[FFmpeg-devel] [PATCH]Write tiff aspect ratio by default instead of dpi

[Reimar Döffinger]

On 03.04.2014, at 05:29, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Tue, Apr 01, 2014 at 12:15:25PM +0200, Carl Eugen Hoyos wrote:
>> Hi!
>> 
>> Attached patch changes the tiff encoder to write the sample 
>> aspect ratio instead of the dpi by default, this is what other 
>> FFmpeg image decoders do.
>> Tested with xv.
>> 
>> Please comment, Carl Eugen
> 
>> Changelog            |    1 +
>> libavcodec/tiffenc.c |   11 ++++++-----
>> libavcodec/version.h |    2 +-
>> tests/ref/lavf/tiff  |    2 +-
>> 4 files changed, 9 insertions(+), 7 deletions(-)
>> 7ddb87691a1e76e7300bdd8d2276a281e8fe6941  patchtiffsar.diff
>> diff --git a/Changelog b/Changelog
>> index 0e38ec8..9d40e53 100644
>> --- a/Changelog
>> +++ b/Changelog
>> @@ -13,6 +13,7 @@ version <next>:
>> - QTKit input device
>> - improvments to OpenEXR image decoder
>> - support decoding 16-bit RLE SGI images
>> +- Write tiff aspect ratio instead of dpi by default
>> 
>> 
>> version 2.2:
>> diff --git a/libavcodec/tiffenc.c b/libavcodec/tiffenc.c
>> index 7b1e510..cccc4bc 100644
>> --- a/libavcodec/tiffenc.c
>> +++ b/libavcodec/tiffenc.c
>> @@ -233,7 +233,8 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
>>     uint8_t *offset;
>>     uint32_t strips;
>>     int bytes_per_row;
>> -    uint32_t res[2] = { s->dpi, 1 };    // image resolution (72/1)
>> +    uint32_t resx[2] = { s->dpi ? s->dpi : avctx->sample_aspect_ratio.den, 1 };
>> +    uint32_t resy[2] = { s->dpi ? s->dpi : avctx->sample_aspect_ratio.num, 1 };
> 
> i think this is wrong
> shouldnt the dpi be adjusted when the aspect differs from 1:1
> i mean 1:2 sample aspect would imply that there are twice as many
> pixels in one direction thus dpi also being different for the two
> axis

Which also kind of leaves the question which to adjust (I'd suggest adjusting both isn't good)
I think scaling y is a bit more common, alternatively scaling so that DPI only decreases probably makes sense, too (which basically means aspect scaling is done by scaling up, not down).