、stb_image都是图像解析的开源库，由于三者我都简单使用过，于是做个总结对比。
二、对比
| 库 | 开源协议 | 编译依赖 | win编译难度 | 使用难度 | 格式支持 |
| libpng | zlib | zlib | 自带vs工程 | 中 | 只支持png |
| FreeImage | 混合 | 自带了7、8个库 | 有dll发行版 | 简单 | 很多 |
| stb_image | MIT | 无 | 只有头文件 | 简单 | 常用的几个 |
它们的官网如下：
GitHub - nothings/stb: stb single-file public domain libraries for C/C++
总的来说，它们的开源协议都问题不大。源码编译stb_image最简单，因为它只有一个头文件。而libpng是操作png文件的库，代码比较复杂，但是它的优点是灵活，速度更快。FreeImage集成了各种加载库，支持的格式比较多。
接下来，我就按使用难度给出它们的基本代码。注意均是读取到uint32_t缓冲区，代表RGBA32位颜色，如下：
class Image
{
//other
uint32_t* _data;
array<unsigned, 2> _size;
}
注意我并没有处理字节序问题，是写死的，在不同大小端的系统运行，应该改进一下代码（我懒得很，要等到遇到问题再解决，就是交换一下单个像素RGBA通道的读取顺序）。
觉得有用，请点赞、收藏、关注。我写这篇文件就是因为有人get了我以前写的libpng的文章。
三、stb_image
读取如下，确实很简单，直接返回的就是RGBA32位颜色：
#define STB_IMAGE_IMPLEMENTATION
#include <stb_image.h>
dnd::Image* Image::Create(string_view path_name)
{
int texWidth, texHeight, texChannels;
stbi_uc* pixels = stbi_load(string{ path_name }.c_str(),
&texWidth, &texHeight, &texChannels, STBI_rgb_alpha);
if (!pixels)
{
debug_err(format("加载图像失败：{}", path_name));
return nullptr;
}
Image* ret = new Image;
ret->_data = (uint32_t*)pixels;
ret->_size = { (unsigned)texWidth, (unsigned)texHeight };
return ret;
}
导出图像数据更简单;
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include <stb_image_write.h>
bool Image::SaveToFile(string_view path_name)
{
return stbi_write_png(string{ path_name }.c_str(), _size[0], _size[1], 4, _data, 0);
}
注意，我们应该使用它的释放函数来释放资源，转回stbi_uc*类型（实际上也就是调用了free函数，不过大家都懂不要自己free）：
stbi_image_free((stbi_uc*)_data);
四、FreeImage
代码中用到的一些函数（语法糖）如下：
/**
* @brief RAII清理操作
*/
template<typename F>
class finally
{
public:
finally(F&& func) : _func(func) {}
~finally() { _func(); }
private:
F _func;
};
//! 对容器任意元素的判断
namespace Any
{
/**
* @brief 判断容器任意元素等于某值
* @param[in] container 操作的容器
* @param[in] v 要比较的值
*/
template<typename C, typename V>
bool Equal(const C& container, const V& v)
{
for (auto& iter : container)
{
if (iter == v)
return true;
}
return false;
}
}
读取代码如下：
#include <FreeImage.h>
string path_name_mb = String::cvt_u8_mb(path_name);
const char* filename = path_name_mb.c_str();
//读文件头判断格式
FREE_IMAGE_FORMAT file_format = FreeImage_GetFileType(filename, 0);
if (file_format == FIF_UNKNOWN)
{//通过文件名读取
file_format = FreeImage_GetFIFFromFilename(filename);
}
if (file_format == FIF_UNKNOWN)
{
debug_err("图像文件格式不支持：" + string{ path_name });
return nullptr;
}
//格式支持读取则读取
FIBITMAP* bitmap = nullptr;
if (FreeImage_FIFSupportsReading(file_format))
{
bitmap = FreeImage_Load(file_format, filename);
}
if (!bitmap)
{
return nullptr;
}
//资源释放
finally f0([&]() {
FreeImage_Unload(bitmap);
});
//
unsigned bpp = FreeImage_GetBPP(bitmap); //取像素深度
FREE_IMAGE_TYPE file_type = FreeImage_GetImageType(bitmap); //取数据类型
BYTE* bits = FreeImage_GetBits(bitmap); //取像素数组
unsigned w = FreeImage_GetWidth(bitmap); //宽
unsigned h = FreeImage_GetHeight(bitmap); //高
unsigned pitch = FreeImage_GetPitch(bitmap); //每行像素(freeimage自动做了32位对齐, gl默认也是32位对齐)
if (bits == 0 || w == 0 || h == 0)
{
debug_err("图像文件基本数据错误：" + string{ path_name });
return nullptr;
}
//
if (file_type != FIT_BITMAP)
{
debug_err("图像文件类型不是位图：" + string{ path_name });
