To apply a brightness filter you simply add a fixed amount to every pixel in the image and then clamp the result to ensure it remains in the range 0 - 255. To apply a contrast filter, you determine if a pixel is lighter or darker than a threshold amount. If it's lighter, you scale the pixel's intensity up otherwise you scale it down. In code this is done by subtracting the threshold from a pixel, multiplying by the contrast factor and adding the threshold value back again. As with the brighness filter the resulting value needs to be clamped to ensure it remains in the range 0 - 255.