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Add/update comments about avifCleanApertureBox #1749

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Add/update comments about avifCleanApertureBox #1749

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41f1839
Add/update comments about avifCleanApertureBox
wantehchang Nov 10, 2023
92dc7bc
Fix a grammatical error, add link to QuickTime doc
wantehchang Nov 13, 2023
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20 changes: 13 additions & 7 deletions include/avif/avif.h
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Original file line number Diff line number Diff line change
Expand Up @@ -428,30 +428,36 @@ typedef uint32_t avifTransformFlags;

typedef struct avifPixelAspectRatioBox
{
// 'pasp' from ISO/IEC 14496-12:2015 12.1.4.3
// 'pasp' from ISO/IEC 14496-12:2022 12.1.4.3

// define the relative width and height of a pixel
uint32_t hSpacing;
uint32_t vSpacing;
} avifPixelAspectRatioBox;

// NOTE: The members of the avifCleanApertureBox struct are declared as uint32_t to match the
// unsigned int(32) type used in ISO/IEC 14496-12:2022 faithfully. However, ISO/IEC 14496-12:2022
// 12.1.4.1 clearly interprets these values as signed int(32) and talk about them being strictly
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// positive, positive, or negative. Cast these struct members to int32_t before use.
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typedef struct avifCleanApertureBox
{
// 'clap' from ISO/IEC 14496-12:2015 12.1.4.3
// 'clap' from ISO/IEC 14496-12:2022 12.1.4.3

// a fractional number which defines the exact clean aperture width, in counted pixels, of the video image
// a fractional number which defines the width of the clean aperture image
uint32_t widthN;
uint32_t widthD;

// a fractional number which defines the exact clean aperture height, in counted pixels, of the video image
// a fractional number which defines the height of the clean aperture image
uint32_t heightN;
uint32_t heightD;

// a fractional number which defines the horizontal offset of clean aperture centre minus (width-1)/2. Typically 0.
// a fractional number which defines the horizontal offset between the clean aperture image
// centre and the full aperture image centre. Typically 0.
uint32_t horizOffN;
uint32_t horizOffD;

// a fractional number which defines the vertical offset of clean aperture centre minus (height-1)/2. Typically 0.
// a fractional number which defines the vertical offset between clean aperture image centre
// and the full aperture image centre. Typically 0.
uint32_t vertOffN;
uint32_t vertOffD;
} avifCleanApertureBox;
Expand Down Expand Up @@ -711,7 +717,7 @@ typedef struct avifImage
// appropriately, but do not impact/adjust the actual pixel buffers used (images won't be
// pre-cropped or mirrored upon decode). Basic explanations from the standards are offered in
// comments above, but for detailed explanations, please refer to the HEIF standard (ISO/IEC
// 23008-12:2017) and the BMFF standard (ISO/IEC 14496-12:2015).
// 23008-12:2017) and the BMFF standard (ISO/IEC 14496-12:2022).
//
// To encode any of these boxes, set the values in the associated box, then enable the flag in
// transformFlags. On decode, only honor the values in boxes with the associated transform flag set.
Expand Down
11 changes: 10 additions & 1 deletion src/avif.c
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Expand Up @@ -697,7 +697,7 @@ avifBool avifCropRectConvertCleanApertureBox(avifCropRect * cropRect,
{
avifDiagnosticsClearError(diag);

// ISO/IEC 14496-12:2020, Section 12.1.4.1:
// ISO/IEC 14496-12:2022, Section 12.1.4.1:
// For horizOff and vertOff, D shall be strictly positive and N may be
// positive or negative. For cleanApertureWidth and cleanApertureHeight,
// N shall be positive and D shall be strictly positive.
Expand All @@ -719,6 +719,15 @@ avifBool avifCropRectConvertCleanApertureBox(avifCropRect * cropRect,
return AVIF_FALSE;
}

// ISO/IEC 23000-22:2019/Amd. 2:2021, Section 7.3.6.7:
// The clean aperture property is restricted according to the chroma
// sampling format of the input image (4:4:4, 4:2:2:, 4:2:0, or 4:0:0) as
// follows:
// - cleanApertureWidth and cleanApertureHeight shall be integers;
// - The leftmost pixel and the topmost line of the clean aperture as
// defined in ISO/IEC 14496-12:2020, Section 12.1.4.1 shall be integers;
// ...

if ((widthN % widthD) != 0) {
avifDiagnosticsPrintf(diag, "[Strict] clap width %d/%d is not an integer", widthN, widthD);
return AVIF_FALSE;
Expand Down