Это основано на алгоритме, который я скопировал десятилетия назад из немецкого c't Magazin , и время от времени использую его для подобных проблем, как описано в OP.
bool scaleDown(
const Image &imgSrc,
Image &imgDst,
int w, int h,
int align)
{
const int wSrc = imgSrc.w(), hSrc = imgSrc.h();
assert(w > 0 && w <= wSrc && h > 0 && h <= hSrc);
// compute scaling factors
const double sx = (double)wSrc / (double)w;
const double sy = (double)hSrc / (double)h;
const double sxy = sx * sy;
// prepare destination image
imgDst.resize(w, h, (w * 3 + align - 1) / align * align);
// cache some data
const uint8 *const dataSrc = imgSrc.data();
const int bPRSrc = imgSrc.bPR();
// perform scaling
for (int y = 0; y < h; ++y) {
const double yStart = sy * y;
const double yEnd = std::min(sy * (y + 1), (double)hSrc);
const int yStartInt = (int)yStart;
const int yEndInt = (int)yEnd - (yEndInt == yEnd);
const double tFrm = 1 + yStartInt - yStart, bFrm = yEnd - yEndInt;
for (int x = 0; x < w; ++x) {
const double xStart = sx * x;
const double xEnd = std::min(sx * (x + 1), (double)wSrc);
const int xStartInt = (int)xStart;
const int xEndInt = (int)xEnd - (xEndInt == xEnd);
double lFrm = 1 + xStartInt - xStart, rFrm = xEnd - xEndInt;
double pixel[3] = { 0.0, 0.0, 0.0 }; // values of target pixel
for (int i = yStartInt; i <= yEndInt; ++i) {
int jData = i * bPRSrc + xStartInt * 3;
for (int j = xStartInt; j <= xEndInt; ++j) {
double pixelAdd[3];
for (int k = 0; k < 3; ++k) {
pixelAdd[k] = (double)dataSrc[jData++] / sxy;
}
if (j == xStartInt) {
for (int k = 0; k < 3; ++k) pixelAdd[k] *= lFrm;
} else if (j == xEndInt) {
for (int k = 0; k < 3; ++k) pixelAdd[k] *= rFrm;
}
if (i == yStartInt) {
for (int k = 0; k < 3; ++k) pixelAdd[k] *= tFrm;
} else if (i == yEndInt) {
for (int k = 0; k < 3; ++k) pixelAdd[k] *= bFrm;
}
for (int k = 0; k < 3; ++k) pixel[k] += pixelAdd[k];
}
}
imgDst.setPixel(x, y,
(uint8)pixel[0], (uint8)pixel[1], (uint8)pixel[2]);
}
}
// done
return true;
}
Если я правильно понял, это реализует билинейную интерполяцию .
Я не смею называть это Минимально завершенный проверяемый пример , хотяэто то, что я намеревался сделать.
Полный пример приложения:
Упрощенный class Image
image.h:
#ifndef IMAGE_H
#define IMAGE_H
#include <vector>
// convenience type for bytes
typedef unsigned char uint8;
// image helper class
class Image {
private: // variables:
int _w, _h; // image size
size_t _bPR; // bytes per row
std::vector<uint8> _data; // image data
public: // methods:
// constructor.
Image(): _w(0), _h(0), _bPR(0) { }
// destructor.
~Image() = default;
// copy constructor.
Image(const Image&) = delete; // = default; would work as well.
// copy assignment.
Image& operator=(const Image&) = delete; // = default; would work as well.
