On Feb 10, 9:45 pm, Brian Holt <bdho...@gmail.com> wrote: <snip>
A line is drawn for each gradient bin with an intensity proportional to the magnitude of that gradient. So, the 'star' shape you see for each cell is just the superimposition of all of these lines. You should expect to see dominant lines perpendicular to lines in the image (parallel to the gradient). Also remember that the default is to use 9 bins, so it may be that the 45degree dominant line you see is the closest approximation to horizontal. You can test this out by trying 8 bins instead of 9.
Okay, I am still having problems understanding visualisation of the HoG. When I plot the visualisation it seem to make sense. It is still not clear why I don't see vertical/perpendicular lines. If you look at http://tinypic.com/r/dy0fud/5 I would expect to see the pole of the street light to show dominant perpendicular lines, but they appear to be more horizontal than vertical. Similar stuff happens if I use buildings, the "edge" of the building, clearly visable in the gradient images, ends up as either tending towards the 45deg (or 135deg). The parameters used for the picture were: Orientations 9, Pixesl Per Cell16x16, Cells per Block 3x3. I will trace the code to try to get a better understanding of the visualisation output, but would appreciate if anyone has any advice or explanation of my misunderstanding of the visualisation. Also what is the best why to link to images in a post? Thanks, Michael. --
On Thu, Feb 16, 2012 at 6:32 AM, bricklemacho <bricklemacho@gmail.com>wrote:
A line is drawn for each gradient bin with an intensity proportional to
On Feb 10, 9:45 pm, Brian Holt <bdho...@gmail.com> wrote: <snip> the
magnitude of that gradient. So, the 'star' shape you see for each cell is just the superimposition of all of these lines. You should expect to see dominant lines perpendicular to lines in the image (parallel to the gradient). Also remember that the default is to use 9 bins, so it may be that the 45degree dominant line you see is the closest approximation to horizontal. You can test this out by trying 8 bins instead of 9.
Okay, I am still having problems understanding visualisation of the HoG. When I plot the visualisation it seem to make sense. It is still not clear why I don't see vertical/perpendicular lines. If you look at http://tinypic.com/r/dy0fud/5 I would expect to see the pole of the street light to show dominant perpendicular lines, but they appear to be more horizontal than vertical. Similar stuff happens if I use buildings, the "edge" of the building, clearly visable in the gradient images, ends up as either tending towards the 45deg (or 135deg). The parameters used for the picture were: Orientations 9, Pixesl Per Cell16x16, Cells per Block 3x3.
I don't actually understand HoG that well, but this result, I believe, is expect. Assuming HoG does what the name suggests, you should expect the lines to align with the direction with the largest gradient. So, if the pole of the street light is a dark gray, and the background is white, then the lines should be perpendicular to the pole since this is the direction of the highest gradient.
I will trace the code to try to get a better understanding of the visualisation output, but would appreciate if anyone has any advice or explanation of my misunderstanding of the visualisation.
Also what is the best why to link to images in a post?
Thanks,
Michael. --
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Tony Yu