Just looking at your image, it seems like an AA (anti aliasing) issue. I would recommend you increasing the AA settings. Without the actual file it is hard to be more specific.
Run the _DocumentProperties command and select the “Rhino Render” tab. From there you can adjust the image size and antialiasing values (a higher value results in greater smoothing). You can also go to the “Mesh” tab and select the “Smooth & Slower” or “Custom” Render Mesh Quality if needing more control. Then run the _Render command to view the result and save out as an image file.
I changed the resolution to 1000 x 1000 pixels as you suggested but it slows down the rendering significantly (20 mins) but the image is much clearer !
You’re welcome. The render resolution can be whatever you’d like to best show your design (the 1000 x 1000 px size just happened to be the resolution of draft renders I was using on a project at the time). Your render time of 20 minutes does seem rather high considering there isn’t that much geometry in your scene (assuming it’s just the one chair from your screenshot). When using Rhino’s built-in render engine, your system’s processor is basically the hardware that dictates how slow or fast something will render. You might try unchecking the “Skylight” option as this increases render time but will also affect how your image looks.