Author Topic: Size/Resolution  (Read 1144 times)

pedwards2932

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Size/Resolution
« on: October 22, 2018, 04:43:33 AM »
I think I had asked about this before but still need some clarity.   I was designing a decal sheet in Inkscape and had the decals precisely sized for the airplane I am building.  When I printed it from Inkscape it was exactly the correct size.  When I pulled it into T2 and traced for cutting out the decals the cut was much smaller than the print.  I remembered from before that I had to change the DPI.  The last time I did I had to bump it to 256 dpi.  On this one 256 was not large enough it turns out it was about 500 dpi so I am guessing by multiples it should be 512.  I found that Inkscape allows you to change the DPI on a png pretty easily.  My question is how do you know what factor of resolution you need.  I just did this by guessing then running the cut to see if it was correct.  Also is there any "undo" function in the trace function on T2.....occasionally you need to undo and retry but I couldn't figure out how....I don't think control Z works.

Zax

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Re: Size/Resolution
« Reply #1 on: October 22, 2018, 05:17:44 AM »
T2Laser supports 2 different ways of dealing with images, the default is to use the pixels.

So if you export an image with 254dpi and import it to T2Laser at 0.1 resolution the size would be correct (since 10 dots per mm is the same as 254 dots per inch). If you are using a different resolution then you would set the export dpi to match, e.g. 127dpi for 0.2 resolution.

The other method (when Use Image DPI is enabled) sets the resolution automatically to match your image, so if your image is exported at 300 dpi the resolution would be changed to 0.085 which results in the image being the same size as it was saved. Of course this probably won't engrave very well since the laser would be repeating over the same area. This works if you export at a valid resolution for the engraver rather than the typical defaults which are for laser printers or screen resolution.

In most cases you can just click the trace icon again to "undo" it and start over, if using manual trace the delete key removes the last trace (you can press it multiple times to keep stepping back and erasing the traced lines).

ggallant571

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Re: Size/Resolution
« Reply #2 on: October 22, 2018, 05:54:34 AM »
I do everything at 256dpi. My host software is more primitive than T2 and does not provide and option to do otherwise.
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pedwards2932

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Re: Size/Resolution
« Reply #3 on: October 22, 2018, 06:34:28 AM »
When I did it at 256 it was about 1/2 the size of the Inkscape print.  500 dpi was about the same as the print so figuring in multiples probably should be 512.  I need precision since I am using the trace to cut these out exactly on their outlines. In the svg file in Inkscape I can export as a png and pretty much select any DPI.  So what I did was try different resolutions then print the png until the size was correct.  On T2 it is set to default 0.2.   When I made the decal set I just pulled in an insignia that was an svg and resized it to fit the plane.  The lettering and numerals were just from a military font.  I guess I just am not getting why if I print them from Inkscape they are exactly correct and when I pulled them into T2 it was so different that I had to play around with the resolution to get it right.

I can work with it the way it is......also be interested to know how you get exact registration on the cut so it will cut exactly on the decals.  I usually zero the laser by moving it fully down and to the left until it stops....I haven't set up zeroing switches.  Is there a way to put in markers on the page that you line the laser up with?

Zax

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Re: Size/Resolution
« Reply #4 on: October 22, 2018, 07:17:11 AM »
The reason it prints correctly in Inkscape is because it has physical dimensions, when you save an image it is stored as pixels (sometimes with an embedded DPI which can be used to translate back to the physical size, which is what T2Laser does if you enable that option - not recommended).

T2Laser / Grbl uses metric units, so a DPI of 254 = 10 DPmm = Resolution of 0.1mm.

If you are using 0.2 resolution then save the image at 127 DPI and it will load the correct size. If this isn't working as expected then the problem is with the export from Inkscape, load the image back to Inkscape or another image viewer and check the pixel count.

Since you are going to trace these you could also do that in Inkscape and save a DXF, this format uses physical dimensions (mm) and doesn't rely on scaling.

pedwards2932

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Re: Size/Resolution
« Reply #5 on: October 22, 2018, 07:57:11 AM »
Ok I pulled the decal.svg into Inkscape and set resolution to 254 dpi and the size is 2100 x 2970 and created a png.  I pulled this into GIMP and verified size and resolution and it is 10px/mm.  So now I will set T2 to 0.1 and it should work.  Will try it when I get home.  If that works then all should be okay except figuring out the best way to check that I have the alignment correct.

