Calculating degree of visual angle for stimuli size

[aa93...@gmail.com]

Hello,

Using the following mointor/viewing distance measurements I needed to calculate the amount of pixels in 1 degree of visual angle:

Monitor: 1680 x 1050 resolution, Monitor width = 477mm (47.7cm), Monitor height = 268mm (26.8cm)

Viewing distance from screen = 530mm

I have been told that, given the above measurements, the pixel densities (x,y) will be different i.e. 32pxls (x) and 36 pxls (y)

The problem is, if I make stimuli corresponding to 1 degree of visual angle, using those pixel densities, the square stimuli
 will be stretched, and therefore of no use. I need them to be of equal side both sides.

Does anyone know a way around this problem? Or is it simply the case that I would need to change the monitor?

Cheers,
Steve

[Michael MacAskill]

> On 5/04/2016, at 11:36, aa93...@gmail.com wrote:
>
> I have been told that, given the above measurements, the pixel densities (x,y) will be different i.e. 32pxls (x) and 36 pxls (y)

Dear Steve,

Physically, screen pixels are pretty much universally square. So:

Is 1680 × 1050 the native resolution of this monitor (assuming it's an LCD)? (i.e. it is possible that the logical resolution doesn't match the native physical resolution).

Are the physical dimensions correct (don't rely on specs: they might not be just of the visible area)?

Regards,

Michael

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University of Otago, Christchurch http://www.nzbri.org/macaskill

[parsnip 1]

Hi Michael,

1680 × 1050 is the native resolution of the monitor. If pixels are pretty much universally square, am I just unlucky to have pixels that have different sized sides? Yes the physical dimensions are correct. Jon has informed me that Psychopy can take care of displaying stimuli at various degrees etc. (monitor centre), which is fantastic. My only problem is.I need my stim to be sized 1 degree of visual angle with equal sides of the png. Does that make sense? e.g. 60x60pixels (or whatever 1 degree is. If it's 32x36 like someone suggested, then I will have stretched stims.

Thanks,

Steve

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[Michael MacAskill]

Hi Steve,

If the pixels aren't square, then you would be seeing distortion effects everywhere, not just in PsychoPy. i.e. pretty much all computer graphics systems are predicated on pixels being square. If they aren't square, then, for example, photos would have incorrect apparent aspect ratios and appear stretched in one direction. All square shapes of any sort displayed in any program would appear as rectangles.

Is that happening?

Perhaps you should make this less theoretical and post the actual name and model of the display.

Regards,

Michael

> On 6/04/2016, at 11:32, parsnip 1 <aa93...@gmail.com> wrote:
>
> Hi Michael,
>
> 1680 × 1050 is the native resolution of the monitor. If pixels are pretty much universally square, am I just unlucky to have pixels that have different sized sides? Yes the physical dimensions are correct. Jon has informed me that Psychopy can take care of displaying stimuli at various degrees etc. (monitor centre), which is fantastic. My only problem is.I need my stim to be sized 1 degree of visual angle with equal sides of the png. Does that make sense? e.g. 60x60pixels (or whatever 1 degree is. If it's 32x36 like someone suggested, then I will have stretched stims.
>
> Thanks,
> Steve

[parsnip 1]

Hi Michael,

Sorry, I think I should have been clearer. It was the amount of pixels (x and y coord) within 1 degree of visual angle that were not equal (because it was a widescreen monitor). The pixels themselves were fine, no distortion etc.

I've now switched to a 1024x768 monitor (57cm viewing distance). This calculates to 28.29 x29.86 pixels per 1 degree visual angle, which is the sort of ratio that I was looking for.

Just thought I should clear that up.

Cheers,

Steve

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[Michael MacAskill]

> On 8/04/2016, at 05:55, parsnip 1 <aa93...@gmail.com> wrote:
>
> Sorry, I think I should have been clearer. It was the amount of pixels (x and y coord) within 1 degree of visual angle that were not equal (because it was a widescreen monitor). The pixels themselves were fine, no distortion etc.

If the pixels are square, their density is independent of whether you are measuring horizontally or vertically: one degree of visual angle will encompass the same number of pixels regardless.

The total width of the display in any direction is irrelevant (except that you use it to calculate the density).

Thought experiment: the monitor suddenly becomes twice as long physically, but also now has twice as many pixels horizontally. So the density is unchanged but the display is now even more radically "widescreen". But the number of pixels contained within one degree of visual angle has not altered, horizontally or vertically.

Second thought experiment: you take a monitor with square pixels. You count the number of pixels within one degree of visual angle, horizontally and vertically. You rotate it 90 degrees and measure again. What has changed?

I would become absolutely comfortable with the geometry before proceeding…

Regards,

Michael

[parsnip 1]

Hi Michael,

Thought experiment: the monitor suddenly becomes twice as long physically, but also now has twice as many pixels horizontally. So the density is unchanged but the display is now even more radically "widescreen". But the number of pixels contained within one degree of visual angle has not altered, horizontally or vertically.

Isn't this only the case if both the height and the width are increased to the same degree?

E.g. 1024x768 res, 340x270mm (w,h), viewing distance = 57cm  ... = 1 degree of visual angle encompasses: 27.86027 x 26.31248 px

You are correct, I do need to properly understand all of this before I continue. It is not something I had previously considered would be an issue.

Thanks,

Steve

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