time fetch and crop for image read

fetch-vs-crop.py

#!/usr/bin/python3

import random
import time
import sys

import pyvips

if len(sys.argv) != 4:
    print("usage: ./fetch-vs-crop.py IMAGE SIZE N-TILES")

size = int(sys.argv[2])
n_tiles = int(sys.argv[3])
print(f"{n_tiles} tiles, each {size}x{size} pixels")

image = pyvips.Image.new_from_file(sys.argv[1])

print(f"random read")

start = time.time()
for _ in range(n_tiles):
    x = random.randint(0, image.width - size)
    y = random.randint(0, image.height - size)
    tile = image.crop(x, y, size, size)
    # necessary to force evaluation
    tile = tile.copy_memory()
end = time.time()
print(f"crop: time per tile = {1000 * (end - start) / n_tiles:.2f}ms")

start = time.time()
region = pyvips.Region.new(image)
for _ in range(n_tiles):
    x = random.randint(0, image.width - size)
    y = random.randint(0, image.height - size)
    p = region.fetch(x, y, size, size)
end = time.time()
print(f"fetch: time per tile = {1000 * (end - start) / n_tiles:.2f}ms")

print(f"row-major sequential read")

image = pyvips.Image.new_from_file(sys.argv[1])

start = time.time()
tile_number = 0
for y in range(0, image.height - size, size):
    for x in range(0, image.width - size, size):
        tile = image.crop(x, y, size, size)
        # necessary to force evaluation
        tile = tile.copy_memory()

        tile_number += 1
        if tile_number > n_tiles:
            break

    if tile_number > n_tiles:
        break
end = time.time()
print(f"crop: time per tile = {1000 * (end - start) / n_tiles:.2f}ms")

start = time.time()
tile_number = 0
region = pyvips.Region.new(image)
for y in range(0, image.height - size, size):
    for x in range(0, image.width - size, size):
        x = random.randint(0, image.width - size)
        y = random.randint(0, image.height - size)
        p = region.fetch(x, y, size, size)

        tile_number += 1
        if tile_number > n_tiles:
            break

    if tile_number > n_tiles:
        break
end = time.time()
print(f"fetch: time per tile = {1000 * (end - start) / n_tiles:.2f}ms")

print(f"column-major sequential read")

image = pyvips.Image.new_from_file(sys.argv[1])

start = time.time()
tile_number = 0
for x in range(0, image.width - size, size):
    for y in range(0, image.height - size, size):
        tile = image.crop(x, y, size, size)
        # necessary to force evaluation
        tile = tile.copy_memory()

        tile_number += 1
        if tile_number > n_tiles:
            break

    if tile_number > n_tiles:
        break
end = time.time()
print(f"crop: time per tile = {1000 * (end - start) / n_tiles:.2f}ms")

start = time.time()
tile_number = 0
region = pyvips.Region.new(image)
for x in range(0, image.width - size, size):
    for y in range(0, image.height - size, size):
        x = random.randint(0, image.width - size)
        y = random.randint(0, image.height - size)
        p = region.fetch(x, y, size, size)

        tile_number += 1
        if tile_number > n_tiles:
            break

    if tile_number > n_tiles:
        break
end = time.time()
print(f"fetch: time per tile = {1000 * (end - start) / n_tiles:.2f}ms")