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fix: preserve indexed PNG transparency to prevent black box regression when stamping PDF #1525

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fix: preserve indexed PNG transparency to prevent black box regression when stamping PDF #1525

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f9de8a2
fix: resolve indexed color image color distortion in PDF output (#1499)
Mar 1, 2026
687cb2a
Merge branch 'LibrePDF:master' into master
wang0331 Mar 2, 2026
efb0457
Merge branch 'LibrePDF:master' into master
wang0331 Mar 27, 2026
297a81b
fix: preserve indexed PNG transparency to prevent black box regressio…
Mar 27, 2026
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130 changes: 77 additions & 53 deletions openpdf-core/src/main/java/org/openpdf/text/Image.java
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Original file line number Diff line number Diff line change
Expand Up @@ -260,6 +260,8 @@ public abstract class Image extends Rectangle {

// member variables
public static final int[] PNGID = {137, 80, 78, 71, 13, 10, 26, 10};
private static final int INDEXED_NO_TRANSPARENCY = -1;
private static final int INDEXED_UNSUPPORTED_TRANSPARENCY = -2;
/**
* a static that is used for attributing a unique id to each image.
*/
Expand Down Expand Up @@ -828,61 +830,68 @@ public static Image getInstance(java.awt.Image image, java.awt.Color color,
forceBW = true;
}

// Handle indexed color images
if (bi.getColorModel() instanceof IndexColorModel && !forceBW) {
IndexColorModel icm = (IndexColorModel) bi.getColorModel();
int mapSize = icm.getMapSize();
int bitsPerPixel = icm.getPixelSize();

// Ensure bits per pixel is valid (1, 2, 4, or 8)
// For PDF indexed images, bpc should be the bits needed to index the palette
if (bitsPerPixel > 8 || bitsPerPixel == 0) {
bitsPerPixel = 8;
} else if (bitsPerPixel > 4) {
bitsPerPixel = 8;
} else if (bitsPerPixel > 2) {
bitsPerPixel = 4;
} else if (bitsPerPixel > 1) {
bitsPerPixel = 2;
} else {
bitsPerPixel = 1;
}
// Handle indexed color images when transparency can be represented directly.
// More complex alpha (for example semi-transparency) must fallback to the generic RGB+SMask path below
if (bi.getColorModel() instanceof IndexColorModel icm && !forceBW && color == null) {
int transparentIndex = getIndexedTransparentPaletteIndex(icm);
if (transparentIndex != INDEXED_UNSUPPORTED_TRANSPARENCY) {
int mapSize = icm.getMapSize();
int bitsPerPixel = icm.getPixelSize();

// Ensure bits per pixel is valid (1, 2, 4, or 8)
// For PDF indexed images, bpc should be the bits needed to index the palette
if (bitsPerPixel > 8 || bitsPerPixel == 0) {
bitsPerPixel = 8;
} else if (bitsPerPixel > 4) {
bitsPerPixel = 8;
} else if (bitsPerPixel > 2) {
bitsPerPixel = 4;
} else if (bitsPerPixel > 1) {
bitsPerPixel = 2;
} else {
bitsPerPixel = 1;
}

// Extract palette data
byte[] reds = new byte[mapSize];
byte[] greens = new byte[mapSize];
byte[] blues = new byte[mapSize];
icm.getReds(reds);
icm.getGreens(greens);
icm.getBlues(blues);

// Build palette as RGB byte array
byte[] palette = new byte[mapSize * 3];
for (int i = 0; i < mapSize; i++) {
palette[i * 3] = reds[i];
palette[i * 3 + 1] = greens[i];
palette[i * 3 + 2] = blues[i];
}
// Extract palette data
byte[] reds = new byte[mapSize];
byte[] greens = new byte[mapSize];
byte[] blues = new byte[mapSize];
icm.getReds(reds);
icm.getGreens(greens);
icm.getBlues(blues);

// Build palette as RGB byte array
byte[] palette = new byte[mapSize * 3];
for (int i = 0; i < mapSize; i++) {
palette[i * 3] = reds[i];
palette[i * 3 + 1] = greens[i];
palette[i * 3 + 2] = blues[i];
}

