如何将多张已有PNG图合并为指定布局？含分辨率优化问题

Hey there! Let's break down how to solve both your layout and resolution issues with your R plots—here's a step-by-step guide tailored to your needs:

First, we'll use the png package to read your existing PNG files and convert them into grid-compatible grob objects, then use gridExtra to arrange them into your desired 2-row layout (two plots side-by-side on the top, one full-width plot on the bottom):

library(png)
library(gridExtra)
library(grid)

# Read your PNG files
img1 <- readPNG("png1.png")
img2 <- readPNG("png2.png")
img3 <- readPNG("png3.png")

# Convert images to grid grob objects
g1 <- rasterGrob(img1)
g2 <- rasterGrob(img2)
g3 <- rasterGrob(img3)

# Define the layout matrix: 2 rows, first row has 2 columns, second row spans 2 columns
layout_matrix <- rbind(c(1, 2), c(3, 3))

# Arrange the plots
grid.arrange(g1, g2, g3, layout_matrix = layout_matrix)

This will produce the exact layout you're targeting—top row with two side-by-side plots, bottom row with a single plot that takes up the full width.

The low-resolution issue happens because the default preview in RStudio or direct grid.arrange output uses low default resolution. To fix this, we'll explicitly create a high-resolution output device before rendering the merged plot:

# Set up a high-resolution PNG output device
# Adjust width/height based on your original image dimensions (in pixels)
# res = 300 gives print-quality resolution; match your original images' DPI if needed
png("merged_high_res.png", 
    width = 1600,  # Example: if each top plot is 800px wide, total width is 1600
    height = 1200, # Example: 600px for top row + 600px for bottom plot
    res = 300)

# Render the arranged plot within the device
grid.arrange(g1, g2, g3, layout_matrix = layout_matrix)

# Close the output device to save the file
dev.off()

For even more precision (to perfectly match your original image dimensions), use the pixel dimensions of your input PNGs directly:

# Get original image dimensions (width in pixels)
width_img1 <- ncol(img1)
width_img3 <- ncol(img3)
height_img1 <- nrow(img1)
height_img3 <- nrow(img3)

# Set output dimensions to match total layout size
png("merged_high_res_exact.png",
    width = width_img1 * 2,  # Two top plots side-by-side
    height = height_img1 + height_img3,  # Top row height + bottom row height
    res = 300)

grid.draw(arrangeGrob(g1, g2, g3, layout_matrix = layout_matrix))
dev.off()

This method ensures the merged plot retains the same high resolution as your original PNGs, matching the quality you referenced.

内容的提问来源于stack exchange，提问作者lanselibai