Zax

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Re: Size/Resolution
« Reply #6 on: October 22, 2018, 08:03:58 AM »
T2Laser will load it as 210 x 297 mm if you use 0.1 resolution, since it needs 10 pixels to make 1 mm.

pedwards2932

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Re: Size/Resolution
« Reply #7 on: October 22, 2018, 09:32:50 AM »
So if I want it to load in T2 at the exact size then I should use 1.0 resolution? or should I change the png resolution?  I need T2 to be the same dimensions as the png.

Zax

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Re: Size/Resolution
« Reply #8 on: October 22, 2018, 09:47:08 AM »
You need to export and import at the same resolution.

Physical size is irrelevant as images are based on pixels (not real world dimension). If you specify the same resolution those pixels will be the same size and thus the resulting image will the the same size.

pedwards2932

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Re: Size/Resolution
« Reply #9 on: October 22, 2018, 10:41:01 AM »
I'm going to just try it when I get home and see what it does.....the math makes my head hurt

pedwards2932

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Re: Size/Resolution
« Reply #10 on: October 23, 2018, 10:05:19 AM »
Ok I think I have it figured out.....I was setting the size of the paper to 8 x 11.5 and this was causing some issues in Gimp.  I set the size to the content and exported the png to Gimp at 256 dpi and according to print output it is 186.10 mm by 128.2 mm resolution 10 px/mm and when I brought it into T2 at 0.1 resolution the size was identical.

The precision for making these cuts pretty much has to be spot on or the decals won't be cut out correctly.  Is there any way to use the laser to accurately align the paper so it will be in the correct alignment for the cut?

ggallant571

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Re: Size/Resolution
« Reply #11 on: October 23, 2018, 11:51:23 AM »
Homing switches and alignment braces. I set the back off for the homing switches to 5mm. That way  it can go slightly negative. Then I screw a piece (or pieces) of scrap 3mm wood to the waste-board. Then set machine at the HOME position and use the laser to cut the X & Y axis. After a while this edge gets ruined and I do it again.  Thinking about it, I should start with a 10mm offset and just re-cut the axis with a smaller offset.

On a machine without homing switches I use a right angle aluminum rod on just the Y-axis. Not as exact but this machine can handle long pieces of wood that slide under the X rails.

For etching slate I have a 100x100mm (4x4 inch) square etched in the waste-board. Then I eyeball center the tile and jog the laser to the lower left corner and call that 0,0. Works good enough.


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Zax

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Re: Size/Resolution
« Reply #12 on: October 23, 2018, 04:18:46 PM »
I use home switches which set an accurate 0,0 and then the preset locations menu for different "jigs".

These jigs are alignment marks or fixtures to hold different items you commonly engrave. A basic straightedge or square (90 degree ruler) would probably be sufficient in your case. You could manually move to this start point but it's less accurate and more time consuming than having home switches.

pedwards2932

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Re: Size/Resolution
« Reply #13 on: October 25, 2018, 03:40:06 AM »
I am going to look into getting homing switches but that may take a while.  I have a couple of questions that may solve this for me.  What are the units of measure in the x/y T2 in the laser control page?  Pixels or mm?  What are the increments of motion for the jog? mm?  Is it possible to move the laser to an exact coordinate in the controls or do you just have to use jog to get it where you want it?

My thought here is I can put a graphic plus sign at specific points on the decal image then when I move the laser to those coordinates I could use a weak laser pulse to make sure it is dead center on the plus sign.  If I put 3 plus signs in the image and align all three it should be dead on aligned.  This is the way it is done in the craft cutter software that I have.

Zax

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Re: Size/Resolution
« Reply #14 on: October 25, 2018, 05:28:35 AM »
Everything is in mm as that's what Grbl uses.

You can use the custom buttons or presets to set specific moves but without home switches there's no reference. In this case you would need to jog to the start position and set 0,0 manually, there's no way you can click on the graphics and move to a location when the laser doesn't know where it is now. I suspect your craft cutter homes itself or has some other method to know it's position.