将透明PNG保存为透明GIF

我正在尝试调整透明png的大小并将其保存为单帧gif图像。 让我们跳过resize的部分，当你尝试将透明png保存为gif时，你会在输出gif中看到一个黑色背景：

Bitmap n = new Bitmap(targetPngPath); n.Save(@"C:\1.gif", ImageFormat.Gif); 

是的，我可以将黑色背景变成白色，但这不是我想要的。 即使我可以使用MakeTransparent方法删除黑色，但它会删除图像中的每个黑色，我们将没有标准的透明图像。

我们也可以做一个TRICK保存gif图像，我们在文件名中保留扩展名，但我们会将其保存为PNG格式，如下所示：

 n.Save(@"C:\1.gif", ImageFormat.Png); 

但它也不是标准的。 那么有没有办法安全地将透明的png保存为具有透明度的gif图像？

PNG = GIF =

用Photoshop =保存GIF

Answer 1

这是因为内置GIF编码器无法很好地处理源，除非它已经是8 bpp图像。 您必须先将PNG图像转换为256色图像，然后才能使用GIF编码器正确保存。

 public static void SaveGif(string fileName, Image image) { int bpp = Image.GetPixelFormatSize(image.PixelFormat); if (bpp == 8) { image.Save(fileName, ImageFormat.Gif); return; } // 1 and 4 bpp images are need to be converted, too; otherwise, gif encoder encodes the image from 32 bpp image resulting 256 color, no transparency if (bpp < 8) { using (Image image8Bpp = ConvertPixelFormat(image, PixelFormat.Format8bppIndexed, null)) { image8Bpp.Save(fileName, ImageFormat.Gif); return; } } // high/true color bitmap: obtaining the colors // Converting always to 8 bpp pixel format; otherwise, gif encoder would convert it to 32 bpp first. // With 8 bpp, gif encoder will preserve transparency and will save compact palette // Note: This works well for 256 color images in a 32bpp bitmap. Otherwise, you might try to pass null as palette so a default palette will be used. Color[] palette = GetColors((Bitmap)image, 256); using (Image imageIndexed = ConvertPixelFormat(image, PixelFormat.Format8bppIndexed, palette)) { imageIndexed.Save(fileName, ImageFormat.Gif); } } // TODO: Use some quantizer private static Color[] GetColors(Bitmap bitmap, int maxColors) { if (bitmap == null) throw new ArgumentNullException("bitmap"); if (maxColors < 0) throw new ArgumentOutOfRangeException("maxColors"); HashSet colors = new HashSet(); PixelFormat pixelFormat = bitmap.PixelFormat; if (Image.GetPixelFormatSize(pixelFormat) <= 8) return bitmap.Palette.Entries; // 32 bpp source: the performant variant if (pixelFormat == PixelFormat.Format32bppRgb || pixelFormat == PixelFormat.Format32bppArgb || pixelFormat == PixelFormat.Format32bppPArgb) { BitmapData data = bitmap.LockBits(new Rectangle(Point.Empty, bitmap.Size), ImageLockMode.ReadOnly, pixelFormat); try { unsafe { byte* line = (byte*)data.Scan0; for (int y = 0; y < data.Height; y++) { for (int x = 0; x < data.Width; x++) { int c = ((int*)line)[x]; // if alpha is 0, adding the transparent color if ((c >> 24) == 0) c = 0xFFFFFF; if (colors.Contains(c)) continue; colors.Add(c); if (colors.Count == maxColors) return colors.Select(Color.FromArgb).ToArray(); } line += data.Stride; } } } finally { bitmap.UnlockBits(data); } } else { // fallback: getpixel for (int y = 0; y < bitmap.Height; y++) { for (int x = 0; x < bitmap.Width; x++) { int c = bitmap.GetPixel(x, y).ToArgb(); if (colors.Contains(c)) continue; colors.Add(c); if (colors.Count == maxColors) return colors.Select(Color.FromArgb).ToArray(); } } } return colors.Select(Color.FromArgb).ToArray(); } private static Image ConvertPixelFormat(Image image, PixelFormat newPixelFormat, Color[] palette) { if (image == null) throw new ArgumentNullException("image"); PixelFormat sourcePixelFormat = image.PixelFormat; int bpp = Image.GetPixelFormatSize(newPixelFormat); if (newPixelFormat == PixelFormat.Format16bppArgb1555 || newPixelFormat == PixelFormat.Format16bppGrayScale) throw new NotSupportedException("This pixel format is not supported by GDI+"); Bitmap result; // non-indexed target image (transparency preserved automatically) if (bpp > 8) { result = new Bitmap(image.Width, image.Height, newPixelFormat); using (Graphics g = Graphics.FromImage(result)) { g.DrawImage(image, 0, 0, image.Width, image.Height); } return result; } int transparentIndex; Bitmap bmp; // indexed colors: using GDI+ natively RGBQUAD[] targetPalette = new RGBQUAD[256]; int colorCount = InitPalette(targetPalette, bpp, (image is Bitmap) ? image.Palette : null, palette, out transparentIndex); BITMAPINFO bmi = new BITMAPINFO(); bmi.icHeader.biSize = (uint)Marshal.SizeOf(typeof(BITMAPINFOHEADER)); bmi.icHeader.biWidth = image.Width; bmi.icHeader.biHeight = image.Height; bmi.icHeader.biPlanes = 1; bmi.icHeader.biBitCount = (ushort)bpp; bmi.icHeader.biCompression = BI_RGB; bmi.icHeader.biSizeImage = (uint)(((image.Width + 7) & 0xFFFFFFF8) * image.Height / (8 / bpp)); bmi.icHeader.biXPelsPerMeter = 0; bmi.icHeader.biYPelsPerMeter = 0; bmi.icHeader.biClrUsed = (uint)colorCount; bmi.icHeader.biClrImportant = (uint)colorCount; bmi.icColors = targetPalette; bmp = (image as Bitmap) ?? new Bitmap(image); // Creating the indexed bitmap IntPtr bits; IntPtr hbmResult = CreateDIBSection(IntPtr.Zero, ref bmi, DIB_RGB_COLORS, out bits, IntPtr.Zero, 0); // Obtaining screen DC IntPtr dcScreen = GetDC(IntPtr.Zero); // DC for the original hbitmap IntPtr hbmSource = bmp.GetHbitmap(); IntPtr dcSource = CreateCompatibleDC(dcScreen); SelectObject(dcSource, hbmSource); // DC for the indexed hbitmap IntPtr dcTarget = CreateCompatibleDC(dcScreen); SelectObject(dcTarget, hbmResult); // Copy content BitBlt(dcTarget, 0, 0, image.Width, image.Height, dcSource, 0, 0, 0x00CC0020 /*TernaryRasterOperations.SRCCOPY*/); // obtaining result result = Image.FromHbitmap(hbmResult); result.SetResolution(image.HorizontalResolution, image.VerticalResolution); // cleanup DeleteDC(dcSource); DeleteDC(dcTarget); ReleaseDC(IntPtr.Zero, dcScreen); DeleteObject(hbmSource); DeleteObject(hbmResult); ColorPalette resultPalette = result.Palette; bool resetPalette = false; // restoring transparency if (transparentIndex >= 0) { // updating palette if transparent color is not actually transparent if (resultPalette.Entries[transparentIndex].A != 0) { resultPalette.Entries[transparentIndex] = Color.Transparent; resetPalette = true; } ToIndexedTransparentByArgb(result, bmp, transparentIndex); } if (resetPalette) result.Palette = resultPalette; if (!ReferenceEquals(bmp, image)) bmp.Dispose(); return result; } private static int InitPalette(RGBQUAD[] targetPalette, int bpp, ColorPalette originalPalette, Color[] desiredPalette, out int transparentIndex) { int maxColors = 1 << bpp; // using desired palette Color[] sourcePalette = desiredPalette; // or, using original palette if it has fewer or the same amount of colors as requested if (sourcePalette == null && originalPalette != null && originalPalette.Entries.Length > 0 && originalPalette.Entries.Length <= maxColors) sourcePalette = originalPalette.Entries; // or, using default system palette if (sourcePalette == null) { using (Bitmap bmpReference = new Bitmap(1, 1, GetPixelFormat(bpp))) { sourcePalette = bmpReference.Palette.Entries; } } // it is ignored if source has too few colors (rest of the entries will be black) transparentIndex = -1; bool hasBlack = false; int colorCount = Math.Min(maxColors, sourcePalette.Length); for (int i = 0; i < colorCount; i++) { targetPalette[i] = new RGBQUAD(sourcePalette[i]); if (transparentIndex == -1 && sourcePalette[i].A == 0) transparentIndex = i; if (!hasBlack && (sourcePalette[i].ToArgb() & 0xFFFFFF) == 0) hasBlack = true; } // if transparent index is 0, relocating it and setting transparent index to 1 if (transparentIndex == 0) { targetPalette[0] = targetPalette[1]; transparentIndex = 1; } // otherwise, setting the color of transparent index the same as the previous color, so it will not be used during the conversion else if (transparentIndex != -1) { targetPalette[transparentIndex] = targetPalette[transparentIndex - 1]; } // if black color is not found in palette, counting 1 extra colors because it can be used in conversion if (colorCount < maxColors && !hasBlack) colorCount++; return colorCount; } private unsafe static void ToIndexedTransparentByArgb(Bitmap target, Bitmap source, int transparentIndex) { int sourceBpp = Image.GetPixelFormatSize(source.PixelFormat); int targetBpp = Image.GetPixelFormatSize(target.PixelFormat); BitmapData dataTarget = target.LockBits(new Rectangle(Point.Empty, target.Size), ImageLockMode.ReadWrite, target.PixelFormat); BitmapData dataSource = source.LockBits(new Rectangle(Point.Empty, source.Size), ImageLockMode.ReadOnly, source.PixelFormat); try { byte* lineSource = (byte*)dataSource.Scan0; byte* lineTarget = (byte*)dataTarget.Scan0; bool is32Bpp = sourceBpp == 32; // scanning through the lines for (int y = 0; y < dataSource.Height; y++) { // scanning through the pixels within the line for (int x = 0; x < dataSource.Width; x++) { // testing if pixel is transparent (applies both argb and pargb) if (is32Bpp && ((uint*)lineSource)[x] >> 24 == 0 || !is32Bpp && ((ulong*)lineSource)[x] >> 48 == 0UL) { switch (targetBpp) { case 8: lineTarget[x] = (byte)transparentIndex; break; case 4: // First pixel is the high nibble int pos = x >> 1; byte nibbles = lineTarget[pos]; if ((x & 1) == 0) { nibbles &= 0x0F; nibbles |= (byte)(transparentIndex << 4); } else { nibbles &= 0xF0; nibbles |= (byte)transparentIndex; } lineTarget[pos] = nibbles; break; case 1: // First pixel is MSB. pos = x >> 3; byte mask = (byte)(128 >> (x & 7)); if (transparentIndex == 0) lineTarget[pos] &= (byte)~mask; else lineTarget[pos] |= mask; break; } } } lineSource += dataSource.Stride; lineTarget += dataTarget.Stride; } } finally { target.UnlockBits(dataTarget); source.UnlockBits(dataSource); } } private static PixelFormat GetPixelFormat(int bpp) { switch (bpp) { case 1: return PixelFormat.Format1bppIndexed; case 4: return PixelFormat.Format4bppIndexed; case 8: return PixelFormat.Format8bppIndexed; case 16: return PixelFormat.Format16bppRgb565; case 24: return PixelFormat.Format24bppRgb; case 32: return PixelFormat.Format32bppArgb; case 48: return PixelFormat.Format48bppRgb; case 64: return PixelFormat.Format64bppArgb; default: throw new ArgumentOutOfRangeException("bpp"); } } 

