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I'm working on this illustration project. It is a VW samba bus from the 50'slike the image below. The part I'm struggling the most with are the air ventilation grills seen in the backside of the bus.

Sketch of the problem

I don't know how to create that shade or hollow effect as depicted in the sample. Because in the original image this part is blurry I cannot distinguish exactly where is the shadow or how the light enters in the object so it creates that hollow effect.

In my research by comparing with pictures of samba VW buses around I found these samples of engine vent trims.

Ventilation trim one

Ventilation trim two

Ventilation trim three

But from the different examples it seems that the vent trims protrude or stick out from the surface. Is it possible to use these ideas the replicate the part which I'm struggling?

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The hole in a real VW is a common sheet metal trick named "Rounded Louver". Realistic drawing of a louver needs complex shading. It's easiest if you can accept it as bent inwards, without shiny glosses and without a chrome edge list. Here are three of them on a flat surface:

enter image description here

One louver is made by interpolating between a black line and blue edge curve (=a line + two quarter circles joined)

enter image description here

The interpolation in Inkscape is Extensions > Modify Path > Interpolate.

The result can be a mess if the line and the edge curve have different directions. Flip the line horizontally if needed.

As you see the top edge isn't perfectly straight. Fix it by inserting a decorative list or simply a colored line which covers the edge.

I had 30 steps in the interpolation. Less can be enough if the line and the curve are wider. If you rasterize the louver shape the number of steps can be even higher without making the system slow.

Much simpler construction is possible if you can accept less realistic version which doesn't have rounded bottom curve :

enter image description here

The louvers are grey groups with blending mode Multiply. They make the paint surface darker.

The job starts again from an union of a rectangle and two quarter circles. Have 2 copies. The red version is used only as clipping path, it's color is meaningless. Fill the other with horizontal 4-stop grey gradient dark-light-light-dark:

enter image description here

Insert a blurred black rectangle on the top edge of the gradient shape. It will be the shadow:

enter image description here

Place the red version on the previous shapes, select all and apply Object > Clip > Set to get rid of the blurred extras. Make a group and give to it blending mode Multiply:

enter image description here

As you see the clipped shapes have a special "clip" symbol in the Layers panel. They can still be selected and adjusted freely in the layers panel. You very likely must adjust the blur of the shadow rectangle and the gradient when you have placed the louver onto the final surface.

Protruded louvers cannot be drawn by somehow reversing the colors. It doesn't look right because the major part of the louver should have same color as the surrounding surface. Here's a simple working version which can be used if the main paint color isn't black. It starts from the same basic forms as the previous version but it's colored differently:

enter image description here

1-2. place quarter circles at the ends of a rectangle and make Path > Union

  1. Fill with car's main paint color

  2. Change the fill to 4 stop horizontal gradient which has darker ends. Step 3 is useful because the stops can get easily the main paint color. Only remove the color transparency after you insert the stops by double-clicking. Then adjust the end stops darker

The louver cannot be seen properly on the final surface as is, it needs 2 additions:

  • a thin stroke of the same color as the dark ends of the fill gradient (see NOTE1)
  • a shadow

In the next image 4 louvers are placed on a flat surface. In the bottom you can see the shadow. It's actually the louver shape, but

  • flipped and squeezed vertically
  • with no stroke
  • with black fill
  • blurred and
  • sent behind the louver

enter image description here

NOTE1 The stroke starts to look wrong if the image is so big that one sees the stroke is uniform. Blurring helps, but the stroke should be a separate curve if it's blurred. Harmfully there's no gradients along a path available. The stroke is not needed if the mid gradient stops of your louver are a little lighter than the main paint surface. But that creates a gloss. The same light condition should be used consistently in the whole car. It can fit because in your drawing example there's glosses caused by somehow downwards directed light. I tried to avoid glosses to keep things easily manageable.

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  • Nice answer but there are a couple of things which I need some explaining. Why did you mentioned this: As you see the top edge isn't perfectly straight. Fix it by inserting a decorative list or simply a colored line which covers the edge. What did you meant?. I don't know how to achieve the addition of a four step gradient, can you show an example of how did you this?. When you mentioned Set to get rid of the blurred extras. Make a group and give to it blending mode Multiply will this add more objects to the group? – Chris Steinbeck Bell Sep 1 at 22:38
  • When you overlap this object which has been blended in layers over the surface which is intended to be used will it be transparent in the background?. I mean will it had the background color which I choose for the car?. – Chris Steinbeck Bell Sep 1 at 22:42
  • Not perfectly straight: In high zoom you should see the ends of the black line do not get transformed to blue exactly horizontally. Insert a blue rectangle over the top edge of the louver hole to hide the error. Multistop gradients can be achieved by clicking new stops with the gradient adjusting tool. Get the manual and watch tutorials, it's basic techniques, not mine. "Object > Clip > Set" That one is a menu operation which should be used to make a clipping path which hides the unwanted part. Multiply is object blending mode. You should see where multiply is in the dialog box in my image. – user287001 Sep 1 at 22:52
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I think it depends on the art style you want to go for. If you're doing a flat design style like in your first image, then you can replicate the vents by using 2 rounded-rectangles that overlap.

vent example

If you're going with the other images, then it would be a similar process. Draw the shapes and use gradients.

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  • It's a nice suggestion but I think what I intended to replicate was a bit more elaborate than overlaping two rounded rectangles. But if you look them at a distance gives some sort of the illusion that it was intended. In your example doesn't seem to be a gradient involved, would it give it more realism?. – Chris Steinbeck Bell Sep 1 at 22:25
  • @ChrisSteinbeckBell It depends on the art style that you want to achieve. In the question above, the first image shows a flat design style and I do not think there are any gradients used in the first image. In my image, the zoom is probably like 600%-800% so the shadow effect would look better at 100%. If you want a much more realistic approach, then check out the answer from user287001. – AndrewH Sep 1 at 22:41

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