BufferedImage inconsistent behavior

Question

After I set a pixel of a java.awt.image.BufferedImage to a value using setRGB, a subsequent call to getRGB returns a different value than I set.

Code:

BufferedImage image = new BufferedImage(1, 1, BufferedImage.TYPE_BYTE_GRAY);
int color1 = -16711423; // corresponds to RGB(1, 1, 1)
image.setRGB(0, 0, color1);
int color2 = image.getRGB(0, 0);
System.out.println(color1);
System.out.println(color2);

It produces the following output

-16711423
-16777216

I think it has to do something with gamma correction, but I couldn't find anything about it in the documentation.

Ideally, I want to change this behavior to return the same value as I set. Is that possible?

Answer 1

The BufferedImage.getRGB() method, always returns a color (as an int in "packed format") in the non-linear sRGB color space (ColorSpace.CS_sRGB). It will do so, regardless of what color space and bits per pixel etc. your image has. Thus, conversion and possible precision loss may occur.

From the JavaDoc:

Returns an integer pixel in the default RGB color model (TYPE_INT_ARGB) and default sRGB colorspace. Color conversion takes place if this default model does not match the image ColorModel.

Your TYPE_BYTE_GRAY image internally uses a linear gray color space (ColorSpace.CS_GRAY), which does not map one-to-one with sRGB.

Also, I suggest using hexadecimal notation for (A)RGB colors, it makes the colors and difference much easier to see:

-16711423 == 0xff010101
-16777216 == 0xff000000

So, there is a minor precision loss here, but nothing unexpected.

If you want direct access to the pixel data, look into the Raster, SampleModel and DataBuffer classes (and their respective subclasses).