The Maxwell Render plug-in for Photoshop provides a couple of useful actions:
Maxwell Layer Adjustments
The former makes easy the task of applying the alpha channel to a given image.
The latter allows you to easily make intensity and color adjustments to the lights of your renders, similar to the Multilight adjustments in Maxwell Render. You only need to launch your renders with the Intensity Multilight option enabled (Color Multilight is not needed here, as the color adjustments will be produced inside Photoshop).
Loading the actions
Open Photoshop, and in the Actions palette menu, select “Load Actions” and browse in the folder where you have the “Maxwell Actions.atn” file. A new action group called “Maxwell Actions” will appear with the “Apply Alpha” and “Maxwell Layer Adjustments” actions inside. Next time you open Photoshop, the new actions will already be loaded in the Actions palette and ready to use.
Windows: The PS action is installed in the following folder (Windows 7 or 10): "C:\Users\username\Documents\Maxwell\Maxwell Plug-in for Photoshop".
Mac OSX: The action is included into the plug-in compressed package. Just unpack it.
Using the Apply Alpha action
For this action to work correctly, you need to open the render and the alpha channel as two different images; the first document has to be the render and the second the alpha channel.
Just select the alpha channel document, select the action and click play (or tap the F12 key if you accepted the shortcut when installing).
The alpha channel image will be used in the other document to cut out the background and the typical black rim at the edge of the opaque part will be removed automatically.
This can be useful too to apply the alpha channel to the denoised image (which is not currently embedded automatically).
Apply Alpha action workflow
Using the Maxwell Layer Adjustments action
After loading your MXI file, you will get one layer for each separate emitter in your render, plus any extra channels you may have. To mimic the Multilight slider feature, do the following for each emitter layer in PS:
Select the emitter layer.
Go to the Actions palette and in the "Maxwell Actions" group, select the "Maxwell Layer Adjustments" action and play it.
This will create two adjustment layers for the emitter layer you had selected. They are named "Color" and "Intensity". These two, in fact, use the adjustment layers found in PS which are "Hue/Saturation" and "Exposure".
Select the emitter layer again and set its blending mode to "Linear Dodge (Add)" mode. Please note that this is valid if you imported the MXI as a 32bit image (highly recommended). If you instead chose 16bit, it's better to use the "Screen" or “Lighten” blending mode.
Now you can move the Exposure slider up and down on the Intensity adjustment layer to increase or reduce your emitter intensity. Move the Hue, Saturation sliders up and down on the Color adjustment layer to change your emitter color.
As Adobe Photoshop cannot manage the spectral information from Maxwell Render images, the Color adjustment layer can only mimic the high precision spectral changes that Maxwell Render performs, so this poses some limitations. The Color adjustment action performs a hue-saturation change, so it is not possible to change the color of a pure white emitter. In this case (and in general with pure white, grey or black desaturated emitters), we strongly recommend replacing the “Color” adjustment layer with a more complex “Channel Mixer” adjustment layer.
The Maxwell Action creates two adjustment layers to allow you to adjust the intensity or color of each emitter separately. Change the Exposure handle to adjust the intensity.
Use the Hue slider to adjust the emitter color.
There is a limitation in CS3 to CS6 in the Photoshop Standard versions (not present in the Extended versions), which means that the Adjustment Layers cannot be created in 32-bits mode. Because this action works in 32-bits and uses Adjustment Layers to allow non-destructive adjustments, it won’t work on the Standard versions of Photoshop.
If you are using the Standard version of Photoshop, you can work around this limitation by converting your render to 16-bits (without merging) to be able to use the Action, as Photoshop is able to create Adjustment Layers in 16-bits. In 16-bits you will get a better result by setting your light layers to “Screen” or “Lighten” blending modes, instead of “Linead Dodge (Add)”. Obviously, converting a 32-bits render to 16-bits will produce a reduction of the tonal range of the render.