contrast with color matrix

Question

Hi I want to implement contrast filter in my application like this link or this contrast but with color matrix and track bar for value
I already found color matrix for it

float c = mytrackbar.value * 0.01f //maxTrackbarValue = 100, minTrackbarValue = -100 
float t = 0.01f;
cmPicture = new ColorMatrix(new float[][] {
    new float[] {c,0,0,0,0},
    new float[] {0,c,0,0,0},
    new float[] {0,0,c,0,0},
    new float[] {0,0,0,1,0},
    new float[] {t,t,t,0,1}
});

but the result is very different. I try to change 0.01f in ~c~ and 0.01f in ~t~ value but it only gives result like brightness (ex : c = mytrackbar.value * 0.04f )

I wonder what ~c~ & ~t~ value and how many max and min range i should used for created contrast

---

Update @Nico

    private void myTrackBar_ValueChanged(object sender, EventArgs e) {
        imageHandle = imageHandleTemp.Bitmap;
        myNumericUpDown.Text = myTrackBar.Value.ToString();
        float value = myTrackBar.Value * 0.01f;

        Bitmap bmpInverted = new Bitmap(imageHandle.Width, imageHandle.Height);
        ImageAttributes ia = new ImageAttributes();
        ColorMatrix cmPicture = new ColorMatrix();

        float c = value;
        float t = 0.01f;
        cmPicture = new ColorMatrix(new float[][] {
            new float[] {c,0,0,0,0},
            new float[] {0,c,0,0,0},
            new float[] {0,0,c,0,0},
            new float[] {0,0,0,1,0},
            new float[] {t,t,t,0,1}
        });

        ia.SetColorMatrix(cmPicture);
        Graphics g = Graphics.FromImage(bmpInverted);
        g.DrawImage(imageHandle, new Rectangle(0, 0, imageHandle.Width, imageHandle.Height), 0, 0, imageHandle.Width, imageHandle.Height, GraphicsUnit.Pixel, ia);
        g.Dispose();

        Image<Bgr, Byte> myImage = new Image<Bgr, byte>(bmpInverted);
        imageBoxCamera.Image = myImage;
    }

Answer 1

The t value must be derived from the new contrast value c. so change its assignment like this:

float t = (1.0f - c) / 2.0f;

According to this nice link to the ColorMatrix Guide the rest of the matrix code seems to be OK.

Note: I was wrong about the range of c!! The value of c is not an absolute value but it is the factor that should be applied! So to double the contrast it should be 2f.

Note 2: Your code is not quite clear about source and target; and since you are changing contrast on the fly with the trackbar it should be clear that until the change is applied,

• The intermediate results must always be calculated from the same original bitmap.

• Also, that the trackbar should be so intialized that it starts with a value of 1.

• And finally, in order to reduce contrast values should translate to 0 < c < 1.

Nico's suggestion c = 1+ value; would work nicely with your original range -100 to +100 with an intial value of 0 and your factor 0.01f.