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Avoid another copy when RGBA is resampled as RGB
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In the case of RGBA, the RGB and A channels are resampled separately,
but they are created as a view on the original to pass to the C++ code.
The C++ code then copies it to a contiguous buffer, but Agg's RGB
resampler supports manually stepping the RGB input by a custom stride.
As this step is a template parameter, we can't handle any arbitraray
array, but can special case steps of 3 or 4 units, which should cover
the common cases of RGB or RGBA-viewed-as-RGB input.
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@QuLogic
QuLogic committed Jan 17, 2025
1 parent 2b73e7e commit 769f7a4
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Showing 2 changed files with 37 additions and 14 deletions.
10 changes: 6 additions & 4 deletions src/_image_resample.h
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Original file line number Diff line number Diff line change
Expand Up @@ -504,7 +504,8 @@ template<typename T> struct is_grayscale<T, std::void_t<decltype(T::r)>> : std::
template<typename T> constexpr bool is_grayscale_v = is_grayscale<T>::value;


template<typename color_type, bool input_has_alpha>
// rgb_step is only used if input_has_alpha=false.
template<typename color_type, bool input_has_alpha, int rgb_step=3>
struct type_mapping
{
using input_blender_type = std::conditional_t<
Expand All @@ -526,7 +527,7 @@ struct type_mapping
std::conditional_t<
input_has_alpha,
agg::pixfmt_alpha_blend_rgba<input_blender_type, agg::rendering_buffer>,
agg::pixfmt_alpha_blend_rgb<input_blender_type, agg::rendering_buffer, 3>
agg::pixfmt_alpha_blend_rgb<input_blender_type, agg::rendering_buffer, rgb_step>
>
>;
using output_blender_type = std::conditional_t<
Expand Down Expand Up @@ -722,13 +723,14 @@ static void get_filter(const resample_params_t &params,
}


template<typename color_type, bool input_has_alpha = true>
// rgb_step is only used if input_has_alpha=false.
template<typename color_type, bool input_has_alpha = true, int rgb_step = 3>
void resample(
const void *input, int in_width, int in_height, int in_stride,
void *output, int out_width, int out_height, int out_stride,
resample_params_t &params)
{
using type_mapping_t = type_mapping<color_type, input_has_alpha>;
using type_mapping_t = type_mapping<color_type, input_has_alpha, rgb_step>;

using input_pixfmt_t = typename type_mapping_t::input_pixfmt_type;
using output_pixfmt_t = typename type_mapping_t::output_pixfmt_type;
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41 changes: 31 additions & 10 deletions src/_image_wrapper.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -107,17 +107,29 @@ image_resample(py::array input_array,
}

py::ssize_t ncomponents = 0;
int rgb_step = 0;
if (ndim == 3) {
ncomponents = input_array.shape(2);
if (ncomponents != 3 && ncomponents != 4) {
if (ncomponents == 3) {
// We special-case a few options in order to avoid copying in the common case.
auto rgb_stride = input_array.strides(1);
auto item_stride = input_array.strides(2);
if (rgb_stride == 3 * item_stride) {
rgb_step = 3;
} else if (rgb_stride == 4 * item_stride) {
rgb_step = 4;
}
} else if (ncomponents != 4) {
throw std::invalid_argument(
"3D input array must be RGB with shape (M, N, 3) or RGBA with shape (M, N, 4), "
"has trailing dimension of {}"_s.format(ncomponents));
}
}

// Ensure input array is contiguous, regardless of dtype
input_array = py::array::ensure(input_array, py::array::c_style);
if (rgb_step == 0) {
// Ensure input array is contiguous, regardless of dtype
input_array = py::array::ensure(input_array, py::array::c_style);
}

// Validate output array
auto out_ndim = output_array.ndim();
Expand Down Expand Up @@ -194,13 +206,22 @@ image_resample(py::array input_array,
dtype.equal(py::dtype::of<double>()) ? resample<agg::rgba64, true> :
nullptr
) : (
dtype.equal(py::dtype::of<std::uint8_t>()) ? resample<agg::rgba8, false> :
dtype.equal(py::dtype::of<std::int8_t>()) ? resample<agg::rgba8, false> :
dtype.equal(py::dtype::of<std::uint16_t>()) ? resample<agg::rgba16, false> :
dtype.equal(py::dtype::of<std::int16_t>()) ? resample<agg::rgba16, false> :
dtype.equal(py::dtype::of<float>()) ? resample<agg::rgba32, false> :
dtype.equal(py::dtype::of<double>()) ? resample<agg::rgba64, false> :
nullptr)))
(rgb_step == 4) ? (
dtype.equal(py::dtype::of<std::uint8_t>()) ? resample<agg::rgba8, false, 4> :
dtype.equal(py::dtype::of<std::int8_t>()) ? resample<agg::rgba8, false, 4> :
dtype.equal(py::dtype::of<std::uint16_t>()) ? resample<agg::rgba16, false, 4> :
dtype.equal(py::dtype::of<std::int16_t>()) ? resample<agg::rgba16, false, 4> :
dtype.equal(py::dtype::of<float>()) ? resample<agg::rgba32, false, 4> :
dtype.equal(py::dtype::of<double>()) ? resample<agg::rgba64, false, 4> :
nullptr
) : (
dtype.equal(py::dtype::of<std::uint8_t>()) ? resample<agg::rgba8, false, 3> :
dtype.equal(py::dtype::of<std::int8_t>()) ? resample<agg::rgba8, false, 3> :
dtype.equal(py::dtype::of<std::uint16_t>()) ? resample<agg::rgba16, false, 3> :
dtype.equal(py::dtype::of<std::int16_t>()) ? resample<agg::rgba16, false, 3> :
dtype.equal(py::dtype::of<float>()) ? resample<agg::rgba32, false, 3> :
dtype.equal(py::dtype::of<double>()) ? resample<agg::rgba64, false, 3> :
nullptr))))
{
Py_BEGIN_ALLOW_THREADS
resampler(
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