# How to use an "eraser" on a colorBuffer

**URL:** <https://openrndr.discourse.group/t/how-to-use-an-eraser-on-a-colorbuffer/700>\
**Category:** Tutorials\
**Created:** [November 9, 2024, 3:16pm UTC](https://openrndr.discourse.group/t/how-to-use-an-eraser-on-a-colorbuffer/700 "2024-11-09T15:16:59Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![abe](https://yyz2.discourse-cdn.com/free1/user_avatar/openrndr.discourse.group/abe/32/700_2.png) [@abe](https://openrndr.discourse.group/u/abe)\
**Post date:** [November 9, 2024, 3:16pm UTC](https://openrndr.discourse.group/t/how-to-use-an-eraser-on-a-colorbuffer/700/1 "2024-11-09T15:16:59Z")

</div>

I find interesting that adding elements to a canvas is easier than removing them, be it on a piece of paper, with oil paint on a canvas, or with a `colorBuffer`.

Now that I think about it, we do have a `drawer` in OPENRNDR, which is used to draw things. But where is our `eraser`?

Today I needed to erase some pixels so I thought I would share here the approach I used.

 ![Erasing a pentagon...](https://global.discourse-cdn.com/free1/uploads/openrndr/original/1X/ec7b38987ba83298f8a37fa4649b01e37d8545cf.png)

> **Imports**
>
> ```kotlin
> import org.openrndr.application
> import org.openrndr.color.ColorRGBa
> import org.openrndr.draw.BlendMode
> import org.openrndr.draw.colorBuffer
> import org.openrndr.drawImage
> import org.openrndr.extensions.Screenshots
> import org.openrndr.extra.fx.patterns.Checkers
> import org.openrndr.extra.shapes.primitives.regularPolygon
> import org.openrndr.shape.Circle
> 
> ```

```kotlin
fun main() = application {
    program {
        // A background checkers pattern
        val checkers = colorBuffer(width, height)
        Checkers().apply(checkers, checkers)

        val img = drawImage(width, height) {
            // Draw a circle
            contour(Circle(bounds.center, 200.0).contour)

            // Erase a pentagon
            drawStyle.blendMode = BlendMode.REPLACE
            fill = ColorRGBa.TRANSPARENT
            contour(regularPolygon(5, bounds.center, 100.0, 5.0).contour)
        }
        extend {
            drawer.image(checkers)
            drawer.image(img)
        }
    }
}

```

The program starts by creating a checkers pattern to use as a background. It will help notice the transparency.

Next we use `drawImage` to create an image with transparent background (its default), then we draw a circle into it, and erase a pentagon. The erasing trick is to use the `REPLACE` blend mode, together with a transparent fill color.

I can’t remember if `drawImage` is available with the `main` branch of `openrndr-template`. If not, you can switch to the `next-version` branch. This method makes it easy to draw an image (instead of loading it from the disk).

### Note 1

One shouldn’t use `drawImage` inside `extend`: it’s a one-time thing. If you need to update the drawing on every frame use a `renderTarget` instead.

### Note 2

```
contour(Circle(bounds.center, 200.0).contour)

```

can be written shorter, as

```
circle(bounds.center, 200.0)

```

I chose the longer one so both shapes are drawn the same way, with `drawer.contour`.

### Note 3

This example assumes we actually want a `colorBuffer` (a bitmap). We could instead create a `Shape` with an outer `ShapeContour` (the circle) and an inner one (the pentagon). That way it would be all vector data, which is better for drawing at different sizes, for pen plotters, or for querying contour locations and vertices, or for adjusting those contours programmatically.

---

<div class="post-metadata">

**Author:** ![dedos](https://avatars.discourse-cdn.com/v4/letter/d/f07891/32.png) [@dedos](https://openrndr.discourse.group/u/dedos)\
**Post date:** [November 16, 2024, 7:04pm UTC](https://openrndr.discourse.group/t/how-to-use-an-eraser-on-a-colorbuffer/700/2 "2024-11-16T19:04:53Z")

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Cool, I was wondering if this could be achieved for a part of a project I’m working on. It’s a thing I will implement in a middle term future but I’ll be sure to keep this in mind. Thanks for your dedication and willingness to share, as always