crop image to 4:3 aspect ratio c#

Question

I'm having a tough time wrapping my head around how to figure out the math for cropping any image that has a higher aspect ratio than 4:3 to 4:3.

For example, I may have some images that are 16:9 that I I want resized and then cropped to 4:3.

The resize bit I already have working, but it's maintaining the same aspect ratio. I know I need to use Graphics.DrawImage() But I'm not entirely sure what the parameters should be nor how I derive those parameters.

Here's what I do know:

var dimension = (double)bigSide/smallSide
if(dimension > 1.4)
{
  Graphics.DrawImage(resImage, new Rectangle(?, ?, ?, ?), ?, ?, ?, ?, GraphicsUnit.Pixel);
}

so all of those question marks are parameters that I do not understand. I also am not sure what the math would need to look like for cutting down the image to 4:3.

Essentially I just want to cut the sides off of an image (center it) that is wider than a 4:3 aspect. Obviously I'd cut the top and bottom of an image that is portrait instead of landscape.

Any help would be greatly appreciated.

TIA

Answer 1

I saw you've commented that you want also to resize the cropped image to a smaller size right?

Image resizeImg(Image img, int width)
    {
        // 4:3 Aspect Ratio. You can also add it as parameters
        double aspectRatio_X = 4;
        double aspectRatio_Y = 3;                        
        double targetHeight = Convert.ToDouble(width) / (aspectRatio_X / aspectRatio_Y);

        img = cropImg(img);
        Bitmap bmp = new Bitmap(width, (int)targetHeight);
        Graphics grp = Graphics.FromImage(bmp);
        grp.DrawImage(img, new Rectangle(0, 0, bmp.Width, bmp.Height), new Rectangle(0, 0, img.Width, img.Height), GraphicsUnit.Pixel);
        return (Image)bmp;

    }

    Image cropImg(Image img)
    {
        // 4:3 Aspect Ratio. You can also add it as parameters
        double aspectRatio_X = 4;
        double aspectRatio_Y = 3;

        double imgWidth = Convert.ToDouble(img.Width);
        double imgHeight = Convert.ToDouble(img.Height);

        if (imgWidth / imgHeight > (aspectRatio_X / aspectRatio_Y))
        {
            double extraWidth = imgWidth - (imgHeight * (aspectRatio_X / aspectRatio_Y));
            double cropStartFrom = extraWidth / 2;
            Bitmap bmp = new Bitmap((int)(img.Width - extraWidth), img.Height);
            Graphics grp = Graphics.FromImage(bmp);                                                
            grp.DrawImage(img, new Rectangle(0, 0, (int)(img.Width - extraWidth), img.Height), new Rectangle((int)cropStartFrom, 0, (int)(imgWidth - extraWidth), img.Height), GraphicsUnit.Pixel);
            return (Image)bmp;
        }
        else
            return null;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        pictureBox2.Image = resizeImg(pictureBox1.Image, 60);
    }

Use the resize method and provide width as parameter. No need to add height because it is done by the crop method.

Answer 2

Here's a method for cropping a wider image, so that you can understand the concept.

First calculate the extra width of the image. i.e how much additional space it is taking more than a 4:3 ratio of the same.

Consider I want to crop a 1366 x 768 picture to 1024 x 768.

Here we can calculate the extra_width with the height (768px):

4 / 3 * (768) = 1024

So, that gives you the target width for a 768 height.

Now the extra width is:

1366 - 1024

Now you can crop the image by putting the starting crop point to 1/2 of the extra_width, and select the full_height.

  Image cropImg(Image img)
    {
        // 4:3 Aspect Ratio. You can also add it as parameters
        double aspectRatio_X = 4;
        double aspectRatio_Y = 3;

        double imgWidth = Convert.ToDouble(img.Width);
        double imgHeight = Convert.ToDouble(img.Height);

        if (imgWidth / imgHeight > (aspectRatio_X / aspectRatio_Y))
        {
            double extraWidth = imgWidth - (imgHeight * (aspectRatio_X / aspectRatio_Y));
            double cropStartFrom = extraWidth / 2;
            Bitmap bmp = new Bitmap((int)(img.Width - extraWidth), img.Height);
            Graphics grp = Graphics.FromImage(bmp);                                                
            grp.DrawImage(img, new Rectangle(0, 0, (int)(img.Width - extraWidth), img.Height), new Rectangle((int)cropStartFrom, 0, (int)(imgWidth - extraWidth), img.Height), GraphicsUnit.Pixel);
            return (Image)bmp;
        }
        else
            return null;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        pictureBox2.Image = cropImg(pictureBox1.Image);
    }

Answer 3

Answers are here and here

Rectangle sourceRectangle = new Rectangle(0, 0, 1600, 900);

Rectangle destinationRectangle = new Rectangle(0, 0, 800, 600);

This code crops image part with required aspect ratio, starting from 0,0. You may set x-coordinate and y-coordinate as you like, calculating it in relation to image center. For example if your image is 1600 x 900 centre point is 800 and 450 so to crop it's center

Rectangle destinationRectangle = new Rectangle(400, 75, 800, 600);

This is somewhat simplified but I hope that you'll catch the point.