Java graphics2D fillRect does not work properly with semi-transparent colors

Question

I was writing a program with some custom rendering, and needed to render a rectangle with a border. I decided to simply call graphics2D.fillRect(), switch to the border color, and call graphics2D.drawRect(). However, even though I make these calls back to back with the same coordinates and sizes, fillRect() does not always fill in the entire area contained by drawRect when the color I'm drawing with is translucent (has alpha). Furthermore, the area painted by fillRect() is sometimes outside of the area contained by drawRect(). Why do these two methods draw things in different places when given different colors? Here is an example to demonstrate the problem. Clicking the mouse in the window will switch between drawing the fill with alpha and without. Notice that there is a row of pixels at the bottom of the rectangle that is white when drawing with alpha, but that row of pixels is not there when drawing without alpha. ``` import java.awt.BasicStroke; import java.awt.Color; import java.…

Accepted Answer

Scrap that answer, I reread your question and copied your code and found what your talking about. The small white line is due to a round-off error in the painting. Very interesting little problem. Add this after creating your Graphics2D

```
g2.setRenderingHint(RenderingHints.KEY_STROKE_CONTROL, RenderingHints.VALUE_STROKE_PURE);
```

Rendering hints tell the painting class how you want certain procedures to work. I have no idea why adding transparency to the color would make the roundoff different. I figure it must have to do with multiple rendering hints combining together like antialiasing.

Java graphics2D fillRect does not work properly with semi-transparent colors

Solution

Problem