src/core/SkBitmapScaler.cpp

#include "SkBitmapScaler.h"
#include "SkBitmapFilter.h"
#include "SkRect.h"
#include "SkTArray.h"
#include "SkErrorInternals.h"
#include "SkConvolver.h"

// SkResizeFilter ----------------------------------------------------------------

// Encapsulates computation and storage of the filters required for one complete
// resize operation.
class SkResizeFilter {
public:
    SkResizeFilter(SkBitmapScaler::ResizeMethod method,
                   int srcFullWidth, int srcFullHeight,
                   float destWidth, float destHeight,
                   const SkRect& destSubset,
                   const SkConvolutionProcs& convolveProcs);
    ~SkResizeFilter() {
        SkDELETE( fBitmapFilter );
    }

    // Returns the filled filter values.
    const SkConvolutionFilter1D& xFilter() { return fXFilter; }
    const SkConvolutionFilter1D& yFilter() { return fYFilter; }

private:

    SkBitmapFilter* fBitmapFilter;

    // Computes one set of filters either horizontally or vertically. The caller
    // will specify the "min" and "max" rather than the bottom/top and
    // right/bottom so that the same code can be re-used in each dimension.
    //
    // |srcDependLo| and |srcDependSize| gives the range for the source
    // depend rectangle (horizontally or vertically at the caller's discretion
    // -- see above for what this means).
    //
    // Likewise, the range of destination values to compute and the scale factor
    // for the transform is also specified.

    void computeFilters(int srcSize,
                        float destSubsetLo, float destSubsetSize,
                        float scale,
                        SkConvolutionFilter1D* output,
                        const SkConvolutionProcs& convolveProcs);

    SkConvolutionFilter1D fXFilter;
    SkConvolutionFilter1D fYFilter;
};

SkResizeFilter::SkResizeFilter(SkBitmapScaler::ResizeMethod method,
                               int srcFullWidth, int srcFullHeight,
                               float destWidth, float destHeight,
                               const SkRect& destSubset,
                               const SkConvolutionProcs& convolveProcs) {

    // method will only ever refer to an "algorithm method".
    SkASSERT((SkBitmapScaler::RESIZE_FIRST_ALGORITHM_METHOD <= method) &&
             (method <= SkBitmapScaler::RESIZE_LAST_ALGORITHM_METHOD));

    switch(method) {
        case SkBitmapScaler::RESIZE_BOX:
            fBitmapFilter = SkNEW(SkBoxFilter);
            break;
        case SkBitmapScaler::RESIZE_TRIANGLE:
            fBitmapFilter = SkNEW(SkTriangleFilter);
            break;
        case SkBitmapScaler::RESIZE_MITCHELL:
            fBitmapFilter = SkNEW_ARGS(SkMitchellFilter, (1.f/3.f, 1.f/3.f));
            break;
        case SkBitmapScaler::RESIZE_HAMMING:
            fBitmapFilter = SkNEW(SkHammingFilter);
            break;
        case SkBitmapScaler::RESIZE_LANCZOS3:
            fBitmapFilter = SkNEW(SkLanczosFilter);
            break;
        default:
            // NOTREACHED:
            fBitmapFilter = SkNEW_ARGS(SkMitchellFilter, (1.f/3.f, 1.f/3.f));
            break;
    }


    float scaleX = destWidth / srcFullWidth;
    float scaleY = destHeight / srcFullHeight;

    this->computeFilters(srcFullWidth, destSubset.fLeft, destSubset.width(),
                         scaleX, &fXFilter, convolveProcs);
    if (srcFullWidth == srcFullHeight &&
        destSubset.fLeft == destSubset.fTop &&
        destSubset.width() == destSubset.height()&&
        scaleX == scaleY) {
        fYFilter = fXFilter;
    } else {
        this->computeFilters(srcFullHeight, destSubset.fTop, destSubset.height(),
                          scaleY, &fYFilter, convolveProcs);
    }
}

// TODO(egouriou): Take advantage of periods in the convolution.
// Practical resizing filters are periodic outside of the border area.
// For Lanczos, a scaling by a (reduced) factor of p/q (q pixels in the
// source become p pixels in the destination) will have a period of p.
// A nice consequence is a period of 1 when downscaling by an integral
// factor. Downscaling from typical display resolutions is also bound
// to produce interesting periods as those are chosen to have multiple
// small factors.
// Small periods reduce computational load and improve cache usage if
// the coefficients can be shared. For periods of 1 we can consider
// loading the factors only once outside the borders.
void SkResizeFilter::computeFilters(int srcSize,
                                  float destSubsetLo, float destSubsetSize,
                                  float scale,
                                  SkConvolutionFilter1D* output,
                                  const SkConvolutionProcs& convolveProcs) {
  float destSubsetHi = destSubsetLo + destSubsetSize;  // [lo, hi)

