我有一个8位PNG图像（参见附件）。但是当我使用Image.FromFile方法读取它时，像素格式为32位。由于这个原因，我无法修改调色板。
请帮帮我。
请参阅下文，了解我用于阅读文件和更新调色板的代码。
public static Image GetPreviewImage()
{
Bitmap updatedImage = null;
try
{
// Reads the colors as a byte array
byte[] paletteBytes = FetchColorPallette();
updatedImage = Image.FromFile(@"C:\Screen-SaverBouncing.png");
ColorPalette colorPalette = updatedImage.Palette;
int j = 0;
if (colorPalette.Entries.Length > 0)
{
for (int i = 0; i < paletteBytes.Length / 4; i++)
{
Byte AValue = Convert.ToByte(paletteBytes[j]);
Byte RValue = Convert.ToByte(paletteBytes[j + 1]);
Byte GValue = Convert.ToByte(paletteBytes[j + 2]);
Byte BValue = Convert.ToByte(paletteBytes[j + 3]);
j += 4;
colorPalette.Entries[i] = Color.FromArgb(AValue, RValue, GValue, BValue);
}
updatedImage.Palette = colorPalette; ;
}
return updatedImage;
}
catch
{
throw;
}
}
答案 0 :(得分：0)
我也有这个问题，似乎任何包含透明度的调色板png图像都不能被.Net框架调整，尽管.Net功能可以完美写这样的文件。相反，如果文件是gif格式，它没有问题。
png中的透明度通过添加可选的＆＃34; tRNS＆＃34;标题中的块，用于指定每个调色板条目的alpha。 .Net类正确地读取和应用它，因此我不太明白为什么他们坚持将图像转换为32位之后。更重要的是，当透明块存在时，总是会发生错误，即使它将所有颜色都标记为完全不透明。
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，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。
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。
我的代码：
/// <summary>
/// Image loading toolset class which corrects the bug that prevents paletted PNG images with transparency from being loaded as paletted.
/// </summary>
public class BitmapLoader
{
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 can be found at http://www.libpng.org/pub/png/book/chapter08.html
/// </summary>
/// <param name="filename">Filename to load</param>
/// <returns>The loaded image</returns>
public static Bitmap LoadBitmap(String filename)
{
Byte[] data = File.ReadAllBytes(filename);
return LoadBitmap(data);
}
/// <summary>
/// Loads an image, checks if it is a PNG containing palette transparency, and if so, ensures it loads correctly.
/// The theory 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;
}
}
}
}
Bitmap loadedImage;
using (MemoryStream ms = new MemoryStream(data))
using (Bitmap tmp = new Bitmap(ms))
loadedImage = ImageUtils.CloneImage(tmp);
ColorPalette pal = loadedImage.Palette;
if (pal.Entries.Length == 0 || transparencyData == null)
return loadedImage;
for (Int32 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;
return loadedImage;
}
/// <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 (chunkName.Length != 4 )
throw new ArgumentException("Chunk must be 4 characters!", "chunkName");
Byte[] chunkNamebytes = Encoding.ASCII.GetBytes(chunkName);
if (chunkNamebytes.Length != 4)
throw new ArgumentException("Chunk must be 4 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);
// Alternative for more visual debugging:
//String currentChunk = Encoding.ASCII.GetString(testBytes);
//if (chunkName.Equals(currentChunk))
// return offset;
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;
}
}
据我所知，提到的ImageUtils.CloneImage是加载位图并将其与任何支持资源（如文件或流）取消链接的唯一100％安全方式。 It can be found here.