FreeImage 生成带透明通道的GIF

主要方法：
1. 加载图像及读取参数
FreeImage_Load
FreeImage_GetWidth
FreeImage_GetHeight
FreeImage_Allocate
FreeImage_GetPixelColor
FreeImage_SetPixelColor
2. 保存GIF
FreeImage_OpenMultiBitmap
FreeImage_SetMetadata
FreeImage_SetTagXX
FreeImage_AppendPage
3. GIF 透明模板
FreeImage_ColorQuantize
FreeImage_GetPalette
FreeImage_SetTransparencyTable

这里需要注意的是，图像通道32位转24位时，不使用FreeImage_ConvertTo24Bits，而通过PNG透明通道进行数据复制，避免背景对后面生成透明表(FreeImage _SetTransparencyTable）造成干扰。此外，注意设置24位图的背景色，避免与表情区域颜色相近。

这里做了如下设置

src_color.rgbRed = 0; 
src_color.rgbBlue = 255; 
src_color.rgbGreen = 0;

完整代码：

#include "FreeImagePlus.h"

#include <iostream>
using namespace std;

FIBITMAP* genhDIB8bpp(FIBITMAP* hDIB32bpp);

int main(int argc, char** argv[])
{
    FreeImage_Initialise(TRUE);
    // load source images
    FIBITMAP* hdib1 = FreeImage_Load(FIF_PNG, "img1.png", PNG_IGNOREGAMMA);
    FIBITMAP* hdib2 = FreeImage_Load(FIF_PNG, "img2.png", PNG_IGNOREGAMMA);
    FIBITMAP* hdib3 = FreeImage_Load(FIF_PNG, "img3.png", PNG_IGNOREGAMMA);

    if (NULL == hdib1 || NULL == hdib2 || NULL == hdib3)
    {
        cout << " load  image(png) error. 
";
        return -1;
    }

    int bpp1 = FreeImage_GetBPP(hdib1);
    int bpp2 = FreeImage_GetBPP(hdib2);
    int bpp3 = FreeImage_GetBPP(hdib3);

    int dib_type = FreeImage_GetImageType(hdib1);
    cout << "Type: " << dib_type << " " << FIT_BITMAP  << endl;
    cout << "BPP: " << bpp1 << " " << bpp2 << " " << bpp3 << endl;
    if (32 != bpp1 || 32 != bpp2 || 32 != bpp3)
        return -1;


    bool bMemoryCache = TRUE;

    FIMULTIBITMAP *multi = FreeImage_OpenMultiBitmap(FIF_GIF, "out.gif", TRUE, FALSE);
    int fps = 3;
    DWORD dwFrameTime = (DWORD)((1000.0f / fps) + 0.5f);

    cout << "gen hDIB8bpp " << endl;
    FIBITMAP* dib[3];
    dib[0] = genhDIB8bpp(hdib1);
    dib[1] = genhDIB8bpp(hdib2);
    dib[2] = genhDIB8bpp(hdib3);

    cout << "AppendPage" << endl;
    for (int i = 0; i < 3; i++ ){
        // clear any animation metadata used by this dib as we’ll adding our own ones
        FreeImage_SetMetadata(FIMD_ANIMATION, dib[i], NULL, NULL);
        // add animation tags to dib[i]
        FITAG *tag = FreeImage_CreateTag();
        if (tag) {
            FreeImage_SetTagKey(tag, "FrameTime");
            FreeImage_SetTagType(tag, FIDT_LONG);
            FreeImage_SetTagCount(tag, 1);
            FreeImage_SetTagLength(tag, 4);
            FreeImage_SetTagValue(tag, &dwFrameTime);
            FreeImage_SetMetadata(FIMD_ANIMATION, dib[i], FreeImage_GetTagKey(tag), tag);
            FreeImage_DeleteTag(tag);
        }
        FreeImage_AppendPage(multi, dib[i]);
//      FreeImage_Unload(dib[i]);
    }
    FreeImage_CloseMultiBitmap(multi);

    FreeImage_Unload(dib[0]);
    FreeImage_Unload(dib[1]);
    FreeImage_Unload(dib[2]);

    FreeImage_Unload(hdib1);
    FreeImage_Unload(hdib2);
    FreeImage_Unload(hdib3);

    FreeImage_DeInitialise();
    return 0;
}

FIBITMAP* genhDIB8bpp(FIBITMAP* hDIB32bpp)
{
    FIBITMAP* hDIB8bpp = NULL;
    int w = FreeImage_GetWidth(hDIB32bpp);
    int h = FreeImage_GetHeight(hDIB32bpp);

//  FIBITMAP *hDIB24bpp  = FreeImage_ConvertTo24Bits(hDIB32bpp);
    FIBITMAP *hDIB24bpp = FreeImage_Allocate(w, h, 24);
    for (int i = h - 1; i >= 0; i--)
        for (int j = 0; j < w; j++)
        {
            RGBQUAD src_color;
            FreeImage_GetPixelColor(hDIB32bpp, j, i, &src_color);
            if ( 255 == src_color.rgbReserved )
                FreeImage_SetPixelColor(hDIB24bpp, j, i, &src_color);
            else
            {
                src_color.rgbRed = 0; 
                src_color.rgbBlue = 255; src_color.rgbGreen = 0;
                FreeImage_SetPixelColor(hDIB24bpp, j, i, &src_color);
            }
        }
    FreeImage_Save(FIF_JPEG, hDIB24bpp, "out.jpg");

    BYTE  Transparency[256];
    if (!hDIB8bpp)
        hDIB8bpp = FreeImage_ColorQuantize(hDIB24bpp, FIQ_WUQUANT);

    RGBQUAD *Palette = FreeImage_GetPalette(hDIB8bpp);
    for (int i = 0; i < 256; i++)
    {
        Transparency[i] = 0xFF;
        if (Palette[i].rgbRed != 0x00 && Palette[i].rgbBlue != 0xFF && Palette[i].rgbGreen > 0x00)
        {
            Transparency[i] = 0x00;
        }
    }

    FreeImage_SetTransparencyTable(hDIB8bpp, Transparency, 256);

//  FreeImage_Save(FIF_PNG, hDIB8bpp, "out.png");
    FreeImage_Unload(hDIB24bpp);

    return hDIB8bpp;
}

原图：

结果：
眉毛缺失眉毛完整