将RGB图像保存为PPM格式

Question

我正在尝试用C++创建RGB图像。我没有使用任何像OpenCv这样的图像库。

首先，我尝试创建灰度图像。我想画一个矩形的图像。我给出了起始点、宽度、高度等函数的参数。这些代码行可以很好地应对灰度挑战，但我正在尝试将颜色通道增加到3个，比如RGB。然后，我设置了红色、绿色和蓝色的值，但它不起作用。这是我的问题。

我如何才能正确工作？

• x=>x坐标的起点
• y=>y坐标的起点
• 宽度=>矩形宽度
• 高度=>矩形高度
• value=>RGB或灰度颜色值

我的代码

Image::Image(int width, int height, int n_channels, int step)
{
cout << "Image constructor is running!" << endl;
m_width = width;
m_height = height;
m_n_channels = n_channels;
m_step = m_width*m_n_channels;
if (m_step < step)
m_step = step;
m_data = new uchar[m_step*height];
}

Image* Image::new_gray(int width, int height)
{
cout << "New gray image is creating!" << endl;
return new Image(width, height, 1);
}
Image* Image::new_rgb(int width, int height)
{
cout << "New RGB image is creating!" << endl;
return new Image(width, height, 3);
} 

void Image::set_rect(int x, int y, int width, int height, uchar value)
{
if (x < 0) {
width += x;
x = 0;
}
if (y < 0) {
height += y;
y = 0;
}
for (int j = y; j < y+height; ++j) {
if (j >= m_height)
break;
uchar* row_data = data(j);
for (int i = x; i < x+width; ++i) {
if (i >= m_width)
break;
for (int c = 0; c < m_n_channels; ++c)
if (c == 0) {
row_data[i*m_n_channels + c] = value;
} else if (c == 1) {
row_data[i*m_n_channels + c] = value;
} else if (c == 2) {
row_data[i*m_n_channels + c] = value;
}
}
}
}

bool Image::write_pnm(const std::string& filename) const
{
if (m_n_channels != 1) {
const string magic_head = "P6";
ofstream fout;
string extended_name = filename + ".ppm";
fout.open(extended_name.c_str(), ios::out | ios::binary);
fout << magic_head << "n";
fout << m_width << " " << m_height << " 255n";
for (int y = 0; y < m_height; ++y) {
const uchar *row_data = data(y);
cout << reinterpret_cast<const char*>(row_data);
fout.write(reinterpret_cast<const char*>(row_data), m_width*sizeof(uchar));
}
fout.close();
return true;
}
const string magic_head = "P5";
ofstream fout;
string extended_name = filename + ".pgm";
fout.open(extended_name.c_str(), ios::out | ios::binary);
fout << magic_head << "n";
fout << m_width << " " << m_height << " 255n";
for (int y = 0; y < m_height; ++y) {
const uchar *row_data = data(y);
fout.write(reinterpret_cast<const char*>(row_data), m_width*sizeof(uchar));
}
fout.close();
return true;
}

我的主要功能

#include <cstdlib>
#include <iostream>
#include "image.h"
using std::cout;
using std::endl;
using ceng391::Image;
int main(int argc, char** argv)
{
Image* gray = Image::new_gray(128, 128);
cout << "(" << gray->w() << "x" << gray->h() << ") channels: "
<< gray->n_ch() << " step: " << gray->step() << endl;
gray->set_zero();
gray->set_rect(32, 32, 64, 64, 255);
gray->write_pnm("/tmp/test_image");

Image* rgb_image = Image::new_rgb(128,128);
cout << "(" << rgb_image->w() << "x" << rgb_image->h() << ") channels: "
<< rgb_image->n_ch() << " step: " << rgb_image->step() << endl;
rgb_image->set_zero_rgb();
rgb_image->set_rect(32, 32, 64, 64, 150);
rgb_image->write_pnm("/tmp/test_image_rgb");

delete gray;
delete rgb_image;

return EXIT_SUCCESS;
}

Answer 1

此代码适用于灰度图像，因为灰度图像具有相同数量的像素和宽度。

fout.write(reinterpret_cast<const char*>(row_data), m_width*sizeof(uchar));

但当我想保存RGB图像时，像素数量增加了3倍。一个像素通过3个通道重新表示，因此需要将流大小乘以一个像素的3个(R、G、B通道(。

fout.write(reinterpret_cast<const char*>(row_data), m_width*sizeof(uchar)*3);