使用ImagickDraw绘制单像素线时出现奇怪的阴影/模糊

Question

So I'm trying to draw a line with Imagick today. Sounds simple enough right?

Here's the code I have:

$image = new Imagick();
$image->newImage(100, 100, 'white');
$image->setImageFormat('png');

$line_color = new ImagickPixel('#555555');
$line_weight = 1;

$line = new ImagickDraw();

$line->setResolution(100, 100);
$line->setStrokeWidth($line_weight);
$line->setStrokeColor($line_color);

$line->line(20, 20, 20, 60);

$image->drawImage($line);
$image->writeImage('test.png');

Nothing surprising, I create a 100x100 white image, I set a line color at 555555, a line weight at 1, I create my ImagickDraw object and tell him to draw a line.

The problem is that the output looks like this: http://i.stack.imgur.com/hM8QS.png

Which looks fine at first, until you zoom in and see this: http://i.stack.imgur.com/O6Yie.png

I tried several settings for the color, the weight and the resolution, but I can't get rid of this weird shadow/blur effect. It does this with pretty much anything I draw (lines, rectangle etc...) except with points. So if I want I can create my single-pixel line with a series of points and a simple for loop, but it will really get tedious to do. Especially since I'm not gonna draw just this single line.

So, anyone has any idea of what is happening? How can I get rid of this behavior?

Answer 1

The effect you are seeing is due to stroke anti-aliasing being enabled. You can disable it with setStrokeAntialias.

$line->setResolution(100, 100);
$line->setStrokeWidth($line_weight);
$line->setStrokeColor($line_color);
$line->setStrokeAntialias(false);

Reenable stroke anti-aliasing whilst drawing curved lines to avoid aliasing artifacts.

Bizarely, in my tests, this causes the line to disappear completely when the stroke color is set to #555555 (or an equivalent representation) as it is in your code sample. However, setting the color to #545454 or #565656 causes the line to be drawn correctly, without the shadow/blur effect you described.