return nullptr;
}
//
vector<unsigned> mul_bpp{ 32, 24, 8 };
if (!Any::Equal(mul_bpp, bpp))
{
debug_err(format("图像文件不支持的色深({})：", bpp, path_name));
return nullptr;
}
//
Image* ret = new Image;
ret->_size = { w, h };
size_t length = size_t(w * h);
ret->_buffer = new uint32[length];
//BGRA => ABGR（RGBA） 单个访问时有字节序问题，需要反过来
if (bpp == 32)
{
for (unsigned x = 0; x < w; ++x)
{
for (unsigned y = 0; y < h; ++y)
{
unsigned p0 = y * w + x;
unsigned p1 = (h - 1 - y) * pitch + x * 4;
char* p = (char*)&(ret->_buffer[p0]);
p[0] = bits[p1 + 2];
p[1] = bits[p1 + 1];
p[2] = bits[p1 + 0];
p[3] = bits[p1 + 3];
}
}
}
else if (bpp == 24)
{
for (unsigned x = 0; x < w; x++)
{
for (unsigned y = 0; y < h; y++)
{
unsigned p0 = y * w + x;
unsigned p1 = (h - 1 - y) * pitch + x * 3;
char* p = (char*)&(ret->_buffer[p0]);
p[0] = bits[p1 + 2];
p[1] = bits[p1 + 1];
p[2] = bits[p1 + 0];
p[3] = (char)255;
}
}
}
else if (bpp == 8)
{
for (unsigned x = 0; x < w; x++)
{
for (unsigned y = 0; y < h; y++)
{
unsigned p0 = y * w + x;
unsigned p1 = (h - 1 - y) * pitch + x * 1;
char* p = (char*)&(ret->_buffer[p0]);
p[0] = bits[p1];
p[1] = bits[p1];
p[2] = bits[p1];
p[3] = (char)255;
}
}
}
return ret;
导出图像数据到文件：
string path_name_mb = String::cvt_u8_mb(path_name);
const char* filename = path_name_mb.c_str();
FREE_IMAGE_FORMAT file_format = FreeImage_GetFIFFromFilename(filename);
if (file_format == FIF_UNKNOWN)
{
debug_err("不支持的图像格式：" + string{ path_name });
return false;
}
unsigned w = _size[0];
unsigned h = _size[1];
uint32_t* bits = new uint32_t[w * h];
//ABGR（RGBA）=> BGRA 单个访问时有字节序问题，需要反过来
for (unsigned x = 0; x < w; ++x)
{
for (unsigned y = 0; y < h; ++y)
{
unsigned index = y * w + x;
char* p_dst = (char*)&(bits[index]);
char* p = (char*)&(_buffer[index]);
p_dst[0] = p[2];
p_dst[1] = p[1];
p_dst[2] = p[0];
p_dst[3] = p[3];
}
}
//
FIBITMAP* bitmap = FreeImage_ConvertFromRawBits((BYTE*)bits,
w, h, w * 4, 32, 0, 0, 0, true);
delete[] bits;
FreeImage_Save(file_format, bitmap, filename);
//释放
FreeImage_Unload(bitmap);
return true;
五、libpng
这里的代码比较陈旧，用了win32的类型，自行修改一下即可。加载代码如下：
#include <png.h>
Image* img = new Image;
///从文件加载/
FILE* fp = NULL;
if (sys->GetFile(path, fp) == -1)
{
debug_err(String(L"DND: Image::Create: 图像文件打开失败: ") + path);
return NULL;
}
//判断是否问 png 文件
size_t number = 8;
png_bytep header = new png_byte[number];
fread(header, 1, number, fp);
bool is_png = !png_sig_cmp(header, 0, number);
if (!is_png)
{
fclose(fp);
debug_err(String(L"DND: Image::Create: 必须是png文件: ") + path);
return NULL;
}
//初始化pnglib
png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
NULL, NULL, NULL);
if (!png_ptr)
{
fclose(fp);
debug_err(L"DND: Image::Create: 初始化pnglib失败！");
return NULL;
}
//创建图像信息 info
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
fclose(fp);
debug_err(L"DND: Image::Create: 创建png_info失败！");
return NULL;
}
//错误处理
if (setjmp(png_jmpbuf(png_ptr)))
{
fclose(fp);
png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
debug_err(L"DND: Image::Create: pnglib出现错误！");
return NULL;
}
//设置数据源
png_init_io(png_ptr, fp);
//表明文件头已处理
png_set_sig_bytes(png_ptr, number);
