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LibGfx: Implement PREDICTOR_TRANSFORM for webp lossless decoder
Very much not written for performance at this point.
This commit is contained in:
Nico Weber
Linus Groh
parent
b1cde0d432
commit
ebbe4dafa1
1 changed files with 233 additions and 1 deletions
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@ -692,6 +692,237 @@ public:
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Transform::~Transform() = default;
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// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#41_predictor_transform
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class PredictorTransform : public Transform {
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public:
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static ErrorOr<NonnullOwnPtr<PredictorTransform>> read(WebPLoadingContext&, LittleEndianInputBitStream&, IntSize const& image_size);
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virtual ErrorOr<void> transform(Bitmap&) override;
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private:
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PredictorTransform(int size_bits, NonnullRefPtr<Bitmap> predictor_bitmap)
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: m_size_bits(size_bits)
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, m_predictor_bitmap(move(predictor_bitmap))
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{
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}
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// These capitalized functions are all from the spec:
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static u8 Average2(u8 a, u8 b)
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{
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return (a + b) / 2;
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}
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static u32 Select(u32 L, u32 T, u32 TL)
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{
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// "L = left pixel, T = top pixel, TL = top left pixel."
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#define ALPHA(x) ((x >> 24) & 0xff)
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#define RED(x) ((x >> 16) & 0xff)
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#define GREEN(x) ((x >> 8) & 0xff)
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#define BLUE(x) (x & 0xff)
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// "ARGB component estimates for prediction."
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int pAlpha = ALPHA(L) + ALPHA(T) - ALPHA(TL);
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int pRed = RED(L) + RED(T) - RED(TL);
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int pGreen = GREEN(L) + GREEN(T) - GREEN(TL);
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int pBlue = BLUE(L) + BLUE(T) - BLUE(TL);
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// "Manhattan distances to estimates for left and top pixels."
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int pL = abs(pAlpha - (int)ALPHA(L)) + abs(pRed - (int)RED(L)) + abs(pGreen - (int)GREEN(L)) + abs(pBlue - (int)BLUE(L));
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int pT = abs(pAlpha - (int)ALPHA(T)) + abs(pRed - (int)RED(T)) + abs(pGreen - (int)GREEN(T)) + abs(pBlue - (int)BLUE(T));
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// "Return either left or top, the one closer to the prediction."
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if (pL < pT) { // "\[AMENDED\]"
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return L;
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} else {
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return T;
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}
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#undef BLUE
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#undef GREEN
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#undef RED
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#undef ALPHA
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}
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// "Clamp the input value between 0 and 255."
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static int Clamp(int a)
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{
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return clamp(a, 0, 255);
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}
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static int ClampAddSubtractFull(int a, int b, int c)
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{
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return Clamp(a + b - c);
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}
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static int ClampAddSubtractHalf(int a, int b)
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{
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return Clamp(a + (a - b) / 2);
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}
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// ...and we're back from the spec!
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static Color average2(Color a, Color b)
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{
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return Color(Average2(a.red(), b.red()),
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Average2(a.green(), b.green()),
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Average2(a.blue(), b.blue()),
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Average2(a.alpha(), b.alpha()));
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}
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static ARGB32 average2(ARGB32 a, ARGB32 b)
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{
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return average2(Color::from_argb(a), Color::from_argb(b)).value();
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}
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static ErrorOr<ARGB32> predict(u8 predictor, ARGB32 TL, ARGB32 T, ARGB32 TR, ARGB32 L);
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static ARGB32 inverse_transform(ARGB32 pixel, ARGB32 prediction);
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int m_size_bits;
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NonnullRefPtr<Bitmap> m_predictor_bitmap;
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};
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ErrorOr<NonnullOwnPtr<PredictorTransform>> PredictorTransform::read(WebPLoadingContext& context, LittleEndianInputBitStream& bit_stream, IntSize const& image_size)
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{
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// predictor-image = 3BIT ; sub-pixel code
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// entropy-coded-image
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int size_bits = TRY(bit_stream.read_bits(3)) + 2;
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int block_size = 1 << size_bits;
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IntSize predictor_image_size { ceil_div(image_size.width(), block_size), ceil_div(image_size.height(), block_size) };
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auto predictor_bitmap = TRY(decode_webp_chunk_VP8L_image(context, ImageKind::EntropyCoded, BitmapFormat::BGRx8888, predictor_image_size, bit_stream));
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return adopt_nonnull_own_or_enomem(new (nothrow) PredictorTransform(size_bits, move(predictor_bitmap)));
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}
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ErrorOr<void> PredictorTransform::transform(Bitmap& bitmap)
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{
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// "There are special handling rules for some border pixels.
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// If there is a prediction transform, regardless of the mode [0..13] for these pixels,
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// the predicted value for the left-topmost pixel of the image is 0xff000000,
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bitmap.scanline(0)[0] = inverse_transform(bitmap.scanline(0)[0], 0xff000000);
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// L-pixel for all pixels on the top row,
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for (int x = 1; x < bitmap.width(); ++x)
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bitmap.scanline(0)[x] = inverse_transform(bitmap.scanline(0)[x], bitmap.scanline(0)[x - 1]);
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// and T-pixel for all pixels on the leftmost column."
