Image rotation with bilinear interpolation and alpha progressive borders

So a blog reader, Fabien Auréjac, emailed me an improvement over the code I posted on image rotation. Here’s the one with bilinear interpolation, and here’s the one with bilinear interpolation and no clipping.

You can find Fabien here and here (warning: French ahead), and also at Stack Overflow.

Fabien translated the core of my code into PHP. The improvement was on assigning alpha values to the edge pixels after rotation. Edge pixels are pixels beside the “blank” pixels (I used black in my code, for instance). The alpha values mean the edge pixels are “softer” and thus the resulting image looks smoother.

I suppose if you really want to, you could also “dumb down” the values of the red, green and blue colour components for more softening (in addition to the alpha component). I say “dumb down” because the blank pixels I used are black (meaning zero for the RGB values). You’re free to go ahead and do more interpolation.

Fabien has given permission for me to post the code here. I’ll leave it as an exercise for you to translate to your programming language.

if (($angleBool+$LsupH)%2==0) {
} else {
$imgRot=imagecreatetruecolor($largeur, $hauteur);
imagealphablending($imgRot, true);
imageantialias($imgRot, true);
for ($i=0; $i<$hauteur; $i++) {
	for ($j=0; $j<$largeur; $j++) {
		// convert raster to Cartesian
        $x = $j - $largeur*0.5;
        $y = $hauteur*0.5 - $i;

        // convert Cartesian to polar
        $fDistance = sqrt($x * $x + $y * $y);
   	 	$fPolarAngle = atan2($y, $x);

        // the crucial rotation part
        // "reverse" rotate, so minus instead of plus
        $fPolarAngle -= $angleRot;
		 // convert polar to Cartesian
        $fTrueX = $fDistance * cos($fPolarAngle);
        $fTrueY = $fDistance * sin($fPolarAngle);

        // convert Cartesian to raster
        $fTrueX = $fTrueX + $size[0]*0.5;
        $fTrueY = $size[1]*0.5 - $fTrueY;

        $iFloorX = (int)(floor($fTrueX));
        $iFloorY = (int)(floor($fTrueY));
        $iCeilingX = (int)(ceil($fTrueX));
        $iCeilingY = (int)(ceil($fTrueY));
        //echo $fTrueX." ".$fTrueY." ".$iFloorX." ".$iCeilingX." ".$iFloorY." ".$iCeilingY."<br>";
		if ($iFloorX >= 0 && $iCeilingX >= 0 && $iFloorX < $size[0] && $iCeilingX < $size[0] && $iFloorY >= 0 && $iCeilingY >= 0 && $iFloorY < $size[1] && $iCeilingY < $size[1]) {
			$fDeltaX = $fTrueX - $iFloorX;
			$fDeltaY = $fTrueY - $iFloorY;
			$clrTopLeft = imagecolorat($img, $iFloorX, $iFloorY);
			$colorsTopLeft = imagecolorsforindex($img, $clrTopLeft);
			$clrTopRight = imagecolorat($img, $iCeilingX, $iFloorY); 
			$colorsTopRight = imagecolorsforindex($img, $clrTopRight);
			$clrBottomLeft = imagecolorat($img, $iFloorX, $iCeilingY);
			$colorsBottomLeft = imagecolorsforindex($img, $clrBottomLeft);
			$clrBottomRight = imagecolorat($img, $iCeilingX, $iCeilingY);
			$colorsBottomRight = imagecolorsforindex($img, $clrBottomRight);
			// linearly interpolate horizontally between top neighbours
			$fTopRed = (1 - $fDeltaX) * $colorsTopLeft['red'] + $fDeltaX * $colorsTopRight['red'];
			$fTopGreen = (1 - $fDeltaX) * $colorsTopLeft['green'] + $fDeltaX * $colorsTopRight['green'];
			$fTopBlue = (1 - $fDeltaX) * $colorsTopLeft['blue'] + $fDeltaX * $colorsTopRight['blue'];
			// linearly interpolate horizontally between bottom neighbours
			$fBottomRed = (1 - $fDeltaX) * $colorsBottomLeft['red'] + $fDeltaX * $colorsBottomRight['red'];
			$fBottomGreen = (1 - $fDeltaX) * $colorsBottomLeft['green'] + $fDeltaX * $colorsBottomRight['green'];
			$fBottomBlue = (1 - $fDeltaX) * $colorsBottomLeft['blue'] + $fDeltaX * $colorsBottomRight['blue'];
			// linearly interpolate vertically between top and bottom interpolated results
			$iRed = (int)(round((1 - $fDeltaY) * $fTopRed + $fDeltaY * $fBottomRed));
			$iGreen = (int)(round((1 - $fDeltaY) * $fTopGreen + $fDeltaY * $fBottomGreen));
			$iBlue = (int)(round((1 - $fDeltaY) * $fTopBlue + $fDeltaY * $fBottomBlue));
			// make sure colour values are valid
			if ($iRed < 0) $iRed = 0;
			if ($iRed > 255) $iRed = 255;
			if ($iGreen < 0) $iGreen = 0;
			if ($iGreen > 255) $iGreen = 255;
			if ($iBlue < 0) $iBlue = 0;
			if ($iBlue > 255) $iBlue = 255;
			if ($iFloorX > 0 && $iCeilingX > 0 && $iFloorX < $size[0]-1 && $iCeilingX < $size[0]-1 && $iFloorY > 0 && $iCeilingY > 0 && $iFloorY < $size[1]-1 && $iCeilingY < $size[1]-1) {
				$colorallocation=imagecolorallocate($imgRot, $iRed, $iGreen, $iBlue);
				imagesetpixel($imgRot, $j, $i, $colorallocation);
			} else if ($iFloorX == 0 && $iFloorY >= 0 && $iCeilingY >= 0 && $iFloorY < $size[1] && $iCeilingY < $size[1]) {//left
				$colorallocation=imagecolorallocatealpha($imgRot, $iRed, $iGreen, $iBlue, $alpha);
				imagesetpixel($imgRot, $j, $i, $colorallocation);
			} else if ($iFloorX >= 0 && $iCeilingX >= 0 && $iFloorX < $size[0] && $iCeilingX < $size[0] && $iFloorY == 0) {//top
				$colorallocation=imagecolorallocatealpha($imgRot, $iRed, $iGreen, $iBlue, $alpha);
				imagesetpixel($imgRot, $j, $i, $colorallocation);
			} else if ($iCeilingX == $size[0]-1 && $iFloorY >= 0 && $iCeilingY >= 0 && $iFloorY < $size[1] && $iCeilingY < $size[1]) {//right
				$colorallocation=imagecolorallocatealpha($imgRot, $iRed, $iGreen, $iBlue, $alpha);
				imagesetpixel($imgRot, $j, $i, $colorallocation);
			} else if ($iFloorX >= 0 && $iCeilingX >= 0 && $iFloorX < $size[0] && $iCeilingX < $size[0] && $iCeilingY == $size[1]-1) {//bottom
				$colorallocation=imagecolorallocatealpha($imgRot, $iRed, $iGreen, $iBlue, $alpha);
				imagesetpixel($imgRot, $j, $i, $colorallocation);

