Resuming my previous monologue...
Digging a bit in the targa plugin i found that no check was done between
the flags of the channels and the actual channels in the file.
I modified the code from
m_alpha = (m_tga.attr & 0x0F) > 0 ? TGA_ALPHA_USEFUL : TGA_ALPHA_NONE;
to
m_alpha = TGA_ALPHA_NONE;
if (((m_tga.type == TYPE_RGB || m_tga.type == TYPE_RGB_RLE) &&
m_tga.bpp == 32 )
|| ((m_tga.type == TYPE_GRAY || m_tga.type == TYPE_GRAY_RLE) &&
m_tga.bpp > 8 )) {
m_alpha = (m_tga.attr & 0x08) > 0 ? TGA_ALPHA_USEFUL :
TGA_ALPHA_NONE;
}
The idea is to make sure there is a channel in the file, even if the
flag says the data is used. In the file I have, the m_tga.attr value is
equal to 8, which is, as far as I understand, absolutely inconsistent
with the channel in the file.
If I understand the meaning of the flag, I should be set to 1, meaning
the first channel has useful information, instead of 8 meaning the last
channel (here, alpha), has useful info.
I'd appreciate very much if someone could tell me if my fix is correct,
and if i understood correctly how the tga header work ?
Thanks.
PS : I used this page http://www.paulbourke.net/dataformats/tga/ to get
TGA specs.
On 4/24/2013 1:30 PM, Lerenard Michel wrote:
I found why OIIO finds two channels in the file. The tga_alpha_type of
the file has been set to TGA_ALPHA_UNDEFINED_RETAIN.
I do not know how TGA works, I then have a question: what is the use
of such a flag ? How is it set ? Can i ignore it ?
To be able to load correctly this type of file (ie discard the alpha
layer), i have to parse the data buffer returned by OIIO, and it costs
quite some time. I there another way to detect such an empty channel
before calling "read_scanlines" which i use ?
2013/4/24 Michel Lerenard <[email protected]
<mailto:[email protected]>>
Hi,
I've spotted a weird behavior when opening grayscale files, i'm
wondering if I have to check something else in the header or if it
is the correct behavior: the ImageSpec structure tells me there
are two channels in the file, one called 'I' and another called
'A', which is absolutely black. (ie empty ?)
The file i'm opening is a simple black and white TGA used as a
mask, with, according to Gimp, photoshop, xnview, etc... a single
color channel. ( 8 bits )
Is there a way to check if a given channel is empty ? Apparently
the file does not hold any data for alpha, so i'm wondering if
OIIO is creating an alpha channel for compatibility and if I
should check some attributes to detect if there is actually any
data in it ?
Thanks for your help.
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