using auto everywhere makes everything more difficult to read :(

[tempdeltavalue]

with specific types for vars it would be easier , it's minor but it can improve clarity

    auto focal_length = 1.0;
    auto viewport_height = 2.0;
    auto viewport_width = viewport_height * (static_cast<double>(image_width)/image_height);
    auto camera_center = point3(0, 0, 0);

    // Calculate the vectors across the horizontal and down the vertical viewport edges.
    auto viewport_u = vec3(viewport_width, 0, 0);
    auto viewport_v = vec3(0, -viewport_height, 0);

    // Calculate the horizontal and vertical delta vectors from pixel to pixel.
    auto pixel_delta_u = viewport_u / image_width;
    auto pixel_delta_v = viewport_v / image_height;

    // Calculate the location of the upper left pixel.
    auto viewport_upper_left = camera_center
                             - vec3(0, 0, focal_length) - viewport_u/2 - viewport_v/2;
    auto pixel00_loc = viewport_upper_left + 0.5 * (pixel_delta_u + pixel_delta_v);