// returns width of image.
int w() const { return _w; }
// returns height of image.
int h() const { return _h; }
// returns bytes per row.
size_t bPR() const { return _bPR; }
// returns pointer to image data.
const uint8* data(
int y = 0) // row number
const {
return &_data[y * _bPR];
}
// returns data size (in bytes).
size_t size() const { return _data.size(); }
// clears image.
void clear();
// resizes image.
uint8* resize( // returns allocated buffer
int w, // image width
int h, // image height
int bPR); // bytes per row
// returns pixel.
int getPixel(
int x, // column
int y) // row
const;
// sets pixel.
void setPixel(
int x, // column
int y, // row
uint8 r, uint8 g, uint8 b);
// sets pixel.
void setPixel(
int x, // column
int y, // row
int value) // RGB value
{
setPixel(x, y, value & 0xff, value >> 8 & 0xff, value >> 16 & 0xff);
}
};
// helper functions:
inline uint8 getR(int value) { return value & 0xff; }
inline uint8 getG(int value) { return value >> 8 & 0xff; }
inline uint8 getB(int value) { return value >> 16 & 0xff; }
#endif // IMAGE_H
image.cc:
#include <cassert>
#include "image.h"
// clears image.
void Image::clear()
{
_data.clear(); _w = _h = _bPR = 0;
}
// allocates image data.
uint8* Image::resize( // returns allocated buffer
int w, // image width
int h, // image height
int bPR) // bits per row
{
assert(w >= 0 && 3 * w <= bPR);
assert(h >= 0);
_w = w; _h = h; _bPR = bPR;
const size_t size = h * bPR;
_data.resize(size);
return _data.data();
}
// returns pixel.
int Image::getPixel(
int x, // column
int y) // row
const {
assert(x >= 0 && x < _w);
assert(y >= 0 && y < _h);
const size_t offs = y * _bPR + 3 * x;
return _data[offs + 0]
| _data[offs + 1] << 8
| _data[offs + 2] << 16;
}
// sets pixel.
void Image::setPixel(
int x, // column
int y, // row
uint8 r, uint8 g, uint8 b) // R, G, B values
{
assert(x >= 0 && x < _w);
assert(y >= 0 && y < _h);
const size_t offs = y * _bPR + 3 * x;
_data[offs + 0] = r;
_data[offs + 1] = g;
_data[offs + 2] = b;
}
Масштабирование изображения
imageScale.h:
#ifndef IMAGE_SCALE_H
#define IMAGE_SCALE_H
#include "image.h"
/* scales an image to a certain width and height.
*
* Note:
* imgSrc and imgDst may not be identical.
*/
bool scaleTo( // returns true if successful
const Image &imgSrc, // source image
Image &imgDst, // destination image
int w, int h, // destination width and height
int align = 4); // row alignment
/* scales an image about a certain horizontal/vertical scaling factor.
*
* Note:
* imgSrc and imgDst may not be identical.
*/
inline bool scaleXY( // returns true if successful
const Image &imgSrc, // source image
Image &imgDst, // destination image
double sX, // horizontal scaling factor (must be > 0 but not too large)
double sY, // vertical scaling factor (must be > 0 but not too large)
int align = 4) // row alignment
{
return sX > 0.0 && sY > 0.0
? scaleTo(imgSrc, imgDst,
(int)(sX * imgSrc.w()), (int)(sY * imgSrc.h()), align)
: false;
}
/* scales an image about a certain scaling factor.
*
* Note:
* imgSrc and imgDst may not be identical.
