[osg-users] DDS with DXT1 with and without alpha

Robert Osfield robert.osfield at gmail.com
Thu Apr 14 06:07:04 PDT 2011


Hi Mikhail,

I think you have mis-intrepted my suggestion that DTX1a and DTX1c have
essential the same header and contents - I mean in that they are the
same format in memory so we can't distinguish between the two.

One would not expect nvcompress to produce the same output for DXT1a
and DTX1c and I didn't mean to imply that it or any other tool would.
Instead I meant to infer that such a tool that generated a DTX1a image
with transparent pixels could be identical to  a DTX1c image with
black pixels.

Robert.

On Thu, Apr 14, 2011 at 1:34 PM, Mikhail I. Izmestev
<izmmishao5 at gmail.com> wrote:
> 14.04.2011 16:16, Robert Osfield wrote:
>
>> The heart of the problem is that the DDS file format doesn't
>> distiguish between the
>> two varients of DXT1, there is an alpha bit in the DDS header that
>> could be used,
>> but 3rd party tools like nvdxt and nvcompress that write DDS files
>> don't set this bit
>> so both DTX1c with black encoded pixels and DTX1a are essentially
>> identical
>> in terms of header and content.
>
> Where from this information?
>
> $ nvdxt.exe -nomipmap -dxt1a -file test_dxt1.png -output test_dxt1a.dds
> Version 8.30
>    Reading test_dxt1.png  [Processing] Writing .\test_dxt1a.dds
>
> $ nvdxt.exe -nomipmap -dxt1c -file test_dxt1.png -output test_dxt1c.dds
> Version 8.30
>    Reading test_dxt1.png  [Processing] Writing .\test_dxt1c.dds
>
> $ md5sum.exe test_dxt1*.dds
> ac1473f8822abbb2d6006de0021c6bc0 *test_dxt1a.dds
> 249071d0b0974593b310656acea73e1d *test_dxt1c.dds
>
> Mikhail
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