View source for Race Condition Image Batch Assignment

=Motivation=
Race conditions can lead to difficult bugs to find and fix.  We gain experience finding them and fixing them.

=Background=
==Pixels==
The word [https://en.wikipedia.org/wiki/Pixel pixel] stands for Picture Element. A pixel is a small part of a picture. Pixels are the little dots that make up computer screens. When you see a low-resolution picture that is grainy, the grains you’re seeing are the pixels.

==Color Models==
There are many different representations of Color. These representations are called [https://en.wikipedia.org/wiki/Color_model color models].

===Subtractive Color Models===
Some color models are [https://en.wikipedia.org/wiki/Subtractive_color subtractive]. These models are useful for printing. They are ''subtractive'' because they model the subtraction of light as each color absorbs light (before what remains to bounce off into your eyes). The most common of these models is [https://en.wikipedia.org/wiki/CMYK_color_model CMYK] which represents '''C'''yan, '''M'''agenta, '''Y'''ellow, blac'''K'''. If you have ever changed the ink in a color printer, you have encountered CMYK cartridges. Truth be told, you don’t need the black. Essentially all of the visible spectrum of light would be absorbed printing layers of cyan, magenta, and yellow. A lot of black tends to be printed, so it is more cost effective to add to add black into the model.

[[File:CMYK subtractive color mixing.svg|CMYK subtractive color mixing]]

===Additive Color Models===
[[File:Component video RCA.jpg|thumb|Component_video_RCA]]

Some color models are [https://en.wikipedia.org/wiki/Additive_color additive]. These models are useful for display devices. They are ''additive'' because they model the addition of light which is thrown at your eyes. 

====RGB====
[[File:RGB color solid cube.png|thumb|RGB_color_solid_cube]]

The most common of these models is [https://en.wikipedia.org/wiki/RGB_color_model RGB] which repesents '''R'''ed, '''G'''reen, '''B'''lue. If you have ever plugged an old device into a TV with a [https://en.wikipedia.org/wiki/Component_video component video cable], that is what the separate cables are carrying: analog signal for red, green, and blue.

[[File:RGB illumination.jpg|RGB_illumination]]

====HSV====
We will use the HSV color model which represents '''H'''ue, '''S'''aturation, '''V'''alue.

[[File:HSV_color_solid_cylinder_saturation_gray.png|thumb|HSV_color_solid_cylinder_saturation_gray]]

This model benefits from often being easier for humans to think about and will enable us to create some compelling image filters with minimal effort. Evidence for it being easier for humans to think about can be found in most defaults for color pickers, including the one which can be found by searching on Google for [https://www.google.com/search?q=color+picker color picker].

Play around with the [https://www.google.com/search?q=color+picker color picker] for a while and watch the hue, saturation, and value change as you interact with it. Try to isolate the saturation by dragging left to right in the rectangle component of the color picker. Isolate changes to the value by dragging up and down. Control the hue with the slider below.

[[File:Google color picker.png|600px]]

=====Hue=====
[https://en.wikipedia.org/wiki/Hue Hue] is cyclical. Red is usually at 0.0 and the hue changes as you go around the ring… returning back to red at 1.0.
=====Saturation=====
Full [https://en.wikipedia.org/wiki/HSL_and_HSV#Saturation saturation] is a completely “vivid” form of the color. No saturation is “dull” gray.

=====Value=====
This component is somewhat uninspiringly named: “[https://en.wikipedia.org/wiki/HSL_and_HSV#Lightness value]”. It is called brightness or lightness in similar color models.  Value can be somewhat confusing since one might expect a full (1.0) "brightness" value to always equate to white, which it does not.