*/
inline bool scale( // returns true if successful
const Image &imgSrc, // source image
Image &imgDst, // destination image
double s, // scaling factor (must be > 0 but not too large)
int align = 4) // row alignment
{
return scaleXY(imgSrc, imgDst, s, s, align);
}
#endif // IMAGE_SCALE_H
imageScale.cc:
#include <cassert>
#include <algorithm>
#include "imageScale.h"
namespace {
template <typename VALUE>
VALUE clip(VALUE value, VALUE min, VALUE max)
{
return value < min ? min : value > max ? max : value;
}
bool scaleDown(
const Image &imgSrc,
Image &imgDst,
int w, int h,
int align)
{
const int wSrc = imgSrc.w(), hSrc = imgSrc.h();
assert(w > 0 && w <= wSrc && h > 0 && h <= hSrc);
// compute scaling factors
const double sx = (double)wSrc / (double)w;
const double sy = (double)hSrc / (double)h;
const double sxy = sx * sy;
// prepare destination image
imgDst.resize(w, h, (w * 3 + align - 1) / align * align);
// cache some data
const uint8 *const dataSrc = imgSrc.data();
const int bPRSrc = imgSrc.bPR();
// perform scaling
for (int y = 0; y < h; ++y) {
const double yStart = sy * y;
const double yEnd = std::min(sy * (y + 1), (double)hSrc);
const int yStartInt = (int)yStart;
const int yEndInt = (int)yEnd - (yEndInt == yEnd);
const double tFrm = 1 + yStartInt - yStart, bFrm = yEnd - yEndInt;
for (int x = 0; x < w; ++x) {
const double xStart = sx * x;
const double xEnd = std::min(sx * (x + 1), (double)wSrc);
const int xStartInt = (int)xStart;
const int xEndInt = (int)xEnd - (xEndInt == xEnd);
double lFrm = 1 + xStartInt - xStart, rFrm = xEnd - xEndInt;
double pixel[3] = { 0.0, 0.0, 0.0 }; // values of target pixel
for (int i = yStartInt; i <= yEndInt; ++i) {
int jData = i * bPRSrc + xStartInt * 3;
for (int j = xStartInt; j <= xEndInt; ++j) {
double pixelAdd[3];
for (int k = 0; k < 3; ++k) {
pixelAdd[k] = (double)dataSrc[jData++] / sxy;
}
if (j == xStartInt) {
for (int k = 0; k < 3; ++k) pixelAdd[k] *= lFrm;
} else if (j == xEndInt) {
for (int k = 0; k < 3; ++k) pixelAdd[k] *= rFrm;
}
if (i == yStartInt) {
for (int k = 0; k < 3; ++k) pixelAdd[k] *= tFrm;
} else if (i == yEndInt) {
for (int k = 0; k < 3; ++k) pixelAdd[k] *= bFrm;
}
for (int k = 0; k < 3; ++k) pixel[k] += pixelAdd[k];
}
}
imgDst.setPixel(x, y,
(uint8)pixel[0], (uint8)pixel[1], (uint8)pixel[2]);
}
}
// done
return true;
}
bool scaleUp(
const Image &imgSrc,
Image &imgDst,
int w, int h,
int align)
{
const int wSrc = imgSrc.w(), hSrc = imgSrc.h();
assert(w && w >= wSrc && h && h >= hSrc);
// compute scaling factors
const double sx = (double)wSrc / (double)w;
const double sy = (double)hSrc / (double)h;
// prepare destination image
imgDst.resize(w, h, (w * 3 + align - 1) / align * align);
// cache some data
const uint8 *const dataSrc = imgSrc.data();
const int bPRSrc = imgSrc.bPR();
// perform scaling
for (int y = 0; y < h; ++y) {
const double yStart = sy * y;
const double yEnd = std::min(sy * (y + 1), (double)hSrc - 1);
const int yStartInt = (int)yStart;
const int yEndInt = (int)yEnd;
if (yStartInt < yEndInt) {
