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Thread: Crop/Expand canvas strange or imprecise behavior...

  1. #1
    Join Date
    Feb 2014
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    4

    Question Crop/Expand canvas strange or imprecise behavior...

    Hi everyone,

    I have a problem with the crop/expand canvas. Artrage 4.5.x is not expanding or cropping the images with the same pixel precision as Photoshop or others similar software. It's very annoying if you paint precise frames and you need to switch between different programs. As a demonstration I have created a green square of 640x480 and then expanded to 822x603. If you crop this picture to 640x480 in Photoshop or Krita (freeware) the result is slightly off center

    Click image for larger version. 

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  2. #2
    Join Date
    Oct 2007
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    NC, USA
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    It's possible there's a location shift occurring during the changes of the canvas size. Have you tried clicking the "Centre" option after you've changed both size options?

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    Nothing is easy to the unwilling.

  3. #3
    Join Date
    Feb 2014
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    Yes, but my guess is that Artrage is less precise in the crop/expand calculation and is rounding decimals earlier that Photoshop or similar software.
    I made the test in the opposite direction: I have created the same picture in Krita and in Photoshop (attached) and then imported the picture in Artrage and cropped to 640x480. Unfortunately you have the same shift occurring and you can see the yellow band on the bottom and on the borders (picture 2).
    I hope that the issue can be fixed in a future update.
    Click image for larger version. 

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    Last edited by steph; 02-01-2016 at 10:59 PM.

  4. #4
    Join Date
    Mar 2006
    Location
    New Zealand
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    I just tried to replicate the problem and didn't get it, so let me just check some steps here:

    1. I create a canvas at 640 x 480 and fill with colour.
    2. I expand that canvas to 822 x 603 and fill the background with a different colour to get a defined shape.
    3. I then crop the canvas back to 640 x 480.

    I don't get the problem you're seeing, I've tried with sub pixel placement on/off, merged layers or different layers and I can't get it to happen so there's probably something I'm doing differently here.

    However, the problem probably lies in the odd vertical size you've chosen there. 603 vertically means you've got an uneven border above and below the shape, so when the app crops the centering is going to have to center to a half pixel which might cause the issue you're seeing. If you manually choose to center the canvas (something I didn't do) then that could well be the problem as, again, having different border sizes above/below your shape means the function has to either position the cropped result on a sub pixel boundary, or choose either the top or bottom pixel to offset.
    Matt
    ArtRage UI
    Ambient Design.

  5. #5
    Join Date
    Feb 2014
    Posts
    4
    Hi,

    1+2 Yes
    3. You crop in another application like Photoshop or Krita, or vice versa (you create and expand the canvas in Photoshop or similar software and you crop in Artrage).
    The result is always a slightly shifted crop even if instead of pixels I choose from the beginning mm or cm.

    My concern is that Artrage behavior is different than in any other similar software tested. And unfortunately I often need to switch from an application to another.

    thank you
    Last edited by steph; 02-02-2016 at 10:41 AM.

  6. #6
    Join Date
    Mar 2006
    Location
    New Zealand
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    3,140
    Okay, I think I understand.

    The image in the first post isn't centered, you've created an image 603 pixels tall so the borders above and below the green rectangles are different. Hence, when you crop in any application, it's not necessarily going to be centered. In Photoshop, if I set canvas size to 640 x 480 with that image, I get a white line at the top because there's one pixel more on top than there is below and that breaks the centering. This is expected, if you crop that image to 640 x 480 in ArtRage the white line doesn't appear at the top because we're handling the rounding of your half pixel differently.

    Your second post, the image you created in Photoshop, isn't actually the same layout - It's the same height, but the bottom border has 1 pixel more than the top border rather than the other way around. Flip it vertically in Photoshop so that the borders are the same as the image in your first post (1 pixel more on the top) then crop in Photoshop and you get the same result as you do with the first image. If you then take the flipped image and import that in to ArtRage and crop you get the expected result, no yellow line at the base, same as with your first image. The whole thing is made slightly harder to see because your second image (green on yellow) also has a third colour in there, I assume a partially transparent edge on the green rectangle made it slightly off-green which exacerbated the yellow line issue you were seeing.

    So, it's not a matter of inaccuracy in either app, it's just about whether the rounding balances to the previous or next pixel when dealing with sub pixel values. In this case, setting canvas size to 640 x 480 in ArtRage actually gives you the better result, the complete green rectangle, whereas in Photoshop it doesn't but that's only because our rounding happens to center it correctly when your borders are uneven with a larger border on top. Neither is incorrect because the layout of the image makes it impossible to automatically decide where to center the crop.

    My suggestion is, if you're going to crop an image by adjusting canvas size and letting the app decide where to center the result, make sure that the borders around your crop area are even left/right and top/bottom otherwise any app is going to make a decision about which border to crop more of, and it may cause problems.
    Matt
    ArtRage UI
    Ambient Design.

  7. #7
    Join Date
    Jun 2007
    Posts
    302
    "I've tried with sub pixel placement on/off"

    Matt I have looked through the manual and can not find a reference to turning sub pixel placement on/off and can't find it anywhere in the interface. How is this done?

  8. #8
    Join Date
    Mar 2006
    Location
    New Zealand
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    Sub pixel placement is in the Advanced section of the Preferences panel (AR4.5). However, now that I understand what you're seeing it's not relevant here.
    Matt
    ArtRage UI
    Ambient Design.

  9. #9
    Join Date
    Feb 2014
    Posts
    4
    Many thanks for your explanations. I have painted a boardgame with Artrage and the game box is framed with Photoshop with very precise dimensions so I will need to carefully check the centering of the prints.

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