Reduced grayscale png file size is larger than original png

[fangqiao]

I am testing libvips' resize API and found out it's really convenient. Great job.

But I found out when png is grayscale, that exported png file with smaller dimension is acutally larger than the original. When it is colorful, the reduced png size is proportional.

Why is it? Is there anyway to tune the code so png file with smalled dimensions is always smaller in term of file size?

Below is the code I used to resize then crop:

in = in.resize(scale).smartcrop(w, h, VImage::option()->
                                            set( "interesting", VIPS_INTERESTING_ATTENTION));

And to export to png:

                in.pngsave( (char*) output.c_str(), VImage::option()
                            ->set("compression", 9)
                            ->set("strip", true)
                    );

Original png, width: 1024

Resized, width: 900

Thanks

[lovell]

This question is asked regularly of sharp - see lovell/sharp#478 (and the linked issues).

[fangqiao]

I used imagick " identiy -verbose " to inspect the two png files.

You can find out that the two have exactly the same colorspace, channels and depth .
I suspect it is the compression engine that caused the difference. Maybe libvips can use a more powerful though costly algorithm?

1. Output for original :

Format: PNG (Portable Network Graphics)
Mime type: image/png
Class: DirectClass
Geometry: 1024x369+0+0
Units: Undefined
Colorspace: sRGB
Type: Bilevel
Base type: Undefined
Endianess: Undefined
Depth: 8-bit
Channel depth:
Red: 1-bit
Green: 1-bit
Blue: 1-bit
Alpha: 8-bit
Channel statistics:
Pixels: 377856
Red:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: 0 (0)
kurtosis: -3
skewness: 0
entropy: 0
Green:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: 0 (0)
kurtosis: -3
skewness: 0
entropy: 0
Blue:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: 0 (0)
kurtosis: -3
skewness: 0
entropy: 0
Alpha:
min: 0 (0)
max: 255 (1)
mean: 61.9682 (0.243013)
standard deviation: 109.039 (0.427606)
kurtosis: -0.557646
skewness: 1.19833
entropy: 0.114823
Image statistics:
Overall:
min: 0 (0)
max: 255 (1)
mean: 15.492 (0.0607531)
standard deviation: 27.2599 (0.106901)
kurtosis: 11.541
skewness: 3.67757
entropy: 0.0287057
Alpha: none #00000000
Colors: 256
Histogram:
284536: ( 0, 0, 0, 0) #00000000 none
90330: ( 0, 0, 0,255) #000000FF black
51: ( 0, 0, 0, 1) #1 srgba(0,0,0,0.00392157)
47: ( 0, 0, 0,254) #000000FE srgba(0,0,0,0.996078)
46: ( 0, 0, 0,253) #000000FD srgba(0,0,0,0.992157)
40: ( 0, 0, 0,252) #000000FC srgba(0,0,0,0.988235)
37: ( 0, 0, 0, 3) #3 srgba(0,0,0,0.0117647)
36: ( 0, 0, 0, 2) #2 srgba(0,0,0,0.00784314)
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Rendering intent: Perceptual
Gamma: 0.454545
Chromaticity:
red primary: (0.64,0.33)
green primary: (0.3,0.6)
blue primary: (0.15,0.06)
white point: (0.3127,0.329)
Matte color: grey74
Background color: white
Border color: srgb(223,223,223)
Transparent color: none
Interlace: None
Intensity: Undefined
Compose: Over
Page geometry: 1024x369+0+0
Dispose: Undefined
Iterations: 0
Compression: Zip
Orientation: Undefined
Properties:
date:create: 2018-04-11T18:20:33+08:00
date:modify: 2018-04-11T18:20:31+08:00
png:bKGD: chunk was found (see Background color, above)
png:IHDR.bit-depth-orig: 8
png:IHDR.bit_depth: 8
png:IHDR.color-type-orig: 6
png:IHDR.color_type: 6 (RGBA)
png:IHDR.interlace_method: 0 (Not interlaced)
png:IHDR.width,height: 1024, 369
png:sRGB: intent=0 (Perceptual Intent)
signature: 7e60fb3a7e217e94451ae496216a9d5666726f77fba2fbbc6f5a0add35464b20
Artifacts:
verbose: true
Tainted: False
Filesize: 12232B
Number pixels: 377856
User time: 0.010u
Elapsed time: 0:01.000
Version: ImageMagick 7.0.7-27 Q16 x86_64 2018-03-23 http://www.imagemagick.org

Output for resized:

Image: nike-logo.png.resized.png
Format: PNG (Portable Network Graphics)
Mime type: image/png
Class: DirectClass
Geometry: 900x324+0+0
Resolution: 28.34x28.34
Print size: 31.7572x11.4326
Units: PixelsPerCentimeter
Colorspace: sRGB
Type: Bilevel
Base type: Undefined
Endianess: Undefined
Depth: 8-bit
Channel depth:
Red: 1-bit
Green: 1-bit
Blue: 1-bit
Alpha: 8-bit
Channel statistics:
Pixels: 291600
Red:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: 0 (0)
kurtosis: -3
skewness: 0
entropy: 0
Green:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: 0 (0)
kurtosis: -3
skewness: 0
entropy: 0
Blue:
min: 0 (0)
max: 0 (0)
mean: 0 (0)
standard deviation: 0 (0)
kurtosis: -3
skewness: 0
entropy: 0
Alpha:
min: 0 (0)
max: 255 (1)
mean: 62.0286 (0.243249)
standard deviation: 108.895 (0.427041)
kurtosis: -0.558805
skewness: 1.19679
entropy: 0.13606
Image statistics:
Overall:
min: 0 (0)
max: 255 (1)
mean: 15.5072 (0.0608124)
standard deviation: 27.2239 (0.10676)
kurtosis: 11.5311
skewness: 3.67533
entropy: 0.034015
Alpha: none #00000000
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[jcupitt]

Another factor is the shape of the edge. Your original has very sharp edges and will be easy to compress well -- after lanczo3 they will be smoother and have a bit of ringing. You could try using bilinear instead.

[jcupitt]

I see:

john@yingna ~/pics $ vips resize nike.png x.png[compression=9] 0.9 --kernel linear
john@yingna ~/pics $ ls -l x.png 
-rw-r--r-- 1 john john 15081 Apr 11 12:45 x.png
john@yingna ~/pics $ vips resize nike.png x.png[compression=9] 0.9 --kernel lanczos3
john@yingna ~/pics $ ls -l x.png 
-rw-r--r-- 1 john john 19446 Apr 11 12:45 x.png

[fangqiao]

Tried linear

in = in.resize(scale, VImage::option()->
                           set("kernel",VIPS_KERNEL_LINEAR) ).
                smartcrop(w, h, VImage::option()->
                          set( "interesting", cropMethod));

Output is smaller than the default kernel, I guess it is lanczos3 but still larger than the original.