const double bFract = clip((double)((yEnd - yEndInt) / sy), 0.0, 1.0);
const double tFract = 1.0 - bFract;
for (int x = 0; x < w; ++x) {
const double xStart = sx * x;
const double xEnd = std::min(sx * (x + 1), (double)wSrc - 1);
const int xStartInt = (int)xStart, xEndInt = (int)xEnd;
double pixel[4];
if (xStartInt < xEndInt) {
const double rFract
= clip((double)((xEnd - xEndInt) / sx), 0.0, 1.0);
const double lFract = 1.0 - rFract;
int jData = yStartInt * bPRSrc + xStartInt * 3;
for (int k = 0; k < 3; ++k) {
pixel[k] = tFract * lFract * dataSrc[jData++];
}
for (int k = 0; k < 3; ++k) {
pixel[k] += tFract * rFract * dataSrc[jData++];
}
jData = yEndInt * bPRSrc + xStartInt * 3;
for (int k = 0; k < 3; ++k) {
pixel[k] += bFract * lFract *dataSrc[jData++];
}
for (int k = 0; k < 3; ++k) {
pixel[k] += bFract * rFract *dataSrc[jData++];
}
} else {
int jData = yStartInt * bPRSrc + xStartInt * 3;
for (int k = 0; k < 3; ++k) {
pixel[k] = tFract * dataSrc[jData++];
}
jData = yEndInt * bPRSrc + xStartInt * 3;
for (int k = 0; k < 3; ++k) {
pixel[k] += bFract * dataSrc[jData++];
}
}
imgDst.setPixel(x, y,
(uint8)pixel[0], (uint8)pixel[1], (uint8)pixel[2]);
}
} else {
for (int x = 0; x < w; ++x) {
const double xStart = sx * x;
const double xEnd = std::min(sx * (x + 1), (double)wSrc - 1);
const int xStartInt = (int)xStart, xEndInt = (int)xEnd;
double pixel[3];
if (xStartInt < xEndInt) {
const double rFract
= clip((double)((xEnd - xEndInt) / sx), 0.0, 1.0);
const double lFract = 1.0 - rFract;
int jData = yStartInt * bPRSrc + xStartInt * 3;
for (int k = 0; k < 3; ++k) {
pixel[k] = lFract * dataSrc[jData++];
}
for (int k = 0; k < 3; ++k) {
pixel[k] += rFract * dataSrc[jData++];
}
} else {
int jData = yStartInt * bPRSrc + xStartInt * 3;
for (int k = 0; k < 3; ++k) pixel[k] = dataSrc[jData++];
}
imgDst.setPixel(x, y,
(uint8)pixel[0], (uint8)pixel[1], (uint8)pixel[2]);
}
}
}
// done
return true;
}
} // namespace
bool scaleTo(const Image &imgSrc, Image &imgDst, int w, int h, int align)
{
Image imgTmp;
return w <= 0 || h <= 0 ? false
: w >= imgSrc.w() && h >= imgSrc.h()
? scaleUp(imgSrc, imgDst, w, h, align)
: w <= imgSrc.w() && h <= imgSrc.h()
? scaleDown(imgSrc, imgDst, w, h, align)
: w >= imgSrc.w()
? scaleUp(imgSrc, imgTmp, w, imgSrc.h(), 1)
&& scaleDown(imgTmp, imgDst, w, h, align)
: scaleDown(imgSrc, imgTmp, w, imgSrc.h(), 1)
&& scaleUp(imgTmp, imgDst, w, h, align);
}
PPM-файл IO
imagePPM.h:
#ifndef IMAGE_PPM_H
#define IMAGE_PPM_H
#include <iostream>
#include "image.h"
// reads a binary PPM file.
bool readPPM( // returns true if successful
std::istream &in, // input stream (must be opened with std::ios::binary)
Image &img, // image to read into
int align = 4); // row alignment
// writes binary PPM file.
bool writePPM( // returns true if successful
std::ostream &out, // output stream (must be opened with std::ios::binary)
const Image &img); // image to write from
#endif // IMAGE_PPM_H
imagePPM.cc:
#include <sstream>
#include <string>
#include "imagePPM.h"
// reads a binary PPM file.
