... how to I tweak the encoder, i.e. funnel command line options from IM to the bpgenc.exe? There are plenty that are important:
Code: Select all
BPG Image Encoder version 0.9.6
usage: bpgenc [options] infile.[jpg|png]
Main options:
-q qp set quantizer parameter (smaller gives better quality,
range: 0-51, default = 29)
-f cfmt set the preferred chroma format (420, 422, 444,
default=420)
-c color_space set the preferred color space (ycbcr, rgb, ycgco,
ycbcr_bt709, ycbcr_bt2020, default=ycbcr)
-b bit_depth set the bit depth (8 to 12, default = 8)
-lossless enable lossless mode
-e encoder select the HEVC encoder (x265, default = x265)
-m level select the compression level (1=fast, 9=slow, default = 8)
Advanced options:
-alphaq set quantizer parameter for the alpha channel (default = same as -q value)
-premul store the color with premultiplied alpha
-limitedrange encode the color data with the limited range of video
-hash include MD5 hash in HEVC bitstream
-keepmetadata keep the metadata (from JPEG: EXIF, ICC profile, XMP, from PNG: ICC profile)
Code: Select all
2) BPG encoder
--------------
The BPG command line encoder is 'bpgenc'. It takes JPEG or PNG images
as input.
- Speed: by default bpgenc uses the x265. You can compile the much
slower but more efficient JCTVC encoder and select it with the '-e
jctvc' option. With x265 you can select the encoding speed with the
'-m' option (1 = fast, but larger image, 9 = slower but smaller
image).
- Bit depth: the default bit depth is 8. You can increase it to 10
('-b 10' option) to slightly increase the compression ratio. For web
publishing it is generally not a good idea because the Javascript
decoder uses more memory. The compiled x265 encoder supports the bit
depth of 8, 10 and 12. The slower JCTVC encoder can be compiled to
support higher bit depths (up to 14) by enabling the Makefile
define: USE_JCTVC_HIGH_BIT_DEPTH.
- Lossless compression is supported as a bonus thru the HEVC lossless
capabilities. Use a PNG input in this case unless you know what you
do ! In case of a JPEG input, the compression is lossless related to
the JPEG YCbCr data, not the RGB data. In any case, the bit depth
should match the one of your picture otherwise the file size
increases a lot. By default the lossless mode sets the bit depth to
8 bits. The prefered color space is set to "rgb". Notes:
- lossless mode is less tested that the lossy mode but it usually
gives better results that PNG on photographic images.
- the JCTVC encoder gives smaller images than the x265 encoder
with lossless compression.
- There is a small difference of interpretation of the quantizer
parameter (-q option) between the x265 and JCTVC encoder.
- Color space and chroma format:
* For JPEG input, the color space of the input image is not
modified (it is YCbCr, RGB, YCbCrK or CMYK). The chroma is
subsampled according to the preferred chroma format ('-f'
option).
* For PNG input, the input image is converted to the preferred
color space ('-c' option). Its chroma is then subsampled
according to the preferred chroma format.
* grayscale images are kept unmodified.
- Premultiplied alpha: by default bpgenc uses non-premultiplied alpha
to preserve the color components. However, premultiplied alpha
('-premul' option) usually gives a better compression at the expense
of a loss in the color components. This loss is not an issue if the
image is not edited.