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Fixed certain pngs not being transparent when they should be. #2521

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Merged
swmal merged 1 commit into from
Sep 16, 2026
+126 −5
Merged

Fixed certain pngs not being transparent when they should be. #2521

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12 changes: 9 additions & 3 deletions src/EPPlus.Export.Pdf.Tests/PdfTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -18,14 +18,11 @@ Date Author Change
using OfficeOpenXml.Export.PdfExport.Data;
using OfficeOpenXml.Export.PdfExport.Layout;
using OfficeOpenXml.Export.PdfExport.Settings;
using OfficeOpenXml.FormulaParsing.Excel.Functions.Information;
using OfficeOpenXml.Interfaces.Fonts;
using OfficeOpenXml.Style;
using OfficeOpenXml.Table;
using System.Data;
using System.Diagnostics;
using System.Globalization;
using System.Reflection.Metadata;
using System.Text;
using System.Text.RegularExpressions;

Expand Down Expand Up @@ -1105,5 +1102,14 @@ private void InitDataTable()
_dataTable.Rows.Add("Thailand", 69950850, 513120);
_dataTable.Rows.Add("Vietnam", 98168833, 331212);
}

[TestMethod]
public void PngTransparentTest()
{
using var p = OpenTemplatePackage("Allsvenskan2001.xlsx");
var ws = p.Workbook.Worksheets[0];
string path = _pdfPath + "Allsvenskan2001.pdf";
ws.SaveAsPdf(path);
}
}
}
14 changes: 12 additions & 2 deletions src/EPPlus.Export.Pdf/DocumentObjects/PdfImageXObject.cs
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Expand Up @@ -101,6 +101,11 @@ public PdfImageXObject(int objectNumber, byte[] imageBytes, int version = 0)
int hival = palette == null || palette.Length < 3 ? 0 : (palette.Length / 3) - 1;
ColorSpace = "[ /Indexed /DeviceRGB " + hival + " <" + PngDecoder.ToHex(palette) + "> ]";
colors = 1;
if (PngDecoder.DecodeIndexedAlpha(imageBytes, width, height, bitDepth, out byte[] indexedAlpha))
{
SoftMaskData = indexedAlpha;
HasSoftMask = true;
}
break;
default:
ColorSpace = "/DeviceRGB";
Expand Down Expand Up @@ -217,8 +222,13 @@ internal static bool ProducesSoftMask(byte[] imageBytes)
{
if (!PngDecoder.ReadPngHeader(imageBytes, out int _, out int _, out int bitDepth, out int colorType, out int interlace))
return false;
if (interlace != 0) return false;
return (colorType == 4 || colorType == 6) && bitDepth == 8;
if (interlace != 0)
return false;
if ((colorType == 4 || colorType == 6) && bitDepth == 8)
return true;
if (colorType == 3)
return PngDecoder.TryReadChunk(imageBytes, "tRNS", out byte[] _);
return false;
}
if (GifDecoder.IsGif(imageBytes))
{
Expand Down
105 changes: 105 additions & 0 deletions src/EPPlus.Export.Pdf/Helpers/PngDecoder.cs
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Expand Up @@ -137,6 +137,111 @@ internal static void DecodePngWithAlpha(byte[] pngBytes, int width, int height,
deflatedAlpha = PdfFlate.CompressLeaveOpen(alpha);
}

internal static bool TryReadChunk(byte[] d, string chunkType, out byte[] data)
{
data = null;
if (!IsPng(d)) return false;
int p = _pngSignature.Length;
while (p + 8 <= d.Length)
{
int len = ReadBE32(d, p);
string type = Ascii(d, p + 4, 4);
int dataStart = p + 8;
if (len < 0 || dataStart + len + 4 > d.Length) break;
if (type == chunkType)
{
data = new byte[len];
System.Array.Copy(d, dataStart, data, 0, len);
return true;
}
if (type == "IEND") break;
p = dataStart + len + 4;
}
return false;
}

internal static bool DecodeIndexedAlpha(byte[] pngBytes, int width, int height, int bitDepth, out byte[] deflatedAlpha)
{
deflatedAlpha = null;
if (!TryReadChunk(pngBytes, "tRNS", out byte[] trns) || trns == null || trns.Length == 0)
return false;

byte[] filtered = PdfFlate.Decompress(ReadPngIdat(pngBytes, out byte[] _));
const int bpp = 1;
int stride = (width * bitDepth + 7) / 8;
var prev = new byte[stride];
var cur = new byte[stride];
var alpha = new byte[width * height];
int pos = 0, ai = 0;
for (int y = 0; y < height; y++)
{
int filter = pos < filtered.Length ? filtered[pos++] : 0;
for (int x = 0; x < stride; x++)
{
int raw = pos < filtered.Length ? filtered[pos++] : 0;
int a = x >= bpp ? cur[x - bpp] : 0;
int b = prev[x];
int c = x >= bpp ? prev[x - bpp] : 0;
int val;
switch (filter)
{
case 1: //Sub
val = raw + a;
break;
case 2: //Up
val = raw + b;
break;
case 3: //Average
val = raw + ((a + b) >> 1);
break;
case 4: //Paeth
val = raw + Paeth(a, b, c);
break;
default: //None
val = raw;
break;
}
cur[x] = (byte)(val & 0xFF);
}
for (int x = 0; x < width; x++)
{
int index = GetSample(cur, x, bitDepth);
alpha[ai++] = index < trns.Length ? trns[index] : (byte)255;
}
var swap = prev; prev = cur; cur = swap;
}
deflatedAlpha = PdfFlate.CompressLeaveOpen(alpha);
return true;
}

private static int GetSample(byte[] scanline, int x, int bitDepth)
{
switch (bitDepth)
{
case 8:
return scanline[x];
case 4:
{
int b = scanline[x >> 1];
return (x & 1) == 0 ? (b >> 4) & 0x0F : b & 0x0F;
}
case 2:
{
int b = scanline[x >> 2];
int shift = 6 - 2 * (x & 3);
return (b >> shift) & 0x03;
}
case 1:
{
int b = scanline[x >> 3];
int shift = 7 - (x & 7);
return (b >> shift) & 0x01;
}
default:
return scanline[x];
}
}

internal static int Paeth(int a, int b, int c)
{
int p = a + b - c;
Expand Down
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