png output with Colorspace Gray

[amitm02]

When packing an image to PNG, sRGB is produced:

Colorspace: sRGB
Depth: 8/1-bit
  Channel depth:
    Red: 1-bit
    Green: 1-bit
    Blue: 1-bit
Colors: 2

It would be great to have the ability to generate Gray colorspace PNG images:

Colorspace: Gray
Depth: 8/1-bit
  Channel depth:
    Gray: 1-bit
Colors: 2

[lovell]

https://sharp.pixelplumbing.com/api-colour#tocolourspace

await sharp(input)
 .toColourspace('b-w')
 .toFile('8-bit-greyscale.png')

[amitm02]

Thanks for the reply.
Howver i get Colormap entries: 256:
out.png PNG 1712x2332 1712x2332+0+0 8-bit Gray 256c 38972B 0.000u 0:00.000.

and if running:

sharp('./in.png')
  .toColorspace('b-w')
  .png({colors: 2})
  .toFile('out.png')

I get sRGB back:
out.png PNG 1712x2332 1712x2332+0+0 8-bit sRGB 2c 33018B 0.000u 0:00.000

---

The main issue I try to tackle is that I fail to find a way to re-encode the following input PNG configuration without it being inflated significantly in size.

Image:
  Filename: in.png
  Format: PNG (Portable Network Graphics)
  Mime type: image/png
  Class: PseudoClass
  Geometry: 1712x2332+0+0
  Resolution: 28.34x28.34
  Print size: 60.4093x82.2865
  Units: PixelsPerCentimeter
  Colorspace: Gray
  Type: Bilevel
  Base type: Undefined
  Endianness: Undefined
  Depth: 8/1-bit
  Channel depth:
    Gray: 1-bit
  Channel statistics:
    Pixels: 3992384
    Gray:
      min: 0  (0)
      max: 255 (1)
      mean: 248.265 (0.973588)
      median: 255 (1)
      standard deviation: 40.8907 (0.160356)
      kurtosis: 32.8894
      skewness: -5.90672
      entropy: 0.176064
  Colors: 2
  Histogram:
    105445: (0,0,0) #000000 gray(0)
    3886939: (255,255,255) #FFFFFF gray(255)
  Colormap entries: 2
  Colormap:
    0: (0,0,0,1) #000000FF graya(0,1)
    1: (255,255,255,1) #FFFFFFFF graya(255,1)
  Rendering intent: Undefined
  Gamma: 0.454545
  Matte color: grey74
  Background color: white
  Border color: srgb(223,223,223)
  Transparent color: none
  Interlace: None
  Intensity: Undefined
  Compose: Over
  Page geometry: 1712x2332+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: Zip
  Orientation: Undefined
  Properties:
    date:create: 2021-08-23T18:06:01+00:00
    date:modify: 2021-08-23T18:06:01+00:00
    png:IHDR.bit-depth-orig: 1
    png:IHDR.bit_depth: 1
    png:IHDR.color-type-orig: 0
    png:IHDR.color_type: 0 (Grayscale)
    png:IHDR.interlace_method: 0 (Not interlaced)
    png:IHDR.width,height: 1712, 2332
    png:pHYs: x_res=2834, y_res=2834, units=1
    signature: f39e65310d6a342089b53b6419fe4e50e680c6d7025aa003ab24c08d2f0fec83
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 22185B
  Number pixels: 3.99238M
  Pixels per second: 97.5965MP
  User time: 0.350u
  Elapsed time: 0:01.040
  Version: ImageMagick 7.1.0-0 Q16 x86_64 2021-06-13 https://imagemagick.org

[lovell]

This example looks like a bug, ostensibly in libvips as png->bitdepth is correctly set from colours then erroneously always overwritten as 8 on the next line.

https://github.com/libvips/libvips/blob/1ce301812d1c44ca6421ed93d6ba1ec4b1d14433/libvips/foreign/pngsave.c#L131-L138

However colours is deprecated, so we should probably pass bitdepth, and have the log2(colours) calculation, in sharp instead.

[lovell]

Commit 3402656 should fix this, and will be in v0.29.1

[lovell]

v0.29.1 now available with this fix, thanks for reporting.