#26 Radius-based box blur for a rectangle or an ellipse

[Thomas Mueller]

TBitmap_BlurRect and TBitmap_BlurEllipse in src/u_dzGraphicsUtils.pas blur with repeated 3x3 steps, so a strong blur takes hundreds of passes (about 800 to make a licence plate in a 4096x2304 photo unreadable, ~2.5 s for two faces).

Proposal: new functions such as TBitmap_BoxBlurRect / TBitmap_BoxBlurEllipse(bmp, Left, Top, Right, Bottom, Radius):

• separable box blur, three passes, with a running sum, so the cost per pixel does not depend on the radius; three passes are nearly a Gaussian blur with sigma ~ sqrt(r(r+1))
• intermediate values with more than 8 bits, rounded once when writing back
• the real neighbouring pixels as input, clamped at the bitmap edge, instead of a fixed unchanged outer ring
• for the ellipse: blur the bounding rectangle and write back only the pixels inside; optionally a soft transition at the edge, since a hard edge is clearly visible

A test program did this in ~100 ms for the two faces at r = 23, which matched 800 of the current passes visually. Details in [#15].

[Thomas Mueller]

Implemented in revision #2377: TBitmap_BoxBlurRect and TBitmap_BoxBlurEllipse(bmp, Left, Top, Right, Bottom, Radius), three passes of a separable box blur with a running sum, reading the pixels around the area. On the test photos r = 23 took about 110 ms for the two faces. The ellipse has a hard edge; the optional soft edge is not done.