一、概述
调整图像大小。函数 resize 将图像 src 的大小缩小到或最大到指定的大小。 请注意，不考虑初始 dst 类型或大小。 相反，大小和类型是从 src、dsize、fx 和 fy 派生的。 如果您想调整 src 的大小以使其适合预先创建的 dst，您可以按如下方式调用该函数：
resize(src, dst, dst.size(), 0, 0, interpolation);如果要在每个方向将图像抽取 2 倍，可以这样调用函数：
resize(src, dst, Size(), 0.5, 0.5, interpolation);要缩小图像，通常使用 INTER_AREA 插值效果最好，而要放大图像，通常使用 INTER_CUBIC（慢）或 INTER_LINEAR（更快但看起来还可以）效果最好。
二、resize函数
1、函数原型
cv::resize (InputArray src, OutputArray dst, Size dsize, double fx=0, double fy=0, int interpolation=INTER_LINEAR)2、参数详解
| src | 输入图像。 |
| dst | 输出图像； 它的大小为 dsize（当它非零时）或由 src.size()、fx 和 fy 计算得出的大小； dst 的类型与 src 的类型相同。 |
| dsize | 输出图像大小； 如果它等于 0（在 Python 中为 None），则计算如下： dsize = Size(round(fx*src.cols), round(fy*src.rows)) dsize 或 fx 和 fy 必须为非零。 |
| fx | 沿水平轴的比例因子； 当它等于 0 时，它被计算为 (double)dsize.width/src.cols |
| fy | 沿垂直轴的比例因子； 当它等于 0 时，它被计算为 (double)dsize.height/src.rows |
| interpolation | 插值方法，请参阅 InterpolationFlags |
三、OpenCV源码
1、源码路径
opencv\modules\imgproc\src\resize.cpp2、源码代码
void cv::resize( InputArray _src, OutputArray _dst, Size dsize,
double inv_scale_x, double inv_scale_y, int interpolation )
{
CV_INSTRUMENT_REGION();
Size ssize = _src.size();
CV_Assert( !ssize.empty() );
if( dsize.empty() )
{
CV_Assert(inv_scale_x > 0); CV_Assert(inv_scale_y > 0);
dsize = Size(saturate_cast<int>(ssize.width*inv_scale_x),
saturate_cast<int>(ssize.height*inv_scale_y));
CV_Assert( !dsize.empty() );
}
else
{
inv_scale_x = (double)dsize.width/ssize.width;
inv_scale_y = (double)dsize.height/ssize.height;
CV_Assert(inv_scale_x > 0); CV_Assert(inv_scale_y > 0);
}
if (interpolation == INTER_LINEAR_EXACT && (_src.depth() == CV_32F || _src.depth() == CV_64F))
interpolation = INTER_LINEAR; // If depth isn't supported fallback to generic resize
CV_OCL_RUN(_src.dims() <= 2 && _dst.isUMat() && _src.cols() > 10 && _src.rows() > 10,
ocl_resize(_src, _dst, dsize, inv_scale_x, inv_scale_y, interpolation))
// Fake reference to source. Resolves issue 13577 in case of src == dst.
UMat srcUMat;
if (_src.isUMat())
srcUMat = _src.getUMat();
Mat src = _src.getMat();
_dst.create(dsize, src.type());
Mat dst = _dst.getMat();
if (dsize == ssize)
{
// Source and destination are of same size. Use simple copy.
