Support for 1-bit Grayscale about image HOT 4 OPEN

A bunch of places in this library require that pixels are an integer number of bytes, which would break for ImageLuma1.

However, that doesn't mean we cannot support Luma1 images for limited cases. We already have ExtendedColorType::L1 to describe the color type and the ImageDecoder::original_color_type method to reveal that a conversion is being applied. We've also been wanting to switch the ImageEncoder trait to allow passing any ExtendedColorType when writing image (while still allowing individual format encoders to reject color types they do not support). At that point, we'd be able to start adding Luma1 support to individual encoders

from image.

In the input image, we see (some fields redacted):

Geometry: 13244x71130+0+0
Channels: 2.0
Channel depth:
    Gray: 1-bit
Channel statistics:
    Pixels: 942045720
Colors: 2
Histogram:
      56641321: (0,0,0) #000000 gray(0)
     885404399: (255,255,255) #FFFFFF gray(255)
Filesize: 11.771MiB

while in the output image (processed by image) we see:

Geometry: 6904x37079+0+0
Channels: 2.0
Channel depth:
    Gray: 8-bit
Channel statistics:
    Pixels: 255993416
Colors: 256
Histogram:
  (many entries)
Filesize: 15.3349MiB

1/4 the number of pixels, yet a larger file size.

from image.

Update on this: as of the 0.25 release, ImageEncoder::encode now takes an ExtendedColorType. If anyone is interested in adding 1-bit grayscale support to any of our encoders, it should now be possible

from image.

Thanks for the update!

from image.