Field Mask value for UpdateImageProperties in Google Slides C#

How do I find out the value for file mask. The property that I am trying to set is brightness I have tried different values but I am unable to figure out. I have tried brightness, imageproperties and nothing works. I am using Google Slides library in .net. Any documentation will be helpfull.

{
      "objectId": "gf9f4feaadd_0_51",
      "size": {
        "width": {
          "magnitude": 21600,
          "unit": "EMU"
        },
        "height": {
          "magnitude": 21600,
          "unit": "EMU"
        }
      },
      "transform": {
        "scaleX": 11.4444,
        "scaleY": 11.4444,
        "translateX": 752160.96500000008,
        "translateY": 3479732.17,
        "unit": "EMU"
      },
      "image": {
        "contentUrl": "https://lh4.googleusercontent.com/hBqJYd2o2hkwRHvHqMTqgSkut0SaC3ii_hztvrq7ogftyaOUX",
        "imageProperties": {
          "brightness": -0.25,
          "contrast": 0.63,
          "recolor": {
            "recolorStops": [
              {
                "color": {
                  "rgbColor": {}
                },
                "alpha": 1
              },
            ],
            "name": "LIGHT4"
          },
        }
      }
    }

Solution

You cannot modify the image's brightness using slides api.

As stated in ImageProperties, the brightness field is a read-only property. (Check other properties that are read-only)