src.copyTo(dst);
return;
}
hal::resize(src.type(), src.data, src.step, src.cols, src.rows, dst.data, dst.step, dst.cols, dst.rows, inv_scale_x, inv_scale_y, interpolation);
}void resize(int src_type,
const uchar * src_data, size_t src_step, int src_width, int src_height,
uchar * dst_data, size_t dst_step, int dst_width, int dst_height,
double inv_scale_x, double inv_scale_y, int interpolation)
{
CV_INSTRUMENT_REGION();
CV_Assert((dst_width > 0 && dst_height > 0) || (inv_scale_x > 0 && inv_scale_y > 0));
if (inv_scale_x < DBL_EPSILON || inv_scale_y < DBL_EPSILON)
{
inv_scale_x = static_cast<double>(dst_width) / src_width;
inv_scale_y = static_cast<double>(dst_height) / src_height;
}
CALL_HAL(resize, cv_hal_resize, src_type, src_data, src_step, src_width, src_height, dst_data, dst_step, dst_width, dst_height, inv_scale_x, inv_scale_y, interpolation);
int depth = CV_MAT_DEPTH(src_type), cn = CV_MAT_CN(src_type);
Size dsize = Size(saturate_cast<int>(src_width*inv_scale_x),
saturate_cast<int>(src_height*inv_scale_y));
CV_Assert( !dsize.empty() );
CV_IPP_RUN_FAST(ipp_resize(src_data, src_step, src_width, src_height, dst_data, dst_step, dsize.width, dsize.height, inv_scale_x, inv_scale_y, depth, cn, interpolation))
static ResizeFunc linear_tab[] =
{
resizeGeneric_<
HResizeLinear<uchar, int, short,
INTER_RESIZE_COEF_SCALE,
HResizeLinearVec_8u32s>,
VResizeLinear<uchar, int, short,
FixedPtCast<int, uchar, INTER_RESIZE_COEF_BITS*2>,
VResizeLinearVec_32s8u> >,
0,
resizeGeneric_<
HResizeLinear<ushort, float, float, 1,
HResizeLinearVec_16u32f>,
VResizeLinear<ushort, float, float, Cast<float, ushort>,
VResizeLinearVec_32f16u> >,
resizeGeneric_<
HResizeLinear<short, float, float, 1,
HResizeLinearVec_16s32f>,
VResizeLinear<short, float, float, Cast<float, short>,
VResizeLinearVec_32f16s> >,
0,
resizeGeneric_<
HResizeLinear<float, float, float, 1,
HResizeLinearVec_32f>,
VResizeLinear<float, float, float, Cast<float, float>,
VResizeLinearVec_32f> >,
resizeGeneric_<
HResizeLinear<double, double, float, 1,
HResizeNoVec>,
VResizeLinear<double, double, float, Cast<double, double>,
VResizeNoVec> >,
0
};
static ResizeFunc cubic_tab[] =
{
resizeGeneric_<
HResizeCubic<uchar, int, short>,
VResizeCubic<uchar, int, short,
FixedPtCast<int, uchar, INTER_RESIZE_COEF_BITS*2>,
VResizeCubicVec_32s8u> >,
0,
resizeGeneric_<
HResizeCubic<ushort, float, float>,
VResizeCubic<ushort, float, float, Cast<float, ushort>,
VResizeCubicVec_32f16u> >,
resizeGeneric_<
HResizeCubic<short, float, float>,
VResizeCubic<short, float, float, Cast<float, short>,
VResizeCubicVec_32f16s> >,
0,
resizeGeneric_<
HResizeCubic<float, float, float>,
VResizeCubic<float, float, float, Cast<float, float>,
VResizeCubicVec_32f> >,
resizeGeneric_<
HResizeCubic<double, double, float>,
VResizeCubic<double, double, float, Cast<double, double>,
VResizeNoVec> >,
0
};
static ResizeFunc lanczos4_tab[] =
{
resizeGeneric_<HResizeLanczos4<uchar, int, short>,
VResizeLanczos4<uchar, int, short,
FixedPtCast<int, uchar, INTER_RESIZE_COEF_BITS*2>,
VResizeNoVec> >,
0,
resizeGeneric_<HResizeLanczos4<ushort, float, float>,
VResizeLanczos4<ushort, float, float, Cast<float, ushort>,
VResizeLanczos4Vec_32f16u> >,
resizeGeneric_<HResizeLanczos4<short, float, float>,
VResizeLanczos4<short, float, float, Cast<float, short>,
VResizeLanczos4Vec_32f16s> >,
0,
resizeGeneric_<HResizeLanczos4<float, float, float>,
