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Image resampling and resizing

phase_v · 2005-10-11T08:03:12

Hey, I have a question about resizing and resampling an image. I am currently working only in the x direction (scaling the width). My resampling function is as follows: unsigned char* resampleX2(unsigned char* source, int newWidth, int oldWidth, int height) { int numPixels = int(newWidth * height * 4 + newWidth); // number of pixels in resized image unsigned char* output = new unsigned char[numPixels]; double deltaX = double(oldWidth /newWidth); double radius = 2; // filter radius double x0 = double(-0.5 +deltaX/2); // first sample point (in original image) cubicFilter f; // a mitchell-netravali cubic filter, takes in and spits out a double for(int i =0; i < numPixels - 1; i+=4) { // iterate through all pixels, increment by 4 for rgba double x = x0 + i*deltaX; // determine current sample point double sr = 0,sg=0,sb=0,sa=0; // sum values for rgba - use for convolution for(double j = x - radius; j < x + radius; j++) { // convolve sr += source[(int)j]*f.compute(x-j); sg += source[(int)j + 1]*f.compute(x-j); sb += source[(int)j + 2]*f.compute(x-j); sa += source[(int)j + 3]*f.compute(x-j); } // set output image to new resized values output = sr; output[i+1] = sg; output[i+2] = sb; output[i+3] = sa; } return output; } The odd thing about it is that if I scale by half, I get perfect results. Anything besdies that (and 10% i think) gives me results that are either skewed or have major color issues. Any help would be appreciated. This is for a project due tomorrow and has been driving me crazy. Thanks.

General and Gameplay Programming Programming

Started by phase_v October 11, 2005 03:04 AM

33 comments, last by phase_v 18 years, 6 months ago

phase_v

121

Author

October 11, 2005 05:28 AM

Hmmm, I don't think so. I believe that j already accounts for the rgba because of it's relation to x and i. i is the index value which is multiplied by 4. Correct me if I'm wrong, however when I implement this I get overflows and if I check against them only the bottom row of the image is drawn and things in that row aren't correct either. Any other ideas?

Progress does not consist in replacing a theory that is wrong with one that is right. It consists in replacing a theory that is wrong with one that is more subtly wrong.- Hawkins' Theory of Progress

Anonymous

October 11, 2005 05:42 AM

Source is a char*.

Each pixel is 4 bytes wide. Therefore pixel 0 is at index 0, pixel 1 is at index 4, pixel 2 at index 8, etc. Therefore when you index individual pixels in your source image they should be 4 bytes apart. You increment j by one each loop, not 4.

You have another problem which I've also hinted at earlier. On the left hand side, x is 0. When you add subtract your radius from this (in yout inner loop) you get -2.

Really, the outer 2 (or radius) pixels are undefined. You need a way of dealing with this.

Anonymous

October 11, 2005 05:51 AM

Oops. My bad, I was 1/2 right anyway.

for(int j = - radius; j < radius; j++) {    sr += source[(j*4)+x] * f.compute(x-j);    sg += source[(j*4)+x+1] * f.compute(x-j);    sb += source[(j*4)+x+2] * f.compute(x-j);    sa += source[(j*4)+x+3] * f.compute(x-j);}

samv

312

October 11, 2005 05:57 AM

What I meant was:

	i = index(col,row,newWidth);	int x = x0 + i * deltaX;

The first line calculates the index, the second scales the index. This is wrong. What you want is to first scale the col, then calculate the index:

	newIndex = index(col,row,newWidth);	oldIndex = index(col * deltaX,row,oldWidth)	int x = x0 + oldIndex;

___________________________Buggrit, millennium hand and shrimp!

phase_v

121

Author

October 11, 2005 06:00 AM

Ok, I edited my loops so it now looks like this:

for(int row = 0; row < height; row++) {		for(int col = 0; col < newWidth; col++) {			float sr = 0,sg = 0,sb = 0,sa = 0;			i = index(col,row,newWidth);			int x = x0 + i * deltaX;			for(int j = x - radius; j < x+radius; j++) {				if((j*4)>0 && (j*4)+3 < oldWidth * height * 4) {					sr += source[(int)(j*4)] * f.compute(x-j);					sg += source[(int)(j*4)+1] * (float)f.compute(x-j);					sb += source[(int)(j*4)+2] * (float)f.compute(x-j);					sa += source[(int)(j*4)+3] * (float)f.compute(x-j);				}			}			result = (int)sr;			result[i+1] = (int)sg;			result[i+2] = (int)sb;			result[i+3] = (int)sa;		}	}

My result is this. The colors are right, but my scaling is odd to say the least. Actually, I guess I do have a scaled x,y image in there...

Progress does not consist in replacing a theory that is wrong with one that is right. It consists in replacing a theory that is wrong with one that is more subtly wrong.- Hawkins' Theory of Progress

phase_v

121

Author

October 11, 2005 06:02 AM

btw I'm playing major catch up with your posts

Progress does not consist in replacing a theory that is wrong with one that is right. It consists in replacing a theory that is wrong with one that is more subtly wrong.- Hawkins' Theory of Progress

samv

312

October 11, 2005 06:05 AM

Have you seen my last post? (next to last on page 1)
I think it explains why your image is scaled in y.

___________________________Buggrit, millennium hand and shrimp!

phase_v

121

Author

October 11, 2005 06:09 AM

Yeah I just changed that, but it still seems to be scaling in y. I need to work with it a little more though as I'm probably screwing something up. Check back in like 5 or 10 minutes.

Progress does not consist in replacing a theory that is wrong with one that is right. It consists in replacing a theory that is wrong with one that is more subtly wrong.- Hawkins' Theory of Progress

phase_v

121

Author

October 11, 2005 06:14 AM

HA HA finally it is working (almost) perfectly. Changing the scaling stuff worked. I still get some odd colors at selected scales (like 33%), but i think this has to do with my radius choice. I'll play with it a little more. Once I have it working I'll post my code. You two (i'm assuming anonymous poster has been one consistent individual) are life savers.

Progress does not consist in replacing a theory that is wrong with one that is right. It consists in replacing a theory that is wrong with one that is more subtly wrong.- Hawkins' Theory of Progress

Anonymous

October 11, 2005 06:20 AM

So, Nick; how's University of Wisconsin-Madison treating you so far?

You might be interested in this snippet from the forum FAQ:
- you shouldn't ask for homework help.

Have a nice day.

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