The scaling in the image is all wrong, but I made everything the way I did to make my point clear. What is important in this example is that I have taken the HDR background image and made a grayscale mask from it. I then spherically mapped this mask image onto a sphere that surrounds the orange box inside it. This mask is applied to a mix material so that the sphere is part transparent, part opaque. Inside the sphere, the orange box is kind of hard to see, but you get the point...Box inside a sphere with holes in the sphere created from a mask, with the mask coming from the original HDR image.
Now here is the frustrating part. Since the sphere is now acting as a shadow maker, partially blocking light from the HDR image, this sphere & mask can be used to cast shadows onto the box...IF ONLY THE SPHERE COULD HAVE AN "UNSEEN BY CAMERA" OPTION, AND AN "UNSEEN IN REFLECTIONS" OPTION. What is cool about this possibility, is that without creating extra geometry to cast shadows, we can let the HDR image provide that information for us. It is not necessarily accurate, unless the box is centered around the world origin, but that point is moot. What is important is that a faithful set of cast shadows can be realistically cast onto the box with minimal work.
This means that HDR images can now be made to cast shadows realistically, though not necessarily precise. But again, that point is moot. What is imnportant is the overall effect. If the box is NOT centered about the world origin, then parallax error between the HDRI and the mask may completely block the direct rays from the Sun. But that is not really an issue when we can rotate the sphere to get the best possible alignment with the Sun and the best looking cast shadows.
Now, if we can ALSO tell Octane to let the holes in the mask be considered as pathways for direct light, we can set the exposure of the light that passes through it. In addition, we can set the sphere opacity to less than "1" to control the darkness in the shadows.
THIS DOES NOT AFFECT THE GENERAL EXPOSURE COMING FROM THE HDRI. It only allows us to add EXTRA exposure control for light passing through the holes in the mask.
Also, the scale of the shadows and their sharpness are easily controlled by the radius of the sphere relative to the box's size.
This means that we can fairly easily, perhaps with some work in Photoshop, stop direct light in the HDRI, like from the Sun, from casting into those areas of the scene that the HDRI is telling us are in shadow. Unfortunately, this method cannot prevent the HDRI Sun from appearing in glossy surfaces. But that is another issue for thought.
Since the "Unseen by Camera/Unseen in Reflections" options are critical here, this whole process only affects the diffuse properties of the box material. So this is all really acting as, I guess you could say, an HDRI occlusion mask.
The ONLY thing preventing this from happening in Octane right now, is that it does not give us the option of making a mesh unseen to the camera, and unseen in reflections. That to me makes this a vital addition to Octane, because the benefits are great for such a simple feature addition.
