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A bitmap flip remaps pixel coordinates without changing the image’s width or height. For width W and height H, a left-right mirror uses dst[y][x] = src[y][W - 1 - x]; a top-bottom flip uses dst[y][x] = src[H - 1 - y][x]; applying both produces a 180-degree rotation of the rectangular pixel grid.
| Visual result | Mapping |
|---|---|
| Horizontal mirror (left ↔ right) | dst[y][x] = src[y][W - 1 - x] |
| Vertical flip (top ↔ bottom) | dst[y][x] = src[H - 1 - y][x] |
| Both axes | dst[y][x] = src[H - 1 - y][W - 1 - x] |
Horizontal versus vertical flipping
Use the visual result, not only an API’s name. A person facing right, an arrow pointing right, or readable text makes the difference obvious: a horizontal mirror reverses left and right, while a vertical flip puts the top at the bottom. Different libraries describe the operation by the image’s movement or by the axis of reflection, so their flags can look counterintuitive.
Flipping both axes is equivalent to a 180-degree rotation for a rectangular raster grid, although a library’s rotation operation may also involve metadata or canvas behavior.
Python with Pillow
Pillow provides explicit helpers and transpose constants. Each operation returns a new image; save it where you need it.
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from PIL import Image, ImageOps
with Image.open("input.png") as image:
left_right = ImageOps.mirror(image)
top_bottom = ImageOps.flip(image)
left_right.save("mirror.png")
top_bottom.save("vertical.png")The equivalent transpose calls are:
from PIL import Image
with Image.open("input.png") as image:
left_right = image.transpose(Image.Transpose.FLIP_LEFT_RIGHT)
top_bottom = image.transpose(Image.Transpose.FLIP_TOP_BOTTOM)See the Pillow ImageOps documentation and Image.transpose reference. Keep the original mode when transparency matters; converting RGBA to RGB can discard alpha.
OpenCV
OpenCV names its argument flipCode by the reflection axis: 0 reflects around the horizontal axis (top ↔ bottom), a positive value such as 1 reflects around the vertical axis (left ↔ right), and a negative value such as -1 reflects around both.
import cv2
image = cv2.imread("input.png", cv2.IMREAD_UNCHANGED)
horizontal = cv2.flip(image, 1) # left ↔ right
vertical = cv2.flip(image, 0) # top ↔ bottom
both = cv2.flip(image, -1) # both axes
cv2.imwrite("horizontal.png", horizontal)
cv2.imwrite("vertical.png", vertical)
cv2.imwrite("both.png", both)The same mapping in C++ is:
#include <opencv2/opencv.hpp>
cv::Mat image = cv::imread("input.png", cv::IMREAD_UNCHANGED);
cv::Mat horizontal, vertical, both;
cv::flip(image, horizontal, 1);
cv::flip(image, vertical, 0);
cv::flip(image, both, -1);
cv::imwrite("horizontal.png", horizontal);Consult the OpenCV array-operation documentation; the cited page is in the 4.13.0 documentation tree, not a guarantee about every installed build.
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ImageMagick from a shell
Current ImageMagick usage uses magick:
magick input.png -flop horizontal.png
magick input.png -flip vertical.png
magick input.png -flop -flip both.png-flop creates the left-right mirror, while -flip produces the top-bottom result. Command names on older installations can differ. The documented meanings are in the ImageMagick command-line options.
Browser JavaScript and Canvas
A canvas transformation changes how an image is rendered; it does not rewrite the source file. Save and restore the context so later drawing is unaffected.
function drawFlippedHorizontally(ctx, image, x, y, width, height) {
ctx.save();
ctx.scale(-1, 1);
ctx.drawImage(image, -(x + width), y, width, height);
ctx.restore();
}
function drawFlippedVertically(ctx, image, x, y, width, height) {
ctx.save();
ctx.scale(1, -1);
ctx.drawImage(image, x, -(y + height), width, height);
ctx.restore();
}For an image covering a canvas from its origin:
// left ↔ right
ctx.save();
ctx.scale(-1, 1);
ctx.drawImage(image, -canvas.width, 0);
ctx.restore();
// top ↔ bottom
ctx.save();
ctx.scale(1, -1);
ctx.drawImage(image, 0, -canvas.height);
ctx.restore();Negative coordinates compensate for the transformed coordinate system. MDN documents this behavior for scale(). Do not use negative dWidth or dHeight in drawImage() as a flip shortcut: MDN states that negative dimensions do not flip image pixels.