=Code To Investigate=
==Images of Rows of Colors==
===interface ImmutableHsvImage and class DefaultImmutableHsvImage===
{{HsvImageWithMethodLinks}}

===interface ImmutableHsvRow and class DefaultImmutableHsvRow===
{{HsvRowWithMethodLinks}}

===class HsvColor===
{{HsvColorWithMethodLinks}}

==SequentialImageFilter==
{{CollapsibleCode|SequentialImageFilter|<syntaxhighlight lang="java">
public class SequentialImageFilter implements ImageFilter {
	@Override
	public ImmutableHsvImage apply(ImmutableHsvImage src, PixelFilter pixelFilter) {
		List<ImmutableHsvRow> dstRows = new ArrayList<>(src.rowCount());
		for (ImmutableHsvRow srcRow : src.rows()) {
			List<HsvColor> dstRowPixels = new ArrayList<>(srcRow.columnCount());
			for (HsvColor srcPixel : srcRow.columnColors()) {
				HsvColor dstPixel = pixelFilter.apply(srcPixel);
				dstRowPixels.add(dstPixel);
			}
			dstRows.add(new DefaultImmutableHsvRow(dstRowPixels));
		}
		return new DefaultImmutableHsvImage(dstRows);
	}
}</syntaxhighlight>}}

==DesaturateAllPixelFilter==
{{CollapsibleCode|class DesaturateAllPixelFilter|<pre>public class DesaturateAllPixelFilter implements PixelFilter {
	@Override
	public HsvColor apply(HsvColor src) {
		return new HsvColor(src.hue(), 0.0, src.value());
	}
}</pre>}}

==DesaturateNonSkinTonePixelFilter==
{{CollapsibleCode|class DesaturateNonSkinTonePixelFilter|<pre>public class DesaturateNonSkinTonePixelFilter implements PixelFilter {
	private static boolean isInRange(double min, double x, double max) {
		return min < x && x < max;
	}
	private static boolean isSkin(HsvColor src) {
		return (src.hue() < 0.1 || src.hue() > 0.9) && isInRange(0.1, src.saturation(), 0.7);
	}
	@Override
	public HsvColor apply(HsvColor src) {
		return new HsvColor(src.hue(), isSkin(src) ? src.saturation() : 0.0, src.value());
	}
}</pre>}}

==SequentialImageBatch==
<syntaxhighlight lang="java">
public class SequentialImageBatch implements ImageBatch {
	@Override
	public ImmutableHsvImage[] filterAllImages(ImmutableHsvImage[] images, ImageFilter imageFilter,
			PixelFilter pixelFilter) throws InterruptedException, ExecutionException {
		ImmutableHsvImage[] filteredImages = new ImmutableHsvImage[images.length];
		for (int i = 0; i < images.length; ++i) {
			filteredImages[i] = imageFilter.apply(images[i], pixelFilter);
		}
		return filteredImages;
	}
}
</syntaxhighlight>

=Code To Debug=
==ImageBatch==
===filterAllImages===
{{CodeToDebug|ParallelImageBatch|filterAllImages|racecondition.image.exercise}}
{{Parallel|ImmutableHsvImage[] filterAllImages(ImmutableHsvImage[] images, ImageFilter imageFilter, PixelFilter pixelFilter)}}

If one had a filter that could be applied to an image independently, it is reasonable to apply the filter to each image in an array of images in parallel.  Unfortunately, the code you must debug has a data race.  Fix the data race in ParallelImageBatch's filterAllImages method.

<syntaxhighlight lang="java">
public class ParallelImageBatch implements ImageBatch {
	@Override
	public ImmutableHsvImage[] filterAllImages(ImmutableHsvImage[] images, ImageFilter imageFilter,
			PixelFilter pixelFilter) throws InterruptedException, ExecutionException {

		@SuppressWarnings("unchecked")
		Future<Void>[] futures = new Future[images.length];
		ImmutableHsvImage[] filteredImages = new ImmutableHsvImage[images.length];
		int[] array = { 0 };
		for (array[0] = 0; array[0] < images.length; ++array[0]) {
			futures[array[0]] = void_fork(() -> {
				filteredImages[array[0]] = imageFilter.apply(images[array[0]], pixelFilter);
			});
		}
		join(futures);
		return filteredImages;

	}
}
</syntaxhighlight>

=Client=

{{Viz|ImageBatchApp|racecondition.image.viz|main}}

=Testing Your Solution=
==Correctness==
{{TestSuite|_ImageBatchTestSuite|racecondition.image.exercise}}

=Pledge, Acknowledgments, Citations=
{{Pledge|race-condition-image-batch}}