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for (int y = 1; y < bitmap.height(); ++y)
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bitmap.scanline(y)[0] = inverse_transform(bitmap.scanline(y)[0], bitmap.scanline(y - 1)[0]);
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ARGB32* bitmap_previous_scanline = bitmap.scanline(0);
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for (int y = 1; y < bitmap.height(); ++y) {
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ARGB32* bitmap_scanline = bitmap.scanline(y);
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ARGB32 TL = bitmap_previous_scanline[0];
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ARGB32 T = bitmap_previous_scanline[1];
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ARGB32 TR = 2 < bitmap.width() ? bitmap_previous_scanline[2] : bitmap_previous_scanline[0];
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ARGB32 L = bitmap_scanline[0];
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int predictor_y = y >> m_size_bits;
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ARGB32* predictor_scanline = m_predictor_bitmap->scanline(predictor_y);
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for (int x = 1; x < bitmap.width(); ++x) {
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int predictor_x = x >> m_size_bits;
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// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#51_roles_of_image_data
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// "The green component of a pixel defines which of the 14 predictors is used within a particular block of the ARGB image."
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u8 predictor = Color::from_argb(predictor_scanline[predictor_x]).green();
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ARGB32 predicted = TRY(predict(predictor, TL, T, TR, L));
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bitmap_scanline[x] = inverse_transform(bitmap_scanline[x], predicted);
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TL = T;
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T = TR;
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// "Addressing the TR-pixel for pixels on the rightmost column is exceptional.
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// The pixels on the rightmost column are predicted by using the modes [0..13] just like pixels not on the border,
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// but the leftmost pixel on the same row as the current pixel is instead used as the TR-pixel."
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TR = x + 2 < bitmap.width() ? bitmap_previous_scanline[x + 2] : bitmap_previous_scanline[0];
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L = bitmap_scanline[x];
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}
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bitmap_previous_scanline = bitmap_scanline;
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}
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return {};
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}
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ErrorOr<ARGB32> PredictorTransform::predict(u8 predictor, ARGB32 TL, ARGB32 T, ARGB32 TR, ARGB32 L)
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{
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switch (predictor) {
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case 0:
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// "0xff000000 (represents solid black color in ARGB)"
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return 0xff000000;
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case 1:
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// "L"
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return L;
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case 2:
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// "T"
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return T;
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case 3:
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// "TR"
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return TR;
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case 4:
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// "TL"
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return TL;
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case 5:
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// "Average2(Average2(L, TR), T)"
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return average2(average2(L, TR), T);
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case 6:
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// "Average2(L, TL)"
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return average2(L, TL);
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case 7:
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// "Average2(L, T)"
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return average2(L, T);
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case 8:
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// "Average2(TL, T)"
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return average2(TL, T);
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case 9:
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// "Average2(T, TR)"
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return average2(T, TR);
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case 10:
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// "Average2(Average2(L, TL), Average2(T, TR))"
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return average2(average2(L, TL), average2(T, TR));
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case 11:
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// "Select(L, T, TL)"
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return Select(L, T, TL);
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case 12: {
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// "ClampAddSubtractFull(L, T, TL)"
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auto color_L = Color::from_argb(L);
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auto color_T = Color::from_argb(T);
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auto color_TL = Color::from_argb(TL);
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return Color(ClampAddSubtractFull(color_L.red(), color_T.red(), color_TL.red()),
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ClampAddSubtractFull(color_L.green(), color_T.green(), color_TL.green()),
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ClampAddSubtractFull(color_L.blue(), color_T.blue(), color_TL.blue()),
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ClampAddSubtractFull(color_L.alpha(), color_T.alpha(), color_TL.alpha()))
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.value();
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}
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case 13: {
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// "ClampAddSubtractHalf(Average2(L, T), TL)"
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auto color_L = Color::from_argb(L);
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auto color_T = Color::from_argb(T);
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auto color_TL = Color::from_argb(TL);
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return Color(ClampAddSubtractHalf(Average2(color_L.red(), color_T.red()), color_TL.red()),
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ClampAddSubtractHalf(Average2(color_L.green(), color_T.green()), color_TL.green()),
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ClampAddSubtractHalf(Average2(color_L.blue(), color_T.blue()), color_TL.blue()),
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ClampAddSubtractHalf(Average2(color_L.alpha(), color_T.alpha()), color_TL.alpha()))
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.value();
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}
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}
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return Error::from_string_literal("WebPImageDecoderPlugin: invalid predictor");
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}
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ARGB32 PredictorTransform::inverse_transform(ARGB32 pixel, ARGB32 prediction)
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{
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auto pixel_color = Color::from_argb(pixel);
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auto prediction_color = Color::from_argb(prediction);
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return Color(pixel_color.red() + prediction_color.red(),
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pixel_color.green() + prediction_color.green(),
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pixel_color.blue() + prediction_color.blue(),
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pixel_color.alpha() + prediction_color.alpha())
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.value();
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}
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// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#43_subtract_green_transform
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class SubtractGreenTransform : public Transform {
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public:
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@ -771,7 +1002,8 @@ static ErrorOr<void> decode_webp_chunk_VP8L(WebPLoadingContext& context, Chunk c
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switch (transform_type) {
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case PREDICTOR_TRANSFORM:
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return context.error("WebPImageDecoderPlugin: VP8L PREDICTOR_TRANSFORM handling not yet implemented");
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TRY(transforms.try_append(TRY(PredictorTransform::read(context, bit_stream, context.size.value()))));
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break;
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case COLOR_TRANSFORM:
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return context.error("WebPImageDecoderPlugin: VP8L COLOR_TRANSFORM handling not yet implemented");
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case SUBTRACT_GREEN_TRANSFORM:
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