Crop

Cropping refers to the removal of the outer parts of an image to improve framing, changing the framesize (also called image or storage aspect ratio). See Wikipedia: Cropping.

Syntax and Parameters

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)

int  left, top =

Cropping of the left and top edges respectively, in pixels.

• See crop modulo restrictions below.

int  width, height =

int  right, bottom =

The third and fourth arguments have different names or aliases, depending on their value:

• If  > zero, these set the width and height of the resulting clip.
• If <= zero, they set the cropping of the right and bottom edges respectively.

Note, there are certain limits:

• clip.Width must be >= (left + width)
• clip.Width must be >  (left + right)
• clip.Height must be >= (top + height)
• clip.Height must be >  (top + bottom)

 ...otherwise it would enlarge ("un-crop") the clip, or reduce width or height to 0, which is not allowed.

• See crop modulo restrictions below.

int  count =

The alternative CropBottom syntax was created to crop garbage off the bottom of a clip captured from VHS tape. It removes count lines from the bottom of the frame.

• See crop modulo restrictions below.

bool  align = false

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 you may get strange artifacts. For a complete discussion on this, see Filter_SDK/Data_storage

Colorspace	Width	Height
		progressive video	interlaced video
RGB	no restriction	no restriction	mod-2
YUY2	mod-2	no restriction	mod-2
YV12	mod-2	mod-2	mod-4
YV411	mod-4	no restriction	mod-2
YV16	mod-2	no restriction	mod-2
YV24	no restriction	no restriction	mod-2

Note: The resize functions allow fractional (sub-pixel) cropping of the frame, with a weighting being applied to the edge pixels being resized. This may be used if the mod-n format dimension restriction of Crop are inconvenient.

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)

Examples

• 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)

• Using the alternative syntax with negative (or zero) values in the last two parameters, they become offsets, VirtualDub-style:

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

# crop 8 off the left, 2 off the top, 9 off the right, and 4 off the bottom
Crop(8, 2, -9, -4)