c++ - 翻转 glReadPixels 的 RGBA 输出(C++ OpenGL)

Question

我想翻转 glReadPixel 的输出图像。我通过将 glReadPixels 保存到文件来检查输出。该图像是正确的，但它是颠倒的。

我尝试过的

int patchSize = 50;
std::vector<unsigned char> rgbdata(4*patchSize*patchSize);
glReadPixels(x-(patchSize/2), y-(patchSize/2)/2), patchSize,patchSize,GL_RGBA,GL_UNSIGNED_BYTE, &rgbdata[0]);

std::vector< unsigned char > temp_rgbdata = rgbdata; // Create a copy of the data
    rgbdata.clear(); // Reset the array
    for (int i=patchSize-1; i >= 0; i--) // Count backwards in order to flip
    {
        for( int j = 0; j < patchSize; j++) {

            rgbdata.push_back(temp_rgbdata[i*patchSize+j*4]);
            rgbdata.push_back(temp_rgbdata[i*patchSize+j*4+1]);
            rgbdata.push_back(temp_rgbdata[i*patchSize+j*4+2]);
            rgbdata.push_back(temp_rgbdata[i*patchSize+j*4+3]);

        }
    }
    temp_rgbdata.clear(); // Clear the temporary array

怎么了

图像结果全错了:

感谢任何帮助和评论。谢谢。

解决方案(鲍里斯)

int patchSize = 50;
std::vector<unsigned char> rgbdata(4*patchSize*patchSize);
glReadPixels(x-(patchSize/2), y-(patchSize/2)), patchSize,patchSize,GL_RGBA,GL_UNSIGNED_BYTE, &rgbdata[0]);

std::vector< unsigned char > temp_rgbdata = rgbdata; // Create a copy of the data
    rgbdata.clear(); // Reset the array
    for (int i=patchSize-1; i >= 0; i--) // Count backwards in order to flip
    {
        for( int j = 0; j < patchSize; j++) {

            rgbdata.push_back(temp_rgbdata[(i*patchSize+j)*4]);
            rgbdata.push_back(temp_rgbdata[(i*patchSize+j)*4+1]);
            rgbdata.push_back(temp_rgbdata[(i*patchSize+j)*4+2]);
            rgbdata.push_back(temp_rgbdata[(i*patchSize+j)*4+3]);

        }
    }
    temp_rgbdata.clear(); // Clear the temporary array

Answer 1

这是一个完整的例子，建立在 Stypox 的精彩片段之上——这也是实际的写作。当然，交换坐标系统是另一种解决方法但我发现左上角的 0,0 更自然。

// https://github.com/nothings/stb/blob/master/stb_image_write.h
#include "stb_image_write.h"

glPixelStorei(GL_PACK_ALIGNMENT, 1);
glReadBuffer(GL_BACK_LEFT);

std::vector<uint8_t> pixels(3 * w * h);
glReadPixels(0, 0, w, h, GL_RGB, GL_UNSIGNED_BYTE, pixels.data());

for(int line = 0; line != h/2; ++line) {
    std::swap_ranges(pixels.begin() + 3 * w * line,
                     pixels.begin() + 3 * w * (line+1),
                     pixels.begin() + 3 * w * (h-line-1));
}

int components = 3;
stbi_write_png("out.png", w, h, components, pixels.data(), 3 * w);
stbi_write_tga("out.tga", w, h, components, pixels.data());