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To create a new PNG in the browser, draw the transformed result to a canvas and export it, for example with const blob = await new Promise(resolve => canvas.toBlob(resolve, "image/png"));.
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For tightly packed data, treat each pixel as a complete channel group. This RGBA example allocates a destination buffer:
def flip_horizontal(src, width, height, channels=4):
dst = bytearray(len(src))
stride = width * channels
for y in range(height):
for x in range(width):
source_offset = y * stride + x * channels
target_offset = y * stride + (width - 1 - x) * channels
dst[target_offset:target_offset + channels] =
src[source_offset:source_offset + channels]
return dst
def flip_vertical(src, width, height, channels=4):
dst = bytearray(len(src))
stride = width * channels
for y in range(height):
source_row = y * stride
target_row = (height - 1 - y) * stride
dst[target_row:target_row + stride] =
src[source_row:source_row + stride]
return dstThe generic mappings are:
horizontal: source_x = W - 1 - x, source_y = y
vertical: source_x = x, source_y = H - 1 - y
both: source_x = W - 1 - x, source_y = H - 1 - yIn-place swaps
A horizontal in-place algorithm swaps symmetric pixels in each row:
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for each row y:
for x from 0 to floor(W / 2) - 1:
swap(pixel[y][x], pixel[y][W - 1 - x])A vertical in-place algorithm swaps complete rows:
for y from 0 to floor(H / 2) - 1:
swap(row[y], row[H - 1 - y])Swap all channel values together, not just one byte. If rows contain padding, use the actual row stride rather than width × channels. A destination buffer is simpler when source and destination coordinates overlap.
Transparency, color, and EXIF orientation
Preserve channels
A flip moves every channel as one pixel. Preserve the source channel count and test transparent edges, where losing or mishandling alpha is easy to see. Premultiplied-alpha buffers used by graphics APIs require the same care. A basic pixel flip does not guarantee that color profiles or other metadata survive encoding, so choose a metadata policy deliberately.
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Normalize camera orientation
Photos can store unrotated pixels plus an EXIF Orientation tag. Normalize that orientation before applying your requested flip when you need visual coordinates to match what viewers display:
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- Load the image.
- Apply or normalize EXIF orientation.
- Perform the horizontal or vertical flip.
- Save with the metadata policy your application requires.
Pillow’s ImageOps.exif_transpose() applies the orientation and removes the orientation tag; see the Pillow documentation. Browser handling of EXIF orientation has varied historically, so do not assume every canvas workflow normalizes it identically; the MDN drawImage reference discusses this issue.
Quality and dimensions
A direct mirror is a pixel rearrangement and needs no interpolation. Blurring or other quality changes usually come from scaling, filtering, color conversion, repeated JPEG encoding, or incorrect alpha handling during a separate step. Width and height should remain unchanged; a changed dimension indicates an additional crop, resize, or canvas-layout operation.
Debugging checklist
- Use an asymmetric test image containing an arrow or text; symmetry can hide a reversed operation.
- Confirm the intended visual result and the coordinate formula.
- For OpenCV, remember
0is top-to-bottom and1is left-to-right. - After a negative canvas scale, draw at the required negative coordinate.
- Do not rely on negative
drawImage()dimensions. - Move complete pixels, preserve alpha, and account for row padding.
- Check EXIF orientation before comparing output with a photo viewer.
- Verify that a rendering transform was not mistaken for a modified image file.
- Confirm the output path, format, width, and height after saving.
Which approach should you use?
| Situation | Best fit |
|---|---|
| Python script processing ordinary image files | Pillow’s ImageOps.mirror() or ImageOps.flip() |
| Computer-vision or NumPy-style array pipeline | OpenCV’s cv2.flip() |
| Shell automation or batch conversion | ImageMagick magick with -flop or -flip |
| Browser display that leaves the source unchanged | Canvas transforms or CSS scaleX(-1)/scaleY(-1) |
| Raw framebuffer, texture, embedded, or systems code | Manual coordinate mapping or channel-aware swaps |
Use a tested library when it already owns decoding, encoding, metadata, and color handling. Implement the loop yourself when you control the buffer layout or need a dependency-free operation.
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