Fabien is French (I think), which is why you get variable names such as distOmbre (shadow distance?), flouOmbre (fuzzy shadow?), largeur (width), hauteur (height). And this one took me a bit more time to translate… LsupH is probably “width greater than height?”. The “L” probably refers to “largeur”, and “H” refers to “hauteur”.

Reading international programming code is fun. *smile*

There’s also an interesting piece of code:

if (($angleBool+$LsupH)%2==0) {
} else {

So here’s your mission, should you choose to accept it (I recently watched Mission Impossible…). What is Fabien trying to accomplish in that section of code? Hint: it has something to do with getting a “nice” resulting image width and height.

I’ll tell you a more “elegant” alternative to that code section. But it’ll involve some mathematics. And drawings. Prepare for poorly drawn diagrams…

Simple excerpts in WordPress

I have finally decided to plunge into some PHP code in my WordPress template. The reason? My home page is a little long. So are my archive and category pages. I write moderately lengthy posts, so even with 5 posts per page, you might have to scroll a bit.

First, I want to say that I have never learnt PHP. I’m also unfamiliar with the innards of WordPress. But I’m a programmer. The code doesn’t look too hard… so I’m still capable of making small changes.

I’ve heard about the use of excerpts, and because I’m too lazy to set them up properly with the more tag, I’m doing it the simple way. This is the code I used:

<?php if(is_category() || is_archive() || ($postcounter!=1 && !is_single() && !is_page())) {
} else {
	the_content(__('Read more &raquo;'));
} ?>

The the_excerpt() function is used to automatically generate an excerpt. It’s automatic, that’s why it’s simple. Otherwise, I’d have to set up more tags. Under the WordPress panel for that particular post, you can also add in specific text you want to use as an excerpt. I’m going for simplicity, so I’m using whatever WordPress defaults for me (55 words stripped of HTML).

The more tag is actually typed out in your WordPress post, so you actually have to (or can) figure where exactly you want the break to be. More control, less automation. It looks like this <!--More-->

The entire if statement is in the main WordPress loop. The is_category() and is_archive() functions should be self-explanatory. The third condition is a little complicated…

What I want is to customise the layout of the home page. The home page is neither a single WordPress post nor a WordPress page, hence the !is_single() and !is_page(). The $postcounter variable is used to keep track of how many posts were already displayed. The declaration and incrementation of the variable is not shown in the code.

The desired effect is that, on the home page, only the first post (which is the latest) is displayed in full. All other posts on the home page is displayed in excerpt form.

In the else part, it should be either a single post or page, so display the full content. The function the_content() takes a single parameter which is the “read more” text.

I have a customised version of the_content() parameter, which I believe will use any localisation files to translate the given text. If you’re not concerned with it, just use something like the_content('Read more...');.

Hope this helped you in some way.

P.S. Currently, I use 5 posts per display page. If I go to the second display page on my home page, the first post (the 6th post) displayed is in full, whereas the rest are in excerpt form. It’s not a critical issue, so I’m letting it be…