How to remove or mask sweat spots on clothing in a photograph?
Here is an example
The image isn't mine, I just grab the firs appeared to be good as an example of the topic issue.
On party photos, I would not be concerned about this... Unless someone pays you for this or you like wasting hours of retouching, this is not a required step.
@TFuto I'm absolutely agree with you. This work will not worth the effort will be needed. But the OT was regarding spot removal, not the strategy :)
The solution was pretty straightforward.
Covering the stain would work, but just using curves and applying them with a history brush in two modes, soft light and saturation removed the offending stains.
It's not particularly easy to remove entirely because where there are sweat stains there are also generally a lot of texture. I would probably try masking the area (making a layer that covers the area of the stain) and then use that mask to brighten the image up and make the stain less noticeable. I'd 'also probably feather the edges a bit to help blend it in as the wet spot tapers off.
It's still not going to be 100% perfect, but should make it less noticeable. It's also still a reasonably challenging thing to do a good job at without making either a dark ring or a light ring around the outside of the stain.
What you need is the High Frequency Separation technique. Then you will be able to edit the sweat spot on the low resolution image very easily, basically a color replacement, and keep the textures of the T-shirt. You just have to select the proper filter frequency (or pixel blur, in other words, in the described technique).
Do not forget to increase the color bit depth to 16 bit when you start, otherwise you will quickly see color banding.
The other way is just to paint over. With a bit of skill, you can do it with a mouse (... well, I can do it, so you can, too ;-), although a tablet and pen makes this easier (or harder, depending on your skills...).
+1 for the freq sep, but there's no good reason to go to 16-bit unless you are adjusting gradients (that is, doing calculations on existing pixel values). In this case, simple cloning or painting with a soft brush on the low frequency layer (or, rather, on a new layer above the LF layer but below the HF layer) will suffice, and there's no reason to worry about banding/posterizing.
You could try a brush with a little exposure compensation which will lighten up the dark patches because the moisture is only darkening those colours.
You won't get it all out without lots of work but it will make a lot of difference.
Will depend on the size and texture of the Stain, either of dodge and burn, exposure brush or layer mask might work, but we'll take efforts and precision.
In case of smaller spots, even spot healing gives decent results.
You're also talking about "exposure brush". Is it any kind of Photoshop tool? I did not find such Brush tool mode...