  // When we're doing a magnification, the scale will be larger than one. This
  // means the destination pixels are much smaller than the source pixels, and
  // that the range covered by the filter won't necessarily cover any source
  // pixel boundaries. Therefore, we use these clamped values (max of 1) for
  // some computations.
  float clampedScale = SkTMin(1.0f, scale);

  // This is how many source pixels from the center we need to count
  // to support the filtering function.
  float srcSupport = fBitmapFilter->width() / clampedScale;

  // Speed up the divisions below by turning them into multiplies.
  float invScale = 1.0f / scale;

  SkTArray<float> filterValues(64);
  SkTArray<short> fixedFilterValues(64);

  // Loop over all pixels in the output range. We will generate one set of
  // filter values for each one. Those values will tell us how to blend the
  // source pixels to compute the destination pixel.
  for (int destSubsetI = SkScalarFloorToInt(destSubsetLo); destSubsetI < SkScalarCeilToInt(destSubsetHi);
       destSubsetI++) {
    // Reset the arrays. We don't declare them inside so they can re-use the
    // same malloc-ed buffer.
    filterValues.reset();
    fixedFilterValues.reset();

    // This is the pixel in the source directly under the pixel in the dest.
    // Note that we base computations on the "center" of the pixels. To see
    // why, observe that the destination pixel at coordinates (0, 0) in a 5.0x
    // downscale should "cover" the pixels around the pixel with *its center*
    // at coordinates (2.5, 2.5) in the source, not those around (0, 0).
    // Hence we need to scale coordinates (0.5, 0.5), not (0, 0).
    float srcPixel = (static_cast<float>(destSubsetI) + 0.5f) * invScale;

    // Compute the (inclusive) range of source pixels the filter covers.
    int srcBegin = SkTMax(0, SkScalarFloorToInt(srcPixel - srcSupport));
    int srcEnd = SkTMin(srcSize - 1, SkScalarCeilToInt(srcPixel + srcSupport));

    // Compute the unnormalized filter value at each location of the source
    // it covers.
    float filterSum = 0.0f;  // Sub of the filter values for normalizing.
    for (int curFilterPixel = srcBegin; curFilterPixel <= srcEnd;
         curFilterPixel++) {
      // Distance from the center of the filter, this is the filter coordinate
      // in source space. We also need to consider the center of the pixel
      // when comparing distance against 'srcPixel'. In the 5x downscale
      // example used above the distance from the center of the filter to
      // the pixel with coordinates (2, 2) should be 0, because its center
      // is at (2.5, 2.5).
      float srcFilterDist =
          ((static_cast<float>(curFilterPixel) + 0.5f) - srcPixel);

      // Since the filter really exists in dest space, map it there.
      float destFilterDist = srcFilterDist * clampedScale;

      // Compute the filter value at that location.
      float filterValue = fBitmapFilter->evaluate(destFilterDist);
      filterValues.push_back(filterValue);

      filterSum += filterValue;
    }
    SkASSERT(!filterValues.empty());

    // The filter must be normalized so that we don't affect the brightness of
    // the image. Convert to normalized fixed point.
    short fixedSum = 0;
    for (int i = 0; i < filterValues.count(); i++) {
      short curFixed = output->FloatToFixed(filterValues[i] / filterSum);
      fixedSum += curFixed;
      fixedFilterValues.push_back(curFixed);
    }

    // The conversion to fixed point will leave some rounding errors, which
    // we add back in to avoid affecting the brightness of the image. We
    // arbitrarily add this to the center of the filter array (this won't always
    // be the center of the filter function since it could get clipped on the
    // edges, but it doesn't matter enough to worry about that case).
    short leftovers = output->FloatToFixed(1.0f) - fixedSum;
    fixedFilterValues[fixedFilterValues.count() / 2] += leftovers;

