C# crop image trapezoid / polygon

Question

I've got an image and I want to cut out a trapezoid from an image.

I've got 8 different coordinates like this (8 values to form a polygon with 4 Points)

pointA(222,222)
pointB(666,234)
pointC(678,235)
pointD(210,220)

I only found out how to crop images with bitmap.Clone like this

var x = pointA.x;
var y = pointA.y;
var height = pointD.y - pointA.y;
var width = pointB.x - pointA.x;
Bitmap image = new Bitmap(imagepath);
var rect = new Rectangle(x, y, wight, height);
var newImage = image.Clone(rect, image.PixelFormat);

This will create a straight rectangle and the important parts of the sub-parts wich I want to cut out are disappearing.

So, how to cut out a trapezoid using c# and .net framework core from a console environment?

I want to cut out a trapezoid but I just figured out the better form to describe what I want is a polygon.

score 2 · Accepted Answer · answered May 17 '21 at 14:46

2

One way to do it is to use the Graphics API. This has a FillPolygon method that takes a list of points and a brush. To use the source bitmap you would use the TextureBrush. Put this together and you would end up with something like this:

    public Bitmap FillPolygon(Bitmap sourceBitmap, PointF[] polygonPoints)
    {
        var targetBitmap = new Bitmap(256, 256);
        using var g = Graphics.FromImage(targetBitmap);
        using var brush = new TextureBrush(sourceBitmap);
        g.FillPolygon(brush, polygonPoints);
        return targetBitmap;
    }

I think this should work from a windows console. There have historically been a few issues using some graphics APIs without a windows sessions, but I do not think this is a problem anymore.

For maximum compatibility you could always triangulate and rasterize the trapezoid yourself and copy the selected pixels, but it would probably be significantly more work.

answered May 17 '21 at 14:46

JonasH

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Thanks for your answer - After I figured out "trapezoid" is a polygon too, I edited my question and also tried to implement the answer from this - https://stackoverflow.com/questions/30954503/how-to-crop-a-polygonal-area-from-an-image-in-a-winform-picturebox but I get only black images – derBjörn May 17 '21 at 15:03
@derbjörn check that the target bitmap is large enough, and try drawing with for example `Brushes.White` to check that the drawing works. And some objects need to be disposed at the correct place for it to work correctly. – JonasH May 17 '21 at 15:09
the target bitmap is large enough, I tried some approaches with the solution. But I need to tinker a little bit more to archive my solution. Meanwhile (and thanks again for this hint with Graphis-API) I figured out how to draw on the image - instead of "brushing" the zone out, I am able to draw a polygon on the image, using the "Pen" - This works - But even with your hint, all "transparent" or "black" images (resolution is "correct" but nothing is in the picture) – derBjörn May 17 '21 at 15:26
_I get only black images_ Please show the code you are using now! - _8 different points_ What does that mean???? The 4(!) points form more like a triangle than a trapez... - Also: How large is the source bitmap? – TaW May 17 '21 at 18:36
I was able to create a solution, thanks to @JonasH <3 – derBjörn May 17 '21 at 23:06

C# crop image trapezoid / polygon

1 Answers1