glPixelStorei(GL_UNPACK_ALIGNMENT, 1) Disadvantages?

Question

What are the disadvantages of always using alginment of 1?

glPixelStorei(GL_UNPACK_ALIGNMENT, 1)
glPixelStorei(GL_PACK_ALIGNMENT, 1)

Will it impact performance on modern gpus?

Answer 1

How can data not be 1-byte aligned?

This strongly suggests a lack of understanding of what the row alignment in pixel transfer operations means.

Image data that you pass to OpenGL is expected to be grouped into rows. Each row contains width number of pixels, with each pixel being the size as defined by the format and type parameters. So a format of GL_RGB with a type of GL_UNSIGNED_BYTE will result in a pixel that is 24-bits in size. Pixels are otherwise expected to be packed, so a row of 16 of these pixels will take up 48 bytes.

Each row is expected to be aligned on a specific value, as defined by the GL_PACK/UNPACK_ALIGNMENT. This means that the value you add to the pointer to get to the next row is: align(pixel_size * width, GL_*_ALIGNMENT). If the pixel size is 3-bytes, the width is 2, and the alignment is 1, the row byte size is 6. If the alignment is 4, the row byte size is eight.

See the problem?

Image data, which may come from some image file format as loaded with some image loader, has a row alignment. Sometimes this is 1-byte aligned, and sometimes it isn't. DDS images have an alignment specified as part of the format. In many cases, images have 4-byte row alignments; pixel sizes less than 32-bits will therefore have padding at the end of rows with certain widths. If the alignment you give OpenGL doesn't match that, then you get a malformed texture.

You set the alignment to match the image format's alignment. If you know or otherwise can ensure that your row alignment is always 1 (and that's unlikely unless you've written your own image format or DDS writer), you need to set the row alignment to be exactly what your image format uses.

Answer 2

Will it impact performance on modern gpus?

No, because the pixel store settings are only relevent for the transfer of data from or to the GPU, namely the alignment of your data. Once on the GPU memory it's aligned in whatever way the GPU and driver desire.

Answer 3

There will be no impact on performance. Setting higher alignment (in openGL) doesn't improve anything, or speeds anything up.

All alignment does is to tell openGL where to expect the next row of pixels. You should always use an alignment of 1, if your image pixels are tightly packed, i.e. if there are no gaps between where a row of bytes ends and where a new row starts.

The default alignment is 4 (i.e. openGL expects the next row of pixels to be after a jump in memory which is divisible by 4), which may cause problems in cases where you load R, RG or RGB textures which are not 4-bytes floats, or the width is not divisible by 4. If your image pixels are tightly packed you have to change alignment to 1 in order for the unpacking to work.

You could (I personally haven’t encountered them) have an image of, say, 3x3 RGB ubyte, whose rows are 4th-aligned with 3 extra bytes used as padding in the end. Which rows might look like this:

R - G - B - R - G - B - R - G - B - X - X - X (16 bytes in total)

The reason for it is that aligned data improves the performance of the processor (not sure how much it's true/justified on todays processors). IF you have any control over how the original image is composed, then maybe aligning it one way or another will improve the handling of it. But this is done PRIOR to openGL. OpenGL has no way of changing anything about this, it only cares about where to find the pixels.

So, back to the 3x3 image row above - setting the alignment to 4 would be good (and necessary) to jump over the last padding. If you set it to 1 then, it will mess your result, so you need to keep/restore it to 4. (Note that you could also use ROW_LENGTH to jump over it, as this is the parameter used when dealing with subsets of the image, in which case you sometime have to jump much more then 3 or 7 bytes (which is the max the alignment parameter of 8 can give you). In our example if you supply a row length of 4 and an alignment of 1 will also work).

Same goes for packing. You can tell openGL to align the pixels row to 1, 2, 4 and 8. If you're saving a 3x3 RGB ubyte, you should set the alignment to 1. Technically, if you want the resulting rows to be tightly packed, you should always give 1. If you want (for whatever reason) to create some padding you can give another value. Giving (in our example) a PACK_ALIGNMENT of 4, would result in creating rows that look like the row above (with the 3 extra padding in the end). Note that in that case your containing object (openCV mat, bitmap, etc.) should be able to receive that extra padding.