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How to efficiently sum 4 x 8bit integers with ARM or NEON

Note: This was originally posted on 17th September 2010 at http://forums.arm.com

Hi,

I am trying to write an ASM function to shrink an 8-bit greyscale image by 4, so I need to get the sum of 4 bytes very quickly. From what I have read, NEON needs atleast 32-bit integers and VFP is for floats, so it looks like I should just stick with ARM (or Thumb-2) instructions.

But I'm just a beginner so I'm wondering if there is a more efficient method of summing 4 consecutive bytes than convert each byte to a 32bit int and then sum them (and then shift right to get the average).

Its for a Cortex-A8 (ARMv7-A), and the data is aligned to 32 bytes or whatever I want.

Cheers,
Shervin Emami
[url="http://www.shervinemami.co.cc/"]http://www.shervinemami.co.cc/[/url]

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Simon Craske over 12 years ago

Note: This was originally posted on 17th September 2010 at http://forums.arm.com

Assuming your four pixels are in a square, i.e.:

[font="Courier New"]+---+---+
|   A   |   B   |
+---+---+   --->    (A+B+C+D)/4
|   C   |   D |
+---+---+[/font]

Then the following [untested] Neon code might not be too far off optimal.

;; Average square of four pixels to single pixel. ;; Produces NxM pixel image from 2Nx2M pixel image. ;; Generates 16 output pixels per loop. ;; May over-read by upto 63 bytes. ;; May over-write by upto 15 bytes. ;; r0 = Input line start address ;; r1 = Input line width in bytes ;; r2 = Input line total size in bytes ;; r3 = Output line start address quad FUNC ;; Compute start of second line and end address ADD r1,r1,r0 ADD r2,r2,r1 1 ;; Load 32 pixels from each of two rows VLD1.8 {Q0,Q1},[r0]! VLD1.8 {Q2,Q3},[r1]! ;; Sum neighbouring 8-bits in each row to 16-bits VPADDL.U8 Q0,Q0 VPADDL.U8 Q1,Q1 VPADDL.U8 Q2,Q2 VPADDL.U8 Q3,Q3 ;; Sum 16-bit values vertically VADD.U16 Q0,Q0,Q2 VADD.U16 Q1,Q1,Q3 ;; Divide each sum of four pixels by 4 and cast to char VSHRN.U16 D0,Q0,#2 VSHRN.U16 D1,Q1,#2 ;; Store 16 pixels of resized image VST1.8 {Q0},[r3]! ;; Loop if not past end of image CMP r1,r2 BLE %b1 ;; Return from function BX lr ENDFUNC

hth
s.
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Simon Craske over 12 years ago

Note: This was originally posted on 17th September 2010 at http://forums.arm.com

Assuming your four pixels are in a square, i.e.:

[font="Courier New"]+---+---+
|   A   |   B   |
+---+---+   --->    (A+B+C+D)/4
|   C   |   D |
+---+---+[/font]

Then the following [untested] Neon code might not be too far off optimal.

;; Average square of four pixels to single pixel. ;; Produces NxM pixel image from 2Nx2M pixel image. ;; Generates 16 output pixels per loop. ;; May over-read by upto 63 bytes. ;; May over-write by upto 15 bytes. ;; r0 = Input line start address ;; r1 = Input line width in bytes ;; r2 = Input line total size in bytes ;; r3 = Output line start address quad FUNC ;; Compute start of second line and end address ADD r1,r1,r0 ADD r2,r2,r1 1 ;; Load 32 pixels from each of two rows VLD1.8 {Q0,Q1},[r0]! VLD1.8 {Q2,Q3},[r1]! ;; Sum neighbouring 8-bits in each row to 16-bits VPADDL.U8 Q0,Q0 VPADDL.U8 Q1,Q1 VPADDL.U8 Q2,Q2 VPADDL.U8 Q3,Q3 ;; Sum 16-bit values vertically VADD.U16 Q0,Q0,Q2 VADD.U16 Q1,Q1,Q3 ;; Divide each sum of four pixels by 4 and cast to char VSHRN.U16 D0,Q0,#2 VSHRN.U16 D1,Q1,#2 ;; Store 16 pixels of resized image VST1.8 {Q0},[r3]! ;; Loop if not past end of image CMP r1,r2 BLE %b1 ;; Return from function BX lr ENDFUNC

hth
s.
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