From LuxRender Wiki
The output tab contains settings related to output resolution and file format, tonemapping, render resuming and colour space.
This defines the resolution (in pixels) of the final image. The x and y values are multiplied by the percentage on the same line. For example, setting resolution to 800 x 600 pixels at 50% will result in a rendering of 400 x 300 pixels.
Setting a value other than zero for this parameter will make LuxRender stop rendering after the desired number of light samples per pixel has been calculated. If the value is left at zero, the rendering process will continue indefinitely.
Tonemapping is the process of converting the luminance value that LuxRender calculates for each pixel into an actual colour on the screen. Just like a photo camera on auto exposure, LuxRender will try to produce a well lit scene, regardless of the intensity of the light sources.
The burn value controls the general brightness of the rendered image. Values lower than the default value (six) result in brighter images, whereas higher values result in darker images. The algorithm that is used aims to retain detail in both bright and dark areas.
Pre scale multiplies the brightness of the image before tonemapping.
Post scale multiplies the brightness of the whole rendering uniformly. The scale of the value is logarithmic, so setting a value of ten results in the brightness value of all pixels to get twice as high.
display and write intervals
The display value indicates how frequently (in seconds) the rendering view in LuxRender is updated. Shorter intervals between updates result in slightly slower rendering as more resources need to be directed to updating the viewport.
The write value indicates how frequently (in seconds) the rendered image should be saved.
The image file format (or formats) in which the rendering will be saved are defined in this section. If multiple formats are chosen, the rendering will be saved in all of them. Available formats are .exr, .png and .tga.
If you would prefer another file format, please use your favourite image editing program to convert it.
There are two buttons in this section: The 'Write/Use FLM' button and the 'Restart/Erase' button. These two buttons control your ability to resume the rendering of a LuxRender scene at a later time. At first their operation can seem a little bit confusing because they do not operate independently of one another. For example, your ability to resume the rendering of a particular scene file depends on the setting of both buttons, not just one of them.
The details for the operation of each button is given below, followed by an example which should hopefully clarify the situation.
In the situation where you want to start a rendering, then switch off the program and resume the render from where you left off (rather than starting over), either on the same or another computer, LuxRender can save the actual state of the rendering in a file with extension .flm.
If this button is pressed, LuxRender will save the state of the render as it goes along in the .flm file, which will be saved at the same interval as the image file is being saved.
In a nutshell, this button controls your ABILITY to resume the rendering of a scene file later. If this button is not pressed then you are not saving the render as you go along and you will not have the ability to resume the render later. The next button, as you will see, dictates whether you actually WILL resume the render, or restart from the beginning when you re-render the scene.
This button tells LuxRender that if this render is stopped, and restarted in LuxRender at a later time, to erase all of the files associated with the past render and start from the very beginning (start over again) rather than resume from where the render was stopped.
Typically, you will keep this function enabled (button pressed) until you are ready to do a final render of your scene.
First a little background: When LuxRender renders a scene it creates several files to support the rendering of that scene. In order to re-render the scene from the beginning without continuing from where the previous render left off, those files need to first be deleted. This can be done by hand or, with the Restart/Erase button pressed, it happens automatically.
Therefore, in the early stages of developing a scene in LuxRender, it is best to keep this button activated because it makes it easy when testing materials, render settings, object placement and so on, because in that situation you will want to re-render the scene many times over -- restarting the render from the beginning each time. Having this button pressed makes this simpler because you don't have to manually delete the files that LuxRender generates with each test render each time.
Once you are ready to initiate a final render of your scene, you can now deselect this button because it is more likely that you will want to resume the render than start it over again from the beginning.
NOTE: If you discover after starting a render that you forgot to turn off "Restart/Erase" button but you wish to resume the render later, you can edit the .lxs file in a text editor like Notepad to change the following flag from this:
bool restart_resume_flm" ["true"]
bool restart_resume_flm" ["false"]
Now you should be able to resume your render.
Putting It All Together
Here is how I typically use the buttons in my everyday work:
When I am first beginning the development of a scene in LuxRender I will keep the 'Write/Use FLM' button deselected and the 'Restart/Erase' selected. This is because I know that I will want to restart my render over and over from the beginning as I test and tweak different materials, lighting, etc. I also know that I will not want to resume any of these renders, and writing the .flm file repeatedly for a scene that won't need it just slows things down for no reason.
Once I get to the point where I feel that I am ready for the final render, I will activate the 'Write/Use FLM' button because at this point I know that I am more likely to want to resume the render later than restart it. I will typically NOT deselect the 'Restart/Erase' button however, because experience has shown me that once I start doing higher-resolution renders I discover artifacts that were not visible in the lower resolution renders I was doing earlier. In other words, most of the time I find that even when I think I'm going to be doing a final render -- I'm wrong and I need to fix something and restart again from the beginning.
Therefore, I typically keep the 'Restart/Erase' button selected all the time and once I actually do get a high-resolution render that looks like a keeper, at that point I will manually alter the.xls file, changing the 'bool restart_resume_flm"' statement from true to false (as detailed in the 'Restart/Erase' button section above). Thus I can now continue on and resume the rendering of this final image.
Resuming The Render
So now you that you understand how to set the buttons correctly in LuxBlend, see [here] for details on resuming a render in LuxRender.
This setting decides after which stage in the rendering process (measured in samples per pixel) new abnormally bright samples will no longer be accepted to contribute to the image. The debug button will switch off the rejection system altogether.
This sets the colour space and white point of the output image, either using a preset value or using user supplied parameters.
This sets the colour gamma of the output image.