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devel-docs: update the XCF docs.

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Various texts were still assuming 8-bit per component.
This commit is contained in:
Jehan
2019-10-31 16:49:53 +01:00
parent cffb86cc6f
commit 672b77dedb
1 changed files with 45 additions and 24 deletions
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69
devel-docs/xcf.txt
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@ -248,32 +248,44 @@ Color
RGB:
Three intensity values for red, green, and blue additive color
components, each on a scale from 0 to 255. The exact color space
is not specified. GIMP displays image data directly on PC
display hardware without any software correction, so in most
cases the intensity values should be considered nonlinear samples
that map to physical light intensities using a power function
with an exponent ("gamma") of about 2.5. (This is how PC hardware
components. The exact format depends on the field 'precision' of the
image header. If this field is absent (i.e. for XCF version 3 or
before), "8-bit gamma integer" is assumed, which means each component is
on a scale from 0 to 255, with the intensity values considered nonlinear
samples that map to physical light intensities using a power function
with an exponent ("gamma") of about 2.5 (this is how PC hardware
commonly treat bit values in the video buffer, which incidentally
also has the property of making each 1/255th step about equally
perceptible to the human eye when the monitor is correctly
adjusted).
Beware, however, that GIMP's compositing algorithms (as described
in the document compositing.txt) implicitly treat the intensities
as _linear_ samples. The XCF file format currently has no support
for storing the intended gamma of the samples.
When the precision field is present though, it defines the storage
format and the exact color space depends on the color profile attached
to the image. The color profile is saved as a parasite named
"icc-profile" on the image. If no profile is set, sRGB is assumed.
TODO: Are the statements about color space, gamma and layer modes still valid?
Beware, however, that **before GIMP 2.10**, GIMP's compositing
algorithms (as described in the document compositing.txt) implicitly
treated the intensities as _linear_ samples. The XCF file format had no
support for storing the intended gamma of the samples.
Since GIMP 2.10.0 and over, you must rely on PROP_COMPOSITE_MODE,
PROP_COMPOSITE_SPACE and PROP_BLEND_SPACE for compositing and blending.
Grayscale:
One intensity value on a scale from 0 (black) to 255 (white).
Gamma considerations as for RGB.
One intensity value. Grayscale has the same precision considerations as
for RGB and it can also have a profile since GIMP 2.10.0, as well as
compositing and blending rules.
On older XCF without precision field, the value was simply on a scale
from 0 (black) to 255 (white).
Indexed:
An 8-bit index into a color map that is shared between all
layers. The color map maps each index to an RGB triple which is
interpreted as in the RGB model.
It is to be noted that Indexed image in GIMP is limited to 8-bit integer
RGB, even in GIMP 2.10.
Pixel data: Tiles
-----------------
@ -1444,13 +1456,19 @@ It appears in a context: each layer and channel has a pointer to its hierarchy.
uint32 width Width of the pixel array
uint32 height Height of the pixel array
uint32 bpp Number of bytes per pixel given
3: RGB color without alpha
4: RGB color with alpha
1: Grayscale without alpha
2: Grayscale with alpha
1: Indexed without alpha
2: Indexed with alpha
uint32 bpp Number of bytes per pixel; this depends on the
color mode and image precision (fields 'base_type'
and 'precision' of the image header). For
instance, some combination values:
3: RGB color without alpha in 8-bit precision
4: RGB color with alpha in 8-bit precision
6: RGB color without alpha in 16-bit precision
16: RGB color with alpha in 32-bit precision
1: Grayscale without alpha in 8-bit precision
4: Grayscale with alpha in 16-bit precision
1: Indexed without alpha (always 8-bit)
2: Indexed with alpha (always 8-bit)
And so on.
pointer lptr Pointer to the "level" structure
,-------- ------ Repeat zero or more times
@ -1501,10 +1519,13 @@ of width*height pixels, extracted from the tile in row-major,
top-to-bottom, left-to-right order (the same as the reading direction
of multi-line English text).
In color modes with alpha information, the alpha value is the last of
the 2 or 4 bytes for each pixel. In RGB color modes, the 3 (first)
bytes for each pixel is the red intensity, the green intensity, and
the blue intensity, in that order.
In color modes with alpha information, the alpha value is the last bytes
for each pixels, after the color information.
In RGB color modes, the color information (first bytes for each pixel)
is the red intensity, the green intensity, and the blue intensity, in
that order.
The exact size of each component depends on the 'precision' field, for
instance 1 byte in 8-bit modes, 2 bytes in 16-bit, and so on.
Tile data, as other data in XCF format, is big-endian. In particular it
means that pixel values are stored as big-endian when the precision is