Crop

Revision as of 17:02, 9 May 2013

Crop(clip clip, int left, int top, int width, int height, bool align)
Crop(clip clip, int left, int top, int -right, int -bottom, bool align)
CropBottom(clip clip, int count)

So if you have a 640x480 video, and you want to crop 8 off the left, 2 off the top, 9 off the right, and 4 off the bottom, you would use this:

Crop(8,2,-9,-4)

Crop crops excess pixels off of each frame. If your source video has 720x480 resolution, and you want to reduce it to 352x240 for VideoCD, here's the correct way to do it:

# Converts CCIR601 to VCD, preserving the correct aspect ratio:
ReduceBy2()
Crop(4, 0, 352, 240)

In AviSynth v2.01 and later, the alternative syntax using negative (or zero) values is possible. In this case, the last two parameters are also treated as offsets, VirtualDub-style. For example:

# Crops 16 pixels all the way around the picture, regardless of image size:
Crop(16, 16, -16, -16)

The alternative CropBottom syntax is useful for cropping garbage off the bottom of a clip captured from VHS tape. It removes count lines from the bottom of each frame.

Memory alignment

In v2.53 an option align (false by default) is added:

Cropping an YUY2/RGB32 image is always mod4 (four bytes). However, when reading x bytes (an int), it is faster when the read is aligned to a modx placement in memory. MMX/SSE likes 8-byte alignment and SSE2 likes 16-byte alignment. If the data is NOT aligned, each read/write operation will be delayed at least 4 cycles. So images are always aligned to mod16 when they are created by AviSynth.

If an image has been cropped, they will sometimes be placed unaligned in memory - "align = true" will copy the entire frame from the unaligned memory placement to an aligned one. So if the penalty of the following filter is larger than the penalty of a complete image copy, using "align=true" will be faster. Especially when it is followed by smoothers.

Crop restrictions

In order to preserve the data structure of the different colorspaces, the following mods should be used. You will not get an error message if they are not obeyed, but it may create strange artifacts. For a complete discussion on this, see DataStorageInAviSynth ...

Note: The resize functions optionally allow fractional cropping of the input frame. This results in a weighting being applied to the edge pixels being resized. These options may be used if the mod-n format dimension restriction of crop are inconvienient. In summary: “For cropping off hard artifacts like VHS head noise or letterbox borders always use Crop. For extracting a portion of an image and to maintain accurate edge resampling use the resize cropping parameters.” (Doom9 thread)