-level <value>
    adjust the level of image contrast
 
    Give three point values delineated with commas: black, mid, and white (e.g. 10,1.0,65000). The white and black points range from 0 to MaxRGB and mid ranges from 0 to 10.  

-brightness-contrast brightness
-brightness-contrast brightness{xcontrast}{%}}
Adjust the brightness and/or contrast of the image.  
 
Brightness and Contrast values apply changes to the input image. They are not absolute settings. A brightness or contrast value of zero means no change. The range of values is -100 to +100 on each. Positive values increase the brightness or contrast and negative values decrease the brightness or contrast. To control only contrast, set the brightness=0. To control only brightness, set contrast=0 or just leave it off.
 
You may also use -channel to control which channels to apply the brightness and/or contrast change. The default is to apply the same transformation to all channels.
 
Brightness and Contrast arguments are converted to offset and slope of a linear transform and applied using -function polynomial "slope,offset".
 
The slope varies from 0 at contrast=-100 to almost vertical at contrast=+100. For brightness=0 and contrast=-100, the result are totally midgray. For brightness=0 and contrast=+100, the result will approach but not quite reach a threshold at midgray; that is the linear transformation is a very steep vertical line at mid gray.
 
Negative slopes, i.e. negating the image, are not possible with this function. All achievable slopes are zero or positive.
 
The offset varies from -0.5 at brightness=-100 to 0 at brightness=0 to +0.5 at brightness=+100. Thus, when contrast=0 and brightness=100, the result is totally white. Similarly, when contrast=0 and brightness=-100, the result is totally black.
 
As the range of values for the arguments are -100 to +100, adding the '%' symbol is no different than leaving it off.