bool readPPM( // returns true if successful
std::istream &in, // input stream (must be opened with std::ios::binary)
Image &img, // image to read into
int align) // row alignment
{
// parse header
std::string buffer;
if (!getline(in, buffer)) return false;
if (buffer != "P6") {
std::cerr << "Wrong header! 'P6' expected.\n";
return false;
}
int w = 0, h = 0, t = 0;
for (int i = 0; i < 3;) {
if (!getline(in, buffer)) return false;
if (buffer.empty()) continue; // skip empty lines
if (buffer[0] == '#') continue; // skip comments
std::istringstream str(buffer);
switch (i) {
case 0:
if (!(str >> w)) continue;
++i;
case 1:
if (!(str >> h)) continue;
++i;
case 2:
if (!(str >> t)) continue;
++i;
}
}
if (t != 255) {
std::cerr << "Unsupported format! t = 255 expected.\n";
return false;
}
// allocate image buffer
uint8 *data = img.resize(w, h, (w * 3 + align - 1) / align * align);
// read data
for (int i = 0; i < h; ++i) {
if (!in.read((char*)data, 3 * img.w())) return false;
data += img.bPR();
}
// done
return true;
}
// writes binary PPM file.
bool writePPM( // returns true if successful
std::ostream &out, // output stream (must be opened with std::ios::binary)
const Image &img) // image to write from
{
// write header
if (!(out << "P6\n" << img.w() << ' ' << img.h() << " 255\n")) return false;
// write image data
for (size_t y = 0; y < img.h(); ++y) {
const uint8 *const data = img.data(y);
if (!out.write((const char*)data, 3 * img.w())) return false;
}
// done
return true;
}
Основное применение
scaleRGBImg.cc:
#include <iostream>
#include <fstream>
#include <string>
#include "image.h"
#include "imagePPM.h"
#include "imageScale.h"
int main(int argc, char **argv)
{
// read command line arguments
if (argc <= 3) {
std::cerr << "Missing arguments!\n";
std::cout
<< "Usage:\n"
<< " scaleRGBImg IN_FILE SCALE OUT_FILE\n";
return 1;
}
const std::string inFile = argv[1];
char *end;
const double s = std::strtod(argv[2], &end);
if (end == argv[2] || *end != '\0') {
std::cerr << "Invalid scale factor '" << argv[2] << "'!\n";
return 1;
}
if (s <= 0.0) {
std::cerr << "Invalid scale factor " << s << "!\n";
return 1;
}
const std::string outFile = argv[3];
// read image
Image imgSrc;
{ std::ifstream fIn(inFile.c_str(), std::ios::binary);
if (!readPPM(fIn, imgSrc)) {
std::cerr << "Reading '" << inFile << "' failed!\n";
return 1;
}
}
// scale image
Image imgDst;
if (!scale(imgSrc, imgDst, s)) {
std::cerr << "Scaling failed!\n";
return 1;
}
// write image
{ std::ofstream fOut(outFile.c_str(), std::ios::binary);
if (!writePPM(fOut, imgDst) || (fOut.close(), !fOut.good())) {
std::cerr << "Writing '" << outFile << "' failed!\n";
return 1;
}
}
// done
return 0;
}
Тест
Скомпилировано в cygwin64 :
$ g++ -std=c++11 -o scaleRGBImg scaleRGBImg.cc image.cc imagePPM.cc imageScale.cc
$
Образец изображения test.ppm для теста - преобразовано в PPM в GIMP :
Тест с образцом изображения:
$ for I in 0.8 0.6 0.4 0.2 ; do echo ./scaleRGBImg test.ppm $I test.$I.ppm ; done
./scaleRGBImg test.ppm 0.8 test.0.8.ppm
./scaleRGBImg test.ppm 0.6 test.0.6.ppm
./scaleRGBImg test.ppm 0.4 test.0.4.ppm
./scaleRGBImg test.ppm 0.2 test.0.2.ppm
$ for I in 0.8 0.6 0.4 0.2 ; do ./scaleRGBImg test.ppm $I test.$I.ppm ; done
$
Этовот что получилось:
test.0.8.ppm:
test.0.6.ppm:
test.0.4.ppm:
test.0.2.ppm:
