How to Generate Zoom Levels 6-12 from Existing Zoom Level 5 XYZ Tiles (.png) Without Losing Image Quality?

[nisha54321256]

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I already have zoom level 5 XYZ tiles in .png format, and I want to generate zoom levels 6 to 12 using these existing tiles. I don’t have any raster files—only the tiles. I'm using Python, QGIS, and OpenLayers 3 (OL3).
Problems I'm Facing:

Image quality becomes blurry when generating higher zoom levels.
Y tile numbering is incorrect (tiles are not arranged properly).

Below is my Python script that reads zoom 5 tiles, splits them into 4 subtiles, and saves them as higher zoom levels. However, I'm facing the above issues. If anyone has a solution, please help me improve it!
`import os
from PIL import Image

input_folder = "5" # Folder where zoom level 5 tiles are stored
output_base = "zoom1_12" # Base folder for output tiles (6-12)

def generate_tiles(input_folder, output_base, start_zoom=5, max_zoom=12):
for z in range(start_zoom, max_zoom): # Loop from zoom 5 to 11
input_zoom_path = os.path.join(output_base, str(z)) # Current zoom level folder
output_zoom_path = os.path.join(output_base, str(z + 1)) # Next zoom level folder

    if not os.path.exists(input_zoom_path):
        continue

    # Loop through X directories
    for x_folder in os.listdir(input_zoom_path):
        x_path = os.path.join(input_zoom_path, x_folder)
        if not os.path.isdir(x_path):
            continue  # Skip non-folder files

        x = int(x_folder)  # Convert folder name to integer (x-coordinate)

        # Loop through Y files inside X directory
        for file in os.listdir(x_path):
            if not file.endswith(".png"):
                continue  # Skip non-PNG files
            
            y = int(file[:-4])  # Extract y-coordinate from filename
            img_path = os.path.join(x_path, file)

            try:
                # Open image and upscale
                img = Image.open(img_path).convert("RGBA")  # Force PNG mode
                img = img.resize((512, 512), Image.LANCZOS)  # Upscale to 512x512
            except Exception as e:
                print(f"Corrupt image at {img_path}, generating blank tile.")
                img = Image.new("RGBA", (512, 512), (0, 0, 0, 0))  # Transparent image

            # Generate four subtiles in XYZ format
            subtiles = [
                ((0, 0, 256, 256), (2 * x, 2 * y)),       # Top-left
                ((256, 0, 512, 256), (2 * x + 1, 2 * y)), # Top-right
                ((0, 256, 256, 512), (2 * x, 2 * y + 1)), # Bottom-left
                ((256, 256, 512, 512), (2 * x + 1, 2 * y + 1)), # Bottom-right
            ]

            # Save subtiles in the next zoom level
            for (crop_box, (new_x, new_y)) in subtiles:
                subtile = img.crop(crop_box)

                # Save as /{z}/{x}/{y}.png (XYZ format)
                output_x_folder = os.path.join(output_zoom_path, str(new_x))
                os.makedirs(output_x_folder, exist_ok=True)  # Create folder if needed

                output_file = os.path.join(output_x_folder, f"{new_y}.png")
                subtile.save(output_file, "PNG")

            #print(f" Zoom {z + 1}: ({x},{y}) → 4 subtiles at ({2*x}, {2*y})")

Run the function

generate_tiles(input_folder, output_base)
`

What I Need Help With:

How can I improve image quality to prevent blurriness?
How do I fix incorrect Y tile naming in the XYZ format?

If anyone has a better approach, please share your suggestions. Thanks in advance!

[Rahul-2025-prog]

Your approach correctly generates subtiles, but the blurriness is due to upscaling before cropping. Instead, crop first, then resize to maintain sharpness. Also, ensure Y tile numbering follows XYZ format by flipping it using y = (2 ** z - 1) - y. Let me know if you need refinements!

[nisha54321256]

Even though the tiles are being generated correctly, they appear blurry when displayed on a map.
`import os
from PIL import Image

Paths

input_folder = "/home/test/data/t3" # Folder where zoom level 5 tiles are stored
output_base = "/home/test/data/t3" # Base folder for output tiles (6-12)

Renaming function

def rename_tile(y):
return y + 1 if y % 2 == 0 else y - 1

def generate_tiles(input_folder, output_base, start_zoom=5, max_zoom=12):
for z in range(start_zoom, max_zoom):
input_zoom_path = os.path.join(output_base, str(z))
output_zoom_path = os.path.join(output_base, str(z + 1))

    if not os.path.exists(input_zoom_path):
        continue

    for x_folder in os.listdir(input_zoom_path):
        x_path = os.path.join(input_zoom_path, x_folder)
        if not os.path.isdir(x_path):
            continue  

        x = int(x_folder)  

        for file in os.listdir(x_path):
            if not file.endswith(".png"):
                continue  

            y = int(file[:-4])  
            img_path = os.path.join(x_path, file)

            try:
                img = Image.open(img_path).convert("RGBA")  
                img = img.resize((1024, 1024), Image.LANCZOS)  
            except Exception:
                img = Image.new("RGBA", (1024, 1024), (0, 0, 0, 0))  

            subtiles = [
                ((0, 0, 512, 512), (2 * x, 2 * y)),       
                ((512, 0, 1024, 512), (2 * x + 1, 2 * y)), 
                ((0, 512, 512, 1024), (2 * x, 2 * y + 1)), 
                ((512, 512, 1024, 1024), (2 * x + 1, 2 * y + 1)), 
            ]

            for (crop_box, (new_x, new_y)) in subtiles:
                subtile = img.crop(crop_box)
                subtile_256 = subtile.resize((256, 256), Image.LANCZOS)  

                renamed_y = rename_tile(new_y)

                output_x_folder = os.path.join(output_zoom_path, str(new_x))
                os.makedirs(output_x_folder, exist_ok=True)  

                output_file_256 = os.path.join(output_x_folder, f"{renamed_y}.png")
                subtile_256.save(output_file_256, "PNG")

generate_tiles(input_folder, output_base)
import os
from PIL import Image

Paths

input_folder = "/home/test/data/t3" # Folder where zoom level 5 tiles are stored
output_base = "/home/test/data/t3" # Base folder for output tiles (6-12)

Renaming function

def rename_tile(y):
return y + 1 if y % 2 == 0 else y - 1

def generate_tiles(input_folder, output_base, start_zoom=5, max_zoom=12):
for z in range(start_zoom, max_zoom):
input_zoom_path = os.path.join(output_base, str(z))
output_zoom_path = os.path.join(output_base, str(z + 1))

    if not os.path.exists(input_zoom_path):
        continue

    for x_folder in os.listdir(input_zoom_path):
        x_path = os.path.join(input_zoom_path, x_folder)
        if not os.path.isdir(x_path):
            continue  

        x = int(x_folder)  

        for file in os.listdir(x_path):
            if not file.endswith(".png"):
                continue  

            y = int(file[:-4])  
            img_path = os.path.join(x_path, file)

            try:
                img = Image.open(img_path).convert("RGBA")  
                img = img.resize((1024, 1024), Image.LANCZOS)  
            except Exception:
                img = Image.new("RGBA", (1024, 1024), (0, 0, 0, 0))  

            subtiles = [
                ((0, 0, 512, 512), (2 * x, 2 * y)),       
                ((512, 0, 1024, 512), (2 * x + 1, 2 * y)), 
                ((0, 512, 512, 1024), (2 * x, 2 * y + 1)), 
                ((512, 512, 1024, 1024), (2 * x + 1, 2 * y + 1)), 
            ]

            for (crop_box, (new_x, new_y)) in subtiles:
                subtile = img.crop(crop_box)
                subtile_256 = subtile.resize((256, 256), Image.LANCZOS)  

                renamed_y = rename_tile(new_y)

                output_x_folder = os.path.join(output_zoom_path, str(new_x))
                os.makedirs(output_x_folder, exist_ok=True)  

                output_file_256 = os.path.join(output_x_folder, f"{renamed_y}.png")
                subtile_256.save(output_file_256, "PNG")

generate_tiles(input_folder, output_base)
`
What would be the best way to maintain image sharpness while generating subtiles? Any insights or suggestions would be greatly appreciated! 🚀