所以我创建了一个索引颜色，每像素8位PNG（如果格式正确，我已经使用ImageMagick检查过）并且我想将它从磁盘加载到System.Drawing.Bitmap，同时保持8bpp像素格式，以查看（和操纵）其调色板。但是，如果我创建这样的位图：
Bitmap bitmap = new Bitmap("indexed-image.png");
生成的Bitmap会自动转换为32bpp图像格式，而bitmap.Palette.Entries字段则显示为空。
回答问题＆＃34;如何在C＃中将32bpp图像转换为8bpp？＆＃34; StackOverflow上说这可能是将其转换回8bpp的有效方法：
bitmap = bitmap.Clone(new Rectangle(0, 0, bitmap.Width, bitmap.Height), PixelFormat.Format8bppIndexed);
然而，这会产生不正确的结果，因为调色板中的某些颜色是完全错误的。
如何将图像本机加载到8bpp，或者至少正确地将32bpp图像加载到8bpp？
答案 0 :(得分：1)
我也有这个问题，似乎任何包含透明度的调色板png图像都无法加载为.Net框架调整，尽管.Net功能完全可以写这样的文件。相反，如果文件是gif格式，或者调色板的png具有 no 透明度，则它没有问题。
调色板png中的透明度通过在标题中添加可选的“tRNS”块来工作，以指定每个调色板条目的alpha。 .Net类正确读取并应用它，所以我真的不明白为什么然后他们坚持将图像转换为32位。
png格式的结构相当简单;在识别字节之后，每个块是内容大小的4个字节（big-endian），然后是块id的4个ASCII字符，然后是块内容本身，最后是4个字节的块CRC值（再次，保存为大-endian）。
鉴于这种结构，解决方案非常简单：
Bitmap创建MemoryStream对象，从而生成正确的8位图像。如果您执行检查和后退，您可以使用此功能加载任何图像，如果它恰好标识为具有透明度信息的调色板png，它将执行修复。
/// <summary>
/// Image loading toolset class which corrects the bug that prevents paletted PNG images with transparency from being loaded as paletted.
/// </summary>
public class BitmapHandler
{
private static Byte[] PNG_IDENTIFIER = {0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A};
/// <summary>
/// Loads an image, checks if it is a PNG containing palette transparency, and if so, ensures it loads correctly.
/// The theory on the png internals can be found at http://www.libpng.org/pub/png/book/chapter08.html
/// </summary>
/// <param name="data">File data to load.</param>
/// <returns>The loaded image.</returns>
public static Bitmap LoadBitmap(Byte[] data)
{
Byte[] transparencyData = null;
if (data.Length > PNG_IDENTIFIER.Length)
{
// Check if the image is a PNG.
Byte[] compareData = new Byte[PNG_IDENTIFIER.Length];
Array.Copy(data, compareData, PNG_IDENTIFIER.Length);
if (PNG_IDENTIFIER.SequenceEqual(compareData))
{
// Check if it contains a palette.
// I'm sure it can be looked up in the header somehow, but meh.
Int32 plteOffset = FindChunk(data, "PLTE");
if (plteOffset != -1)
{
// Check if it contains a palette transparency chunk.
Int32 trnsOffset = FindChunk(data, "tRNS");
if (trnsOffset != -1)
{
// Get chunk
Int32 trnsLength = GetChunkDataLength(data, trnsOffset);
transparencyData = new Byte[trnsLength];
Array.Copy(data, trnsOffset + 8, transparencyData, 0, trnsLength);
// filter out the palette alpha chunk, make new data array
Byte[] data2 = new Byte[data.Length - (trnsLength + 12)];
Array.Copy(data, 0, data2, 0, trnsOffset);
Int32 trnsEnd = trnsOffset + trnsLength + 12;
Array.Copy(data, trnsEnd, data2, trnsOffset, data.Length - trnsEnd);
data = data2;
}
}
}
}
using(MemoryStream ms = new MemoryStream(data))
using(Bitmap loadedImage = new Bitmap(ms))
{
if (loadedImage.Palette.Entries.Length != 0 && transparencyData != null)
{
ColorPalette pal = loadedImage.Palette;
for (int i = 0; i < pal.Entries.Length; i++)
{
if (i >= transparencyData.Length)
break;
Color col = pal.Entries[i];
pal.Entries[i] = Color.FromArgb(transparencyData[i], col.R, col.G, col.B);
}
loadedImage.Palette = pal;
}
// Images in .Net often cause odd crashes when their backing resource disappears.
// This prevents that from happening by copying its inner contents into a new Bitmap object.
return CloneImage(loadedImage, null);
}
}
/// <summary>
/// Finds the start of a png chunk. This assumes the image is already identified as PNG.
/// It does not go over the first 8 bytes, but starts at the start of the header chunk.
