Editing nXtImage directly and "Average Luminance"

Hello Rich, Al,

I went and opened a nxtImage file to muck with it's lighting. To my surprise it doesn't work anything like when the image is initially rendered. So wanted to bring this to your attention to find out if this is just how it is, or if these are problems in the software...

Here's an image that was taken during the rendering process...

And this is a screen capture after opening the nxtImage...

There are subtle lighting intensity differences, the names of the channels have reverted to their defaults, and the Average Luminance has gone to 0.

Below, I tweak the numbers back to how they were originally, then turn off all the lights except the HDRI background so it should be a very dark picture (only sunset light)...

As you can see, its made this a daylight picture. And it doesn't matter what "Average Luminance" is set to - it has no effect when working directly on the nxtImage file. In short, I cannot make a low-light image.

So is this how the software is supposed to behave? If so, mxtImage aren't too useful (to me) since I can't really control the lighting at all, the light intensities are reset and the channel names are lost. It means I really can't go back and "tweak" lighting. I have to re-render from scratch to really take control over the lighting again.

regards,

-randall

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Permalink Reply by Render Plus on September 7, 2012 at 7:21am

Don;'t switch to nXtImage mode just to use the lighting channels. The lighting channels icon is available directly from the rendering without switching to nXtmage. (We will need to look and see why illumination changed as you switched to nXtImage mode)

If you want to make the whole scene darker - including the sky, use the brightness slider above the rendering window.

If you want to see the sun in the background, but have it not make the bathroom brighter, use a .JPG or .PNG image of the sun as a standard background. (using the Background tab). A standard background is not used for illumination, and won't add light to the batkrooom itself.

Here I placed a JPG image as a background, rather than an HDRI sky.

Note that section plane, used to view the inside of the bathroom, also clips the ground plane, so the whole thing looks strange. But when I zoom into the room it looks better. I zoomed in, and then used the layer wizard to resize and position the background.

Here is the new rendered image:

Now, since the background image does not illuminate the scene at all, I can use the brightness slider to get the desired brightness. (If I had more than one light type, I could also use lighting channels)

If I had wanted to make the sun darker as well, I could have used the layer wizard to make the background darker:

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Permalink Reply by Randy S on September 7, 2012 at 5:43pm

Hi Al, thanx for this thorough explanation. Although in my case I do want to use HDRI because I want the faint lighting it provides.

But your post has given me an idea (using a section plane). I seem to recall reading a long time ago, that there were some idiosyncrasies to keep in mind when dealing with section planes and rendering. I'll have to do a search to see what they were.

regards,

-randall

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Permalink Reply by Render Plus on September 12, 2012 at 7:50am

Basically, we cannot determine which section plane is active in SketchUp, so we either process the first visible section plane, all visible section planes, or all section planes (whether visible or not). (An option on the nXt Setup tab).

So you have to be careful about leaving non-active section planes around which you do not want to use.

On the other hand, the ability to process multiple section planes can be handy for some scenes.