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validity
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... itself, the chunk type code, or the CRC. Zero is a valid
length. Although encoders and decoders ...
... All implementations must understand and successfully render the
standard critical chunks. A valid PNG image must contain an IHDR
...
... integer giving the number of bits
per sample or per palette index (not per pixel). Valid values
are 1, 2, 4, 8, and 16, although not all values are allowed for
all color types.
...
... sums of the following values: 1 (palette used), 2 (color used),
and 4 (alpha channel used). Valid values are 0, 2, 3, 4, and 6.
Bit ...
... decoder need not pay attention to sBIT: the stored image is a
valid PNG file of the sample depth indicated by IHDR. However,
if the decoder ...
... critical
chunk type, because there is no way to be certain that a
valid file will result from modifying a file containing such
a chunk. (Note that simply discarding the chunk is not good
enough, because it might have unknown implications for the
...
... Decoders must not assume more about the positioning of any
ancillary chunk than is specified by the chunk ordering rules. In
particular, it is never valid to assume that a specific ancillary
chunk type occurs with any particular positioning relative to
other ancillary chunks. (For example, it is unsafe to assume that
...
... by "left bit replication", which is shifting the valid bits to
begin in the most significant bit ...
... monitor you are using. This is often because they are working in
some device-independent space internally. Such programs have
enough information to write valid cHRM and gAMA chunks, and should
do so automatically.
...
... bit depth, and
rounding to the nearest integer. In valid premultiplied data, the
sample values never exceed their corresponding alpha values, so
the result of the division should always be in the range ...
... image, as well as any transparent
pixels within the image. (Thus, bKGD is valid and useful even
when the image does not use transparency.) If no bKGD chunk is
...
... Exponentiation; a raised to the power b. C programmers should be
careful not to misread this notation as exclusive-or. Note that
in gamma-related calculations, zero raised to any power is valid
and must give a zero result.
...