和本机类型和方法：

 private const int BI_RGB = 0; private const int DIB_RGB_COLORS = 0; [DllImport("gdi32.dll", CharSet = CharSet.Auto)] private static extern IntPtr CreateDIBSection(IntPtr hdc, [In] ref BITMAPINFO pbmi, int iUsage, out IntPtr ppvBits, IntPtr hSection, uint dwOffset); [DllImport("user32.dll", CharSet = CharSet.Auto)] private static extern IntPtr GetDC(IntPtr hWnd); [DllImport("gdi32.dll", SetLastError = true)] private static extern IntPtr SelectObject(IntPtr hdc, IntPtr hgdiobj); [DllImport("gdi32.dll", SetLastError = true)] private static extern IntPtr CreateCompatibleDC(IntPtr hdc); [DllImport("gdi32.dll", SetLastError = true)] [return: MarshalAs(UnmanagedType.Bool)] private static extern bool BitBlt(IntPtr hdc, int nXDest, int nYDest, int nWidth, int nHeight, IntPtr hdcSrc, int nXSrc, int nYSrc, uint dwRop); [DllImport("gdi32.dll")] private static extern bool DeleteDC(IntPtr hdc); [DllImport("gdi32.dll", SetLastError = true)] private static extern bool DeleteObject(IntPtr hObject); [DllImport("user32.dll")] private static extern bool ReleaseDC(IntPtr hWnd, IntPtr hDC); [StructLayout(LayoutKind.Sequential)] private struct RGBQUAD { internal byte rgbBlue; internal byte rgbGreen; internal byte rgbRed; internal byte rgbReserved; internal RGBQUAD(Color color) { rgbRed = color.R; rgbGreen = color.G; rgbBlue = color.B; rgbReserved = 0; } } [StructLayout(LayoutKind.Sequential)] private struct BITMAPINFO { public BITMAPINFOHEADER icHeader; [MarshalAs(UnmanagedType.ByValArray, SizeConst = 256)] public RGBQUAD[] icColors; } [StructLayout(LayoutKind.Sequential)] private struct BITMAPINFOHEADER { internal uint biSize; internal int biWidth; internal int biHeight; internal ushort biPlanes; internal ushort biBitCount; internal uint biCompression; internal uint biSizeImage; internal int biXPelsPerMeter; internal int biYPelsPerMeter; internal uint biClrUsed; internal uint biClrImportant; } 

Answer 2

这可能有所帮助。 Bitmap类无法正确保存透明度。 您需要将Bitmap转换为Image。

c＃Bitmap.Save transparancy不保存在png中

互联网上有关于.NET没有正确保存透明度位图的评论。

这是一个很好的链接，可以进一步阅读，过多的代码发布。

http://forums.asp.net/t/1057792.aspx?ASP+NET+C+Making+an+Image+transparent