For RGB(A) images that contain gray colors, this reduction can achieve
significant space savings. However, in the absence of gamma correction
data, some PNG decoders assume more exotic color spaces for grayscale
images instead of the ubiquitous sRGB. This results in gamma
miscorrection, and for the end user this means that colors will look
wrong, like "washed-out". Java's ImageIO class, which is popular in the
JVM world to read PNG files, uses rather unconventional defaults, as
explained in this StackOverflow question: https://stackoverflow.com/questions/31312645/java-imageio-grayscale-png-issue
Gamma miscorrection problems aside, OxiPNG currently tries hard to
reduce RGB(A) images to grayscale, because it expects that reduction to
be quite effective. However, in some cases, OxiPNG generates smaller
PNG files when reducing grasycale RGB(A) images to paletted color than
actual grayscale color. For example, let's say that "~/gray.png" is a
256x256 RGBA image entirely filled with (119, 119, 119, 255) pixels.
OxiPNG, by default, reduces this image to grayscale and achieves a
68.23% decrease:
$ cargo build --release && target/release/oxipng -omax --out ~/out.png ~/gray.png
Processing: /home/user/gray.png
256x256 pixels, PNG format
4x8 bits/pixel, RGBA
IDAT size = 604 bytes
File size = 661 bytes
Reducing image to 1x4 bits/pixel, Grayscale
Trying: 144 combinations
Found better combination:
zc = 6 zs = 0 f = 0 153 bytes
IDAT size = 153 bytes (451 bytes decrease)
file size = 210 bytes (451 bytes = 68.23% decrease)
Output: /home/user/out.png
However, if the --ng option that this commit adds is used to skip the
grayscale reduction step, OxiPNG reduces to a single color palette
instead, which is much more efficient, achieving a 84.42% decrease:
$ cargo build --release && target/release/oxipng -omax --ng --out ~/out.png ~/gray.png
Processing: /home/alejandro/gray.png
256x256 pixels, PNG format
4x8 bits/pixel, RGBA
IDAT size = 604 bytes
File size = 661 bytes
Reducing image to 1 bits/pixel, 1 colors in palette
Trying: 144 combinations
Found better combination:
zc = 3 zs = 3 f = 0 31 bytes
IDAT size = 31 bytes (573 bytes decrease)
file size = 103 bytes (558 bytes = 84.42% decrease)
Output: /home/alejandro/out.png
While OxiPNG should arguably be made smarter to better handle these
cases, in the meantime, adding an option to manually skip that grayscale
reduction can't hurt. In fact, it may even help users achieving the most
out of current versions of OxiPNG, and developers reasoning about what
makes a grayscale-like RGB(A) image compress better with a color
palette.
Due to the reasons stated above, this adds a simple "grayscale_reduction"
option to the Options struct, and a "no-grayscale-reduction" command
line switch, that makes OxiPNG not try this problematic grayscale
reduction on RGB(A) images.
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