// Extract pixel indices
int width = bi.getWidth();
int height = bi.getHeight();
byte[] pixelData = generateIndexedColorPixelData(width, bitsPerPixel, height, bi.getRaster());
// Create indexed image with palette
Image img = Image.getInstance(width, height, 1, bitsPerPixel, pixelData);

// Set up indexed colorspace: [/Indexed /DeviceRGB maxIndex palette]
PdfArray indexed = new PdfArray();
indexed.add(PdfName.INDEXED);
indexed.add(PdfName.DEVICERGB);
indexed.add(new PdfNumber(mapSize - 1));
indexed.add(new PdfString(palette));

PdfDictionary additional = new PdfDictionary();
additional.put(PdfName.COLORSPACE, indexed);
img.setAdditional(additional);
if (transparentIndex >= 0) {
img.setTransparency(new int[]{transparentIndex, transparentIndex});
}

// Extract pixel indices
int width = bi.getWidth();
int height = bi.getHeight();
byte[] pixelData = generateIndexedColorPixelData(width, bitsPerPixel, height, bi.getRaster());
// Create indexed image with palette
Image img = Image.getInstance(width, height, 1, bitsPerPixel, pixelData);

// Set up indexed colorspace: [/Indexed /DeviceRGB maxIndex palette]
PdfArray indexed = new PdfArray();
indexed.add(PdfName.INDEXED);
indexed.add(PdfName.DEVICERGB);
indexed.add(new PdfNumber(mapSize - 1));
indexed.add(new PdfString(palette));

PdfDictionary additional = new PdfDictionary();
additional.put(PdfName.COLORSPACE, indexed);
img.setAdditional(additional);

return img;
return img;
}
// Unsupported indexed transparency falls through to the generic RGB+SMask path below.
}
}

Expand Down Expand Up @@ -1099,6 +1108,21 @@ private static byte[] generateIndexedColorPixelData(int width, int bitsPerPixel,
return pixelData;
}

private static int getIndexedTransparentPaletteIndex(IndexColorModel colorModel) {
int transparentIndex = INDEXED_NO_TRANSPARENCY;
for (int index = 0; index < colorModel.getMapSize(); index++) {
int alpha = colorModel.getAlpha(index);
if (alpha == 0xFF) {
continue;
}
if (alpha != 0x00 || transparentIndex != INDEXED_NO_TRANSPARENCY) {
return INDEXED_UNSUPPORTED_TRANSPARENCY;
}
transparentIndex = index;
}
return transparentIndex;
}

/**
* Packs a single pixel index into the target byte array based on specified bit depth (PDF MSB-first rule).
*/
Expand Down
35 changes: 35 additions & 0 deletions openpdf-core/src/test/java/org/openpdf/text/ImageTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -177,4 +177,39 @@ void shouldDetectIndexedColorFromBufferedImage() throws Exception {
assertThat(image.getAdditional().get(PdfName.COLORSPACE)).isNotNull();
}

@Test
void shouldFallbackToRgbWhenIndexedColorHasTransparency() throws Exception {
int width = 10;
int height = 10;

byte[] reds = {(byte) 255, 0, 0, 0};
byte[] greens = {0, (byte) 255, 0, 0};
byte[] blues = {0, 0, (byte) 255, 0};
byte[] alphas = {0, (byte) 128, (byte) 255, (byte) 255};

IndexColorModel colorModel = new IndexColorModel(
2,
4,
reds, greens, blues, alphas
);

BufferedImage bufferedImage = new BufferedImage(
width, height, BufferedImage.TYPE_BYTE_INDEXED, colorModel
);

for (int y = 0; y < height; y++) {
for (int x = 0; x < width; x++) {
bufferedImage.getRaster().setSample(x, y, 0, (x + y) % 4);
}
}

Image image = Image.getInstance(bufferedImage, null);

assertNotNull(image);
assertThat(image.getColorspace()).isEqualTo(3);
assertThat(image.getImageMask()).isNotNull();
assertThat(image.getImageMask().isMask()).isTrue();
assertThat(image.isSmask()).isTrue();
}

}
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