I want to make a mosaic, SA stretch all the images to brush size,
How Can I force it to use the original size of each image not stretched size?
Path start regionization used with region fill as brush pen mode is not going to respect the original brush image aspect ratio in most cases. This is because the generated regions adapt to the source image characteristics, and region fill as brush pen mode overrides the source brush with a temporary region, and any images used as a fill will auto-size to fill the rectangular boundary area associated with the temporary region being filled.
Now some of the path start regionization algorithms break down the canvas into sub-divided areas based on splitting the canvas repeatedly. So you could be smart about sizing the canvas accordingly, and building your image folder brush accordingly, so that the math works out and the aspect ratio is preserved for the sub-divided areas. Takes some planning and forethought, but is achievable.
Of course if you use normal tiling with normal non region fill as brush pen modes (like interactive pen for example), then single brush nibs will be laid down (assuming max path length is set to 1), and then the aspect ratio of any images used in an image brush will be preserved.
You can do sub tilig with this approach, but you need to do multitple passes with to pull it off.
This is still not understandable. Charis achieves this in all his work. There most be a way beyond the technical explanation you have shared above. I will appreciate you kindly share more straight forward approach.
Some works of charis with unstretched images
I also noticed that no matter how I render the canvas, the images are pixellated which doesn't happen on Charis project. I have used a canvas size of 3600px x 3600px with a DPI of 300, 600 & 1200px experimentally yet no visible results. My image folder containes images of at least 3200px x 2000px. Please how can I fix this.
Than you in anticipation
Don't confuse path start regionization mosaic presets with single paint nib tiling mosaic presets.
If you are using region fill as brush pen mode to fill regions with an image brush, then the image is scaled to fit the size of the region. If the image was smaller then this size, then it would be interpolated to fit. So you'd need to make sure your image was large enough to not scale up if you want to avoid interpolation artifacts in the region image fill.
Single nib tiling (typically in a grid path start scan pattern), just lays down a single brush nib of the image. It might be scaled down in size by settings in the Brush Modulation control panel, but it's not going to scale up unless you have a max brush size in there larger than 100%.
Charis typically uses single brush nib tiling in grid patterns. So if you are using path start regionization presets, you are doing something completely different.
If you don't understand paint synthesizer pen modes, here's a tip on them.
Again, path start regionization mosaic presets are typically using path start regionization in combination with region fill as brush pen mode.
Region fill as brush pen mode makes a temporary brush source based on the region you manually paint or studio artist automatically generates.
Charis uses something like interactive pen mode with a path length of 1, so that a single paint nib is laid down at each start point. In combination with grid tiling path start generators, or grid tiling settings in the Path Application control panel.
Here are some blog posts that discuss different studio artist effects. If you click Mosaics to just get the mosaic articles, there are several in there on sub-nested tiling, which is what Charis typically does.
Here are a lot of article links that discuss how to duplicate the kind of thing Charis does with mosaics. Again, pay close attention to the articles on sub-nested tiling. He's doing it using grid tiling with single paint nibs composed of images (or movie brush frames since he started doing this before we had image folder brushes so a lot of his work used movie brushes). The article links above specifically point to this one on sub-nested tiling.