Convert color conversion internals to matrices stored as 1D array

[vrabaud]

This is to prevent passing M[3][3] and make sure they are const, cf https://c-faq.com/ansi/constmismatch.html

[wantehchang]

This kind of conversion is not ideal.

[vrabaud]

Done

[wantehchang]

Thank you for writing this pull request. The new code is not as readable as the original code, so I think we should ask in the C/C++ discussion group at Google about this problem. The two dimensional array is not exactly the same as https://c-faq.com/ansi/constmismatch.html. Hopefully there is a better solution.

I am also wondering why none of the compilers we use regularly warn about this. Is it because we do not enable a certain warning option?

[wantehchang]

I found a Stack Overflow question about this issue:
https://stackoverflow.com/questions/28062095/pass-a-two-dimensional-array-to-a-function-of-constant-parameter

[wantehchang]

If the original still works, the original code seems to be a better initializer for std::array<std::array<double, 3>, 3> (see line 19).

This comment also applies to the second arguments for the ExpectMatrixNear() calls below.

[vrabaud]

I tried to play with clang options like '-Wincompatible-pointer-types-discards-qualifiers' to no avail. Ditto for gcc.
The simplest solution is to remove the const from the functions. I just thought we might want to preserve it. It's all internal so we might as well remove the const and preserve the 2d semantics.

[wantehchang]

Vincent: Removing the const from the function prorotypes is fine by me.

My suggestion was to wrap the two-dimensional array in a struct, e.g.,

typedef struct {
    double x[3][3];
} avifMat;

But this requires more changes to the code, because coeffs[i][j] will become coeffs.d[i][j].

[vrabaud]

Right, but the problem would be the same: a const avifMat offers no const guarantee on the data in x.

[vrabaud]

Closing in favor of #1946

[wantehchang]

Right, but the problem would be the same: a const avifMat offers no const guarantee on the data in x.

Are you sure? I guess you may be thinking of a struct like avifRWData:

typedef struct avifRWData
{
    uint8_t * data;
    size_t size;
} avifRWData;

then the following is allowed:

    uint8_t data[16];
    const avifRWData rw = { data, sizeof(data) };
    rw.data[0] = 0;

On the other hand, if we have a struct like the following:

typedef struct FixedSizedData16
{
    uint8_t data[16];
} FixedSizedData16;

then the following is not allowed:

    const FixedSizedData16 fixed;
    fixed.data[0] = 0; 

[vrabaud]

I thought the same issue would pop with 2d but it indeed works.