VResizeLanczos4<float, float, float, Cast<float, float>,
VResizeLanczos4Vec_32f> >,
resizeGeneric_<HResizeLanczos4<double, double, float>,
VResizeLanczos4<double, double, float, Cast<double, double>,
VResizeNoVec> >,
0
};
static ResizeAreaFastFunc areafast_tab[] =
{
resizeAreaFast_<uchar, int, ResizeAreaFastVec<uchar, ResizeAreaFastVec_SIMD_8u> >,
0,
resizeAreaFast_<ushort, float, ResizeAreaFastVec<ushort, ResizeAreaFastVec_SIMD_16u> >,
resizeAreaFast_<short, float, ResizeAreaFastVec<short, ResizeAreaFastVec_SIMD_16s> >,
0,
resizeAreaFast_<float, float, ResizeAreaFastVec_SIMD_32f>,
resizeAreaFast_<double, double, ResizeAreaFastNoVec<double, double> >,
0
};
static ResizeAreaFunc area_tab[] =
{
resizeArea_<uchar, float>, 0, resizeArea_<ushort, float>,
resizeArea_<short, float>, 0, resizeArea_<float, float>,
resizeArea_<double, double>, 0
};
static be_resize_func linear_exact_tab[] =
{
resize_bitExact<uchar, interpolationLinear<uchar> >,
resize_bitExact<schar, interpolationLinear<schar> >,
resize_bitExact<ushort, interpolationLinear<ushort> >,
resize_bitExact<short, interpolationLinear<short> >,
resize_bitExact<int, interpolationLinear<int> >,
0,
0,
0
};
double scale_x = 1./inv_scale_x, scale_y = 1./inv_scale_y;
int iscale_x = saturate_cast<int>(scale_x);
int iscale_y = saturate_cast<int>(scale_y);
bool is_area_fast = std::abs(scale_x - iscale_x) < DBL_EPSILON &&
std::abs(scale_y - iscale_y) < DBL_EPSILON;
Mat src(Size(src_width, src_height), src_type, const_cast<uchar*>(src_data), src_step);
Mat dst(dsize, src_type, dst_data, dst_step);
if (interpolation == INTER_LINEAR_EXACT)
{
// in case of inv_scale_x && inv_scale_y is equal to 0.5
// INTER_AREA (fast) is equal to bit exact INTER_LINEAR
if (is_area_fast && iscale_x == 2 && iscale_y == 2 && cn != 2)//Area resize implementation for 2-channel images isn't bit-exact
interpolation = INTER_AREA;
else
{
be_resize_func func = linear_exact_tab[depth];
CV_Assert(func != 0);
func(src_data, src_step, src_width, src_height,
dst_data, dst_step, dst_width, dst_height,
cn, inv_scale_x, inv_scale_y);
return;
}
}
if( interpolation == INTER_NEAREST )
{
resizeNN( src, dst, inv_scale_x, inv_scale_y );
return;
}
if( interpolation == INTER_NEAREST_EXACT )
{
resizeNN_bitexact( src, dst, inv_scale_x, inv_scale_y );
return;
}
int k, sx, sy, dx, dy;
{
// in case of scale_x && scale_y is equal to 2
// INTER_AREA (fast) also is equal to INTER_LINEAR
if( interpolation == INTER_LINEAR && is_area_fast && iscale_x == 2 && iscale_y == 2 )
interpolation = INTER_AREA;
// true "area" interpolation is only implemented for the case (scale_x >= 1 && scale_y >= 1).
// In other cases it is emulated using some variant of bilinear interpolation
if( interpolation == INTER_AREA && scale_x >= 1 && scale_y >= 1 )
{
if( is_area_fast )
{
int area = iscale_x*iscale_y;
size_t srcstep = src_step / src.elemSize1();
AutoBuffer<int> _ofs(area + dsize.width*cn);
int* ofs = _ofs.data();
int* xofs = ofs + area;
ResizeAreaFastFunc func = areafast_tab[depth];
CV_Assert( func != 0 );
for( sy = 0, k = 0; sy < iscale_y; sy++ )
for( sx = 0; sx < iscale_x; sx++ )
ofs[k++] = (int)(sy*srcstep + sx*cn);
for( dx = 0; dx < dsize.width; dx++ )
{
int j = dx * cn;
sx = iscale_x * j;
for( k = 0; k < cn; k++ )
xofs[j + k] = sx + k;
}
func( src, dst, ofs, xofs, iscale_x, iscale_y );
return;
}
ResizeAreaFunc func = area_tab[depth];
CV_Assert( func != 0 && cn <= 4 );
