png中的透明度通過在標題中添加可選的「tRNS」塊來指定每個調色板條目的alpha值。 .Net類正確地讀取和應用這個，所以我不明白他們爲什麼堅持把圖像轉換爲32位。更重要的是，當存在透明度塊時，即使它將所有顏色標記爲完全不透明，也會發生錯誤總是。
png格式的結構相當簡單;在識別字節之後，每個塊是內容大小的4個字節，然後是塊ID的4個ASCII字符，然後是塊內容本身，最後是4個字節的塊CRC值。
鑑於這種結構，該解決方案是相當簡單：
- 文件讀入到一個字節數組。
- 通過分析標頭確保它是一個調色板的png文件。
- 通過從塊標題跳轉到塊標題來查找「tRNS」塊。
- 從塊中讀取alpha值。
- 創建一個包含圖像數據的新字節數組，但切割了「tRNS」塊。
- 使用從調整後的字節數據創建的
MemoryStream創建Bitmap對象，從而生成正確的8位圖像。
- 使用提取的alpha數據修復調色板。
如果您執行檢查和回退功能，您可以使用此功能加載任何圖像，並且如果它恰好用透明度信息標識爲調色板png，它將執行修復。
我的代碼：
/// <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;
}
}
所提ImageUtils.CloneImage是，據我所知，加載位圖和從任何後備資源，如文件或流取消鏈接它的只有100％安全的方式。 It can be found here.
您的代碼不顯示如何閱讀圖像。 –
問題是「Screen-SaverBouncing.png」是一個8位圖像（從Windows 7的屬性窗口中可以看到），但colorPalette.Entries.Length始終爲零。另外，如果我在IrfanViewer中打開這個圖像，我可以看到調色板，修改它並保存。如果我在這個新保存的文件上運行上面的代碼，那麼colorPalette.Entries.Length是256。 –
現在解決了這個問題，因爲問題出在png圖片上，而不是代碼上。 –