Issue 225 in webp: Lossless spec does not specify uint8 to int8 conversion

[we...@googlecode.com]

Status: New
Owner: ----
Labels: Type-Defect Priority-Medium

New issue 225 by lukas.pe...@seznam.cz: Lossless spec does not specify
uint8 to int8 conversion
https://code.google.com/p/webp/issues/detail?id=225

What steps will reproduce the problem?

1. The spec webp-lossless-bitstream-spec.txt says:

We assume that each color component (e.g. alpha, red, blue and green) is
represented using an 8-bit byte. We define the corresponding type as
uint8.

Also later on, function ColorTransform is using uint8 arguments:

void ColorTransform(uint8 red, uint8 blue, uint8 green,
ColorTransformElement *trans,
uint8 *new_red, uint8 *new_blue)

However, that function calls function ColorTransformDelta, which has int8
arguments:

int8 ColorTransformDelta(int8 t, int8 c)

The spec does not specify how the uint8 value should be converted to int8
value.

What is the expected output? What do you see instead?

The spec should describe the expected conversion from uint8 to int8 to be
used.

What version of the product are you using? On what operating system?

date Thu Sep 18 06:21:02 2014

Please provide any additional information below.

The spec does not say that C language conversion rules should apply,
however I looked what should be the standard behaviour of C code in this
case. The conversion from uint8 to int8 actually is only defined if the
value fits into the int8 range (i.e. here for the values 0 - 127), for
other values it is implementation defined.

See for example
http://unspecified.wordpress.com/2011/08/08/integer-conversions-in-c/
Quote:
However, a narrowing conversion of signed integers is undefined if the
value cannot be represented in the new type (for example, converting the
value 256 to a signed char is undefined, assuming an 8-bit char type). In
almost all compilers, you can assume two's complement format, which means
the behaviour is nearly uniform, but the language standard itself does not
mandate two's complement representation for signed integers, and therefore
cannot specify the behaviour when a signed integer is narrowed.



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Updates:
Owner: vikaas.a...@gmail.com

Comment #1 on issue 225 by jz...@google.com: Lossless spec does not specify

uint8 to int8 conversion
https://code.google.com/p/webp/issues/detail?id=225

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