/// </summary>
/// <param name="data">The bytes of the png image.</param>
/// <param name="chunkName">The name of the chunk to find.</param>
/// <returns>The index of the start of the png chunk, or -1 if the chunk was not found.</returns>
private static Int32 FindChunk(Byte[] data, String chunkName)
{
if (data == null)
throw new ArgumentNullException("data", "No data given!");
if (chunkName == null)
throw new ArgumentNullException("chunkName", "No chunk name given!");
// Using UTF-8 as extra check to make sure the name does not contain > 127 values.
Byte[] chunkNamebytes = Encoding.UTF8.GetBytes(chunkName);
if (chunkName.Length != 4 || chunkNamebytes.Length != 4)
throw new ArgumentException("Chunk name must be 4 ASCII characters!", "chunkName");
Int32 offset = PNG_IDENTIFIER.Length;
Int32 end = data.Length;
Byte[] testBytes = new Byte[4];
// continue until either the end is reached, or there is not enough space behind it for reading a new header
while (offset < end && offset + 8 < end)
{
Array.Copy(data, offset + 4, testBytes, 0, 4);
if (chunkNamebytes.SequenceEqual(testBytes))
return offset;
Int32 chunkLength = GetChunkDataLength(data, offset);
// chunk size + chunk header + chunk checksum = 12 bytes.
offset += 12 + chunkLength;
}
return -1;
}
private static Int32 GetChunkDataLength(Byte[] data, Int32 offset)
{
if (offset + 4 > data.Length)
throw new IndexOutOfRangeException("Bad chunk size in png image.");
// Don't want to use BitConverter; then you have to check platform endianness and all that mess.
Int32 length = data[offset + 3] + (data[offset + 2] << 8) + (data[offset + 1] << 16) + (data[offset] << 24);
if (length < 0)
throw new IndexOutOfRangeException("Bad chunk size in png image.");
return length;
}
/// <summary>
/// Clones an image object to free it from any backing resources.
/// Code taken from http://stackoverflow.com/a/3661892/ with some extra fixes.
/// </summary>
/// <param name="sourceImage">The image to clone.</param>
/// <returns>The cloned image.</returns>
public static Bitmap CloneImage(Bitmap sourceImage)
{
Rectangle rect = new Rectangle(0, 0, sourceImage.Width, sourceImage.Height);
Bitmap targetImage = new Bitmap(rect.Width, rect.Height, sourceImage.PixelFormat);
targetImage.SetResolution(sourceImage.HorizontalResolution, sourceImage.VerticalResolution);
BitmapData sourceData = sourceImage.LockBits(rect, ImageLockMode.ReadOnly, sourceImage.PixelFormat);
BitmapData targetData = targetImage.LockBits(rect, ImageLockMode.WriteOnly, targetImage.PixelFormat);
Int32 actualDataWidth = ((Image.GetPixelFormatSize(sourceImage.PixelFormat) * rect.Width) + 7) / 8;
Int32 h = sourceImage.Height;
Int32 origStride = sourceData.Stride;
Int32 targetStride = targetData.Stride;
Byte[] imageData = new Byte[actualDataWidth];
IntPtr sourcePos = sourceData.Scan0;
IntPtr destPos = targetData.Scan0;
// Copy line by line, skipping by stride but copying actual data width
for (Int32 y = 0; y < h; y++)
{
Marshal.Copy(sourcePos, imageData, 0, actualDataWidth);
Marshal.Copy(imageData, 0, destPos, actualDataWidth);
sourcePos = new IntPtr(sourcePos.ToInt64() + origStride);
destPos = new IntPtr(destPos.ToInt64() + targetStride);
}
targetImage.UnlockBits(targetData);
sourceImage.UnlockBits(sourceData);
// For indexed images, restore the palette. This is not linking to a referenced
// object in the original image; the getter of Palette creates a new object when called.
if ((sourceImage.PixelFormat & PixelFormat.Indexed) != 0)
targetImage.Palette = sourceImage.Palette;
// Restore DPI settings
targetImage.SetResolution(sourceImage.HorizontalResolution, sourceImage.VerticalResolution);
return targetImage;
}
}
但是这个方法似乎只解决了8位和4位png的问题。由Gimp重新保存的只有4种颜色的png变为2位png，尽管不包含任何透明度，仍然以32位颜色打开。
实际上存在类似的问题，即保存调色板大小; .Net框架可以完美地处理加载png文件，其调色板不是完整大小（8位小于256，4位小于16），但保存文件时，它会将其填充到完整的调色板。这可以通过类似的方式修复by post-processing the chunks after saving to a MemoryStream。但这需要计算CRC。
另请注意，虽然这应该可以加载任何图像类型，但它无法在动画GIF文件上正常工作，因为最后使用的CloneImage函数只能复制单个图像。
答案 1 :(得分：0)
据我所知，默认的8bpp调色板不会作为PNG的索引图像加载。您可以修复文件中的调色板，或将文件转换为GIF，BMP或TIFF。
修复调色板代码：
ColorPalette pal;
pal = bmp.Palette;
for (int i = 16; i < 40; i++)
pal.Entries[i] = Color.FromArgb(i, i, i);
bmp.Palette = pal;