AutoBuffer<DecimateAlpha> _xytab((src_width + src_height)*2);
DecimateAlpha* xtab = _xytab.data(), *ytab = xtab + src_width*2;
int xtab_size = computeResizeAreaTab(src_width, dsize.width, cn, scale_x, xtab);
int ytab_size = computeResizeAreaTab(src_height, dsize.height, 1, scale_y, ytab);
AutoBuffer<int> _tabofs(dsize.height + 1);
int* tabofs = _tabofs.data();
for( k = 0, dy = 0; k < ytab_size; k++ )
{
if( k == 0 || ytab[k].di != ytab[k-1].di )
{
assert( ytab[k].di == dy );
tabofs[dy++] = k;
}
}
tabofs[dy] = ytab_size;
func( src, dst, xtab, xtab_size, ytab, ytab_size, tabofs );
return;
}
}
int xmin = 0, xmax = dsize.width, width = dsize.width*cn;
bool area_mode = interpolation == INTER_AREA;
bool fixpt = depth == CV_8U;
float fx, fy;
ResizeFunc func=0;
int ksize=0, ksize2;
if( interpolation == INTER_CUBIC )
ksize = 4, func = cubic_tab[depth];
else if( interpolation == INTER_LANCZOS4 )
ksize = 8, func = lanczos4_tab[depth];
else if( interpolation == INTER_LINEAR || interpolation == INTER_AREA )
ksize = 2, func = linear_tab[depth];
else
CV_Error( CV_StsBadArg, "Unknown interpolation method" );
ksize2 = ksize/2;
CV_Assert( func != 0 );
AutoBuffer<uchar> _buffer((width + dsize.height)*(sizeof(int) + sizeof(float)*ksize));
int* xofs = (int*)_buffer.data();
int* yofs = xofs + width;
float* alpha = (float*)(yofs + dsize.height);
short* ialpha = (short*)alpha;
float* beta = alpha + width*ksize;
short* ibeta = ialpha + width*ksize;
float cbuf[MAX_ESIZE] = {0};
for( dx = 0; dx < dsize.width; dx++ )
{
if( !area_mode )
{
fx = (float)((dx+0.5)*scale_x - 0.5);
sx = cvFloor(fx);
fx -= sx;
}
else
{
sx = cvFloor(dx*scale_x);
fx = (float)((dx+1) - (sx+1)*inv_scale_x);
fx = fx <= 0 ? 0.f : fx - cvFloor(fx);
}
if( sx < ksize2-1 )
{
xmin = dx+1;
if( sx < 0 && (interpolation != INTER_CUBIC && interpolation != INTER_LANCZOS4))
fx = 0, sx = 0;
}
if( sx + ksize2 >= src_width )
{
xmax = std::min( xmax, dx );
if( sx >= src_width-1 && (interpolation != INTER_CUBIC && interpolation != INTER_LANCZOS4))
fx = 0, sx = src_width-1;
}
for( k = 0, sx *= cn; k < cn; k++ )
xofs[dx*cn + k] = sx + k;
if( interpolation == INTER_CUBIC )
interpolateCubic( fx, cbuf );
else if( interpolation == INTER_LANCZOS4 )
interpolateLanczos4( fx, cbuf );
else
{
cbuf[0] = 1.f - fx;
cbuf[1] = fx;
}
if( fixpt )
{
for( k = 0; k < ksize; k++ )
ialpha[dx*cn*ksize + k] = saturate_cast<short>(cbuf[k]*INTER_RESIZE_COEF_SCALE);
for( ; k < cn*ksize; k++ )
ialpha[dx*cn*ksize + k] = ialpha[dx*cn*ksize + k - ksize];
}
else
{
for( k = 0; k < ksize; k++ )
alpha[dx*cn*ksize + k] = cbuf[k];
for( ; k < cn*ksize; k++ )
alpha[dx*cn*ksize + k] = alpha[dx*cn*ksize + k - ksize];
}
}
for( dy = 0; dy < dsize.height; dy++ )
{
if( !area_mode )
{
fy = (float)((dy+0.5)*scale_y - 0.5);
sy = cvFloor(fy);
fy -= sy;
}
else
{
sy = cvFloor(dy*scale_y);
fy = (float)((dy+1) - (sy+1)*inv_scale_y);
fy = fy <= 0 ? 0.f : fy - cvFloor(fy);
}
yofs[dy] = sy;
if( interpolation == INTER_CUBIC )
interpolateCubic( fy, cbuf );
else if( interpolation == INTER_LANCZOS4 )
interpolateLanczos4( fy, cbuf );
else
{
cbuf[0] = 1.f - fy;
cbuf[1] = fy;
}
if( fixpt )
{
for( k = 0; k < ksize; k++ )
ibeta[dy*ksize + k] = saturate_cast<short>(cbuf[k]*INTER_RESIZE_COEF_SCALE);
}
else
{
for( k = 0; k < ksize; k++ )
beta[dy*ksize + k] = cbuf[k];
}
}
func( src, dst, xofs, fixpt ? (void*)ialpha : (void*)alpha, yofs,
fixpt ? (void*)ibeta : (void*)beta, xmin, xmax, ksize );
}四、效果图像示例
