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. \n";
		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：