为啥我设置的opengl视口宽度变小了，读取像素不对齐？

Posted 2023-03-05

【中文标题】为啥我设置的opengl视口宽度变小了，读取像素不对齐？

【英文标题】：Why the width of the opengl viewport I set is smaller and the reading pixels are not aligned？为什么我设置的opengl视口宽度变小了，读取像素不对齐？

【发布时间】：2021-11-09 14:54:18

【问题描述】：

代码如下（使用enter link description here的demo）渲染字形

Luint rbcolor;
    glGenRenderbuffers(1, &rbcolor);
    glBindRenderbuffer(GL_RENDERBUFFER, rbcolor);
    glRenderbufferStorage(GL_RENDERBUFFER, GL_RED, width, height);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP );
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
    glBindRenderbuffer(GL_RENDERBUFFER, 0);

    GLuint rbds;
    glGenRenderbuffers(1, &rbds);
    glBindRenderbuffer(GL_RENDERBUFFER, rbds);
    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_STENCIL, width, height);
    glBindRenderbuffer(GL_RENDERBUFFER, 0);

    GLuint fbo;
    glGenFramebuffers(1, &fbo);
    glBindFramebuffer(GL_FRAMEBUFFER, fbo);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, rbcolor);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_RENDERBUFFER, rbds);
//render
   while(...)
 ...
 glviewport(0,0,width,height);
  ...
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glReadPixels(0, 0, width, height, GL_RED, GL_UNSIGNED_BYTE, picbuf);
// render in screen 
 glBindFramebuffer(GL_FRAMEBUFFER, 0);



// Flipping the picture vertically

    uint8_t *row_swap = (uint8_t*) malloc( width );

    for ( int iy = 0; iy < height / 2; ++iy ) 
        uint8_t* row0 = picbuf + iy * width;
        uint8_t* row1 = picbuf + ( height - 1 - iy ) * width;
        memcpy( row_swap, row0, width );
        memcpy( row0, row1, width );
        memcpy( row1, row_swap, width );
    

    free( row_swap );

    // Saving the picture

    std::string png_filename = res_filename + ".png";
    if ( !stbi_write_png( png_filename.c_str(), width, height, 1, picbuf, 0) ) 
        std::cout << "Error writing png file." << std::endl;
        exit( 1 );
    

当我尝试将视口宽度设置为恰到好处的渲染文本大小时，我收到错误 malloc check header error 或获取图像未对齐 Misplaced picture，但是当我设置的宽度足够大，比如1024，图片就可以正常显示了。

【参考方案1】：

当您从 GPU 内存读取像素时，您必须设置 GL_PACK_ALIGNMENT 而不是 GL_UNPACK_ALIGNMENT（请参阅 glPixelStore）：

glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

glPixelStorei(GL_PACK_ALIGNMENT, 1);
glReadPixels(0, 0, width, height, GL_RED, GL_UNSIGNED_BYTE, picbuf);

【讨论】：

好的，谢谢！