//读png 这一步会实际分配内存
png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_EXPAND, 0);
fclose(fp);
//从info查询数据
unsigned w = png_get_image_width(png_ptr, info_ptr); //获得图片宽度
unsigned h = png_get_image_height(png_ptr, info_ptr); //获得图片高度
int color_type = png_get_color_type(png_ptr, info_ptr); //获得图片颜色类型
//赋值image
img->_size.w = w;
img->_size.h = h;
img->_buffer = new DWORD[w*h];
//从info 复制到 image
png_bytep *row_point = NULL;
row_point = png_get_rows(png_ptr, info_ptr);
int block_size = (color_type == 6 ? 4 : 3);
//(A)RGB
unsigned pos = 0;
for (unsigned x = 0; x < h; ++x)
for (unsigned y = 0; y < w*block_size; y += block_size)
{
((unsigned char*)img->_buffer)[pos + 0] = row_point[x][y + 2];//b;
((unsigned char*)img->_buffer)[pos + 1] = row_point[x][y + 1];//g
((unsigned char*)img->_buffer)[pos + 2] = row_point[x][y + 0];//r
if (color_type == 6)
((unsigned char*)img->_buffer)[pos + 3] = row_point[x][y + 3];//a
else
((unsigned char*)img->_buffer)[pos + 3] = 0xff;
pos += 4;
}
//释放png内存
png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
return img;
导出图像数据到文件：
FILE* fp;
png_infop info_ptr;
char cpath[MAX_PATH] = { NULL };
path.GetMultiByteStr(cpath, MAX_PATH);
fopen_s(&fp, cpath, "wb");
if (fp == NULL)
{
debug_err(L"DND: Image::SaveToPNG: 创建文件失败！");
return;
}
//初始化pnglib
static png_structp png_ptr = NULL;
if (!png_ptr)
png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
NULL, NULL, NULL);
if (!png_ptr)
{
debug_err(L"DND: Image::SaveToPNG: 创建文件时初始化pnglib失败！");
return;
}
info_ptr = png_create_info_struct(png_ptr);
if (info_ptr == NULL)
{
fclose(fp);
debug_err(L"DND: Image::SaveToPNG: png_create_info_struct失败！");
return;
}
//错误处理
if (setjmp(png_jmpbuf(png_ptr)))
{
fclose(fp);
png_destroy_write_struct(&png_ptr, (png_infopp)NULL);
debug_err(L"DND: Image::SaveToPNG: pnglib 出现错误！");
return;
}
unsigned bit_depth = 8;
unsigned pixel_byte = 4;
unsigned row_byte = _size.w * pixel_byte;
//设置输出控制
png_init_io(png_ptr, fp);
//设置图像属性
png_set_IHDR(png_ptr, info_ptr, _size.w, _size.h, bit_depth,
PNG_COLOR_TYPE_RGB_ALPHA, PNG_INTERLACE_NONE, //交错无
PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
//写头部
png_write_info(png_ptr, info_ptr);
//获取行指针
png_bytepp row_pointers = (png_bytep*)malloc(_size.h*sizeof(png_bytep));
for (unsigned x = 0; x < _size.h; ++x)
{//分配一行
row_pointers[x] = (png_bytep)malloc(row_byte);
for (unsigned y = 0; y < row_byte; y += pixel_byte)
{
row_pointers[x][y + 2] = ((unsigned char*)_buffer)[x * row_byte + y + 0];
row_pointers[x][y + 1] = ((unsigned char*)_buffer)[x * row_byte + y + 1];
row_pointers[x][y + 0] = ((unsigned char*)_buffer)[x * row_byte + y + 2];
row_pointers[x][y + 3] = ((unsigned char*)_buffer)[x * row_byte + y + 3];
/*row_pointers[x][y + 2] =
row_pointers[x][y + 1] =
row_pointers[x][y + 0] =
row_pointers[x][y + 3] = 0xff;*/
}
}
//写入全部
png_write_image(png_ptr, row_pointers);
//写尾部
png_write_end(png_ptr, info_ptr);
//释放png内存
png_destroy_write_struct(&png_ptr, (png_infopp) NULL);
/* delete[] row_pointers;
delete[] image;*/
for (unsigned x = 0; x < _size.h; ++x)
{//释放每行
free(row_pointers[x]);
}
free(row_pointers);
fclose(fp);