    // Now it's ready to go.
    output->AddFilter(srcBegin, &fixedFilterValues[0],
                      static_cast<int>(fixedFilterValues.count()));
  }

  if (convolveProcs.fApplySIMDPadding) {
      convolveProcs.fApplySIMDPadding( output );
  }
}

static SkBitmapScaler::ResizeMethod ResizeMethodToAlgorithmMethod(
                                    SkBitmapScaler::ResizeMethod method) {
    // Convert any "Quality Method" into an "Algorithm Method"
    if (method >= SkBitmapScaler::RESIZE_FIRST_ALGORITHM_METHOD &&
    method <= SkBitmapScaler::RESIZE_LAST_ALGORITHM_METHOD) {
        return method;
    }
    // The call to SkBitmapScalerGtv::Resize() above took care of
    // GPU-acceleration in the cases where it is possible. So now we just
    // pick the appropriate software method for each resize quality.
    switch (method) {
        // Users of RESIZE_GOOD are willing to trade a lot of quality to
        // get speed, allowing the use of linear resampling to get hardware
        // acceleration (SRB). Hence any of our "good" software filters
        // will be acceptable, so we use a triangle.
        case SkBitmapScaler::RESIZE_GOOD:
            return SkBitmapScaler::RESIZE_TRIANGLE;
        // Users of RESIZE_BETTER are willing to trade some quality in order
        // to improve performance, but are guaranteed not to devolve to a linear
        // resampling. In visual tests we see that Hamming-1 is not as good as
        // Lanczos-2, however it is about 40% faster and Lanczos-2 itself is
        // about 30% faster than Lanczos-3. The use of Hamming-1 has been deemed
        // an acceptable trade-off between quality and speed.
        case SkBitmapScaler::RESIZE_BETTER:
            return SkBitmapScaler::RESIZE_HAMMING;
        default:
#ifdef SK_HIGH_QUALITY_IS_LANCZOS
            return SkBitmapScaler::RESIZE_LANCZOS3;
#else
            return SkBitmapScaler::RESIZE_MITCHELL;
#endif
    }
}

// static
bool SkBitmapScaler::Resize(SkBitmap* resultPtr,
                            const SkBitmap& source,
                            ResizeMethod method,
                            float destWidth, float destHeight,
                            const SkConvolutionProcs& convolveProcs,
                            SkBitmap::Allocator* allocator) {

  SkRect destSubset = { 0, 0, destWidth, destHeight };

  // Ensure that the ResizeMethod enumeration is sound.
    SkASSERT(((RESIZE_FIRST_QUALITY_METHOD <= method) &&
        (method <= RESIZE_LAST_QUALITY_METHOD)) ||
        ((RESIZE_FIRST_ALGORITHM_METHOD <= method) &&
        (method <= RESIZE_LAST_ALGORITHM_METHOD)));

    SkRect dest = { 0, 0, destWidth, destHeight };
    if (!dest.contains(destSubset)) {
        SkErrorInternals::SetError( kInvalidArgument_SkError,
                                    "Sorry, the destination bitmap scale subset "
                                    "falls outside the full destination bitmap." );
    }

    // If the size of source or destination is 0, i.e. 0x0, 0xN or Nx0, just
    // return empty.
    if (source.width() < 1 || source.height() < 1 ||
        destWidth < 1 || destHeight < 1) {
        // todo: seems like we could handle negative dstWidth/Height, since that
        // is just a negative scale (flip)
        return false;
    }

    method = ResizeMethodToAlgorithmMethod(method);

    // Check that we deal with an "algorithm methods" from this point onward.
    SkASSERT((SkBitmapScaler::RESIZE_FIRST_ALGORITHM_METHOD <= method) &&
        (method <= SkBitmapScaler::RESIZE_LAST_ALGORITHM_METHOD));

    SkAutoLockPixels locker(source);
    if (!source.readyToDraw() ||
        source.colorType() != kN32_SkColorType) {
        return false;
    }

    SkResizeFilter filter(method, source.width(), source.height(),
                          destWidth, destHeight, destSubset, convolveProcs);

    // Get a source bitmap encompassing this touched area. We construct the
    // offsets and row strides such that it looks like a new bitmap, while
    // referring to the old data.
    const unsigned char* sourceSubset =
        reinterpret_cast<const unsigned char*>(source.getPixels());

    // Convolve into the result.
    SkBitmap result;
    result.setInfo(SkImageInfo::MakeN32(SkScalarCeilToInt(destSubset.width()),
                                        SkScalarCeilToInt(destSubset.height()),
                                        source.alphaType()));
    result.allocPixels(allocator, NULL);
    if (!result.readyToDraw()) {
        return false;
    }

    BGRAConvolve2D(sourceSubset, static_cast<int>(source.rowBytes()),
        !source.isOpaque(), filter.xFilter(), filter.yFilter(),
        static_cast<int>(result.rowBytes()),
        static_cast<unsigned char*>(result.getPixels()),
        convolveProcs, true);

    *resultPtr = result;
    resultPtr->lockPixels();
    SkASSERT(NULL != resultPtr->getPixels());
    return true;
}