yj1214,
Now i'm little confused...
According to these posts,
[snip]
And rightly so.
What you see there is a problem where a row of elements (pixels) does not
need to be a multiple of the required alignment. Just think of 24-bit (3 byte) pixels. If you just put the first pixel of the second line directly behind the last pixel of the first line it might not start on an alignment boundary. In such cases the video card may require (to speed up
processing) every line of pixels being started on an alignment boundary (padding the previous line of pixels to become a multiple of the alignment).
Actually, if you take a look at the BMP file format, thats exactly how an
image is stored in it (a single line of pixels is padded to become a
multiple of the alignment, in this case 4 bytes). :-)
I think this illustrates your explanation of how alignment works,
[Snip]
Am I not correct? i'm really confused about how alignment works...
Well, although that wasn't exactly what I ment (I was thinking about the alignment of a full block of data/pixels, not its individual elements), I
can imagine that a per-pixel alignment could be possible/required somewhere
too (it could speed up the provcessing). It all depends on how the data is used (which functions are going to use the stored data/how its provided to
the video-card).
In short, an alignment is a requirement of called functions, and below that
of the hardware under it (the video-card or other). There is therefore no strictly defined rule about when to use an alignment, and which alignment is needed.
Regards,
Rudy Wieser
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yj1214 <
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R.Wieser wrote:
The alignment has got nothing to do with the number of elements/bytes in
a
row, but everything with the size of a single element.
Now i'm little confused...
According to these posts,
StackOverflow:
http://stackoverflow.com/questions/11042027/glpixelstoreigl-unpack-alignment -1-disadvantages
OpenGL and JOGL:
http://www.felixgers.de/teaching/jogl/imageStorageMode.html
OpenGL official website:
https://www.opengl.org/discussion_boards/showthread.php/197557-What-does-glP ixelStore()-do?p=1279209#post1279209
It says alignment works with row, not with elements...
From OpenGL post,
**************************************************************************** ***************************************************
Suppose that you have texture data with GL_RGB format (3 bytes per
component) and a width of 123, with no padding between rows (i.e. the first byte of the first pixel of one row immediately follows the last byte of the last pixel of the previous row). The stride between rows will be 3*123 = 369 bytes. Note that 369 is not a multiple of 2 or 4.
If GL_UNPACK_ALIGNMENT is 4, and you try to upload the texture data with
glTexImage2D(), the implementation will round the length of each row up to
the next multiple of 4 (i.e. 372 bytes).
**************************************************************************** ***************************************************
It says alignment will round up each row's length(byte) depends on
GL_UNPACK_ALIGNMENT number. It didn't say it will round up each element's length...
I think this illustrates your explanation of how alignment works,
Let's say each pixel has value of RGB. So each pixel has 3 bytes.
GL_UNPACK_ALIGNMENT, 4;
<address- 1><address- 2><address- 3><address-4><address- 5><address-
<address- 7><address- 8>
<myPixel-R><myPixel-G><myPixel-B><alignment><nextPixel-R><nextPixel-G><nextP ixel-B><alignment>
This illustrates explanation of alignment from different posts.
GL_UNPACK_ALIGNMENT, 4;
<address- 1><address- 2><address- 3><address-4><address- 5><address-
<address- 7><address- 8>
<myPixel-R><myPixel-G><myPixel-B><nextPixel-R><nextPixel-G><nextPixel-B><ali gnment><alignment>
Am I not correct? i'm really confused about how alignment works...
Again thanks for the help!
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