"Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to
allocate 13770 bytes) in C:\Program Files\Apache Software"

I'm sorry but it has nothing to do with GD. imagecreate tries to allocate the memory it needs for an image but your application seems to have already used most of the available limit (32M in this case).

By the way, fatal errors are not crashes or do you have a real crash? (segfault). If yes post a comment I will reopen this bug and move it to the engine category.

You have ignored the other submitted info that shows (via call to memory_get_usage(true)) that the memory in use is actually less than 7M.

Either the function call is reporting incorrect values, or there is a problem in the memory manager believing 32M is in use.

I have subsequently added a call to memory_get_peak_usage(true), and it reports the same value as memory_get_usage(true), indicating I have come nowhere near using 32M.

Your image takes 13770*990*4 bytes, which is ca. 54mb. That doesn't fit in 32 mb memory limit.

With understanding of how much memory GD uses per pixel (4 bytes), I am able to scale image request size to fit in memory, but result is that image is not large enough to build tree.

When increasing memory_limit to 128M, I still run into 32M limit, and upon looking further, it appears that default Apache Windows distro is built with MAX_PROCESS_SIZE = 32M.

This implies that the default max image GD can handle (for height of 1024) is about 32M/1024/4 = 8K.

When looking further into GD documentation, there is a reason why I am using imagecreate instead of imagecreatetruecolor.  The GD docs say gdImageCreate(sx, sy) has a 256 color limit, or 1 byte per pixel, while gdImageCreateTrueColor(sx, sy) has "an essentially unlimited number of colors" - presumably 32 bit or 4 bytes per pixel.

When I change my call to use imagecreatetruecolor instead of imagecreate, the memory usage is exactly the same, suggesting the PHP imagecreate is calling the GD gdImageCreateTrueColor and reducing the available image size by a factor of 4.  If this is the case, why?

"When I change my call to use imagecreatetruecolor instead of
imagecreate, the memory usage is exactly the same, suggesting the PHP
imagecreate is calling the GD gdImageCreateTrueColor and reducing the
available image size by a factor of 4.  If this is the case, why?"

That's not the case. You consider the values returned by the memory info functions are accurate, they are not.

"This implies that the default max image GD can handle (for height of
1024) is about 32M/1024/4 = 8K."

Completely wrong, you are looking at the wrong place to fix your problem. If you have doubt about the cause (memory limit), try to run your script (and only what you show here) using CLI (command line).

On looking further, both imagecreate andimagecreatetruecolor were working, and use different amounts of memory.  However, the amounts are different than 4 bytes in both cases.

For imagecreate, 2 bytes (+marginal overhead) are allocated.
For imagecreatetruecolor, 5 bytes (+about .15 per pixel) are allocated.

These were calculated using the following script;

<?php
    for ($i=0;$i<2;$i++) {
        println("iteration = $i");
        $mUsed = memory_get_usage(true);
        $mPeak = memory_get_peak_usage(true);
        println(" $mUsed, $mPeak");
        
        $xSize=8192;
        $ySize=1024;
        if ($i==0) $im = imagecreate($xSize,$ySize);
        else $im = imagecreatetruecolor($xSize,$ySize);
        
        $mUsed2 = memory_get_usage(true);
        $mPeak2 = memory_get_peak_usage(true);
        println(" $mUsed2, $mPeak2");
        $iUsed = $mUsed2-$mUsed;
        $perPixel = $iUsed/($xSize*$ySize);
        println(" incremental memory - $iUsed ($perPixel per pixel)");
        
        imagedestroy($im);
        
        $mUsed3 = memory_get_usage(true);
        $mPeak3 = memory_get_peak_usage(true);
        println(" $mUsed3, $mPeak3");
        println;
    }
    
function println($text) {print("$text\n<BR>");}
?>

From this, it appears there is 1 or 4 bytes for color, plus 1 byte for something else.  This should be in the docs.

Consider the item closed.

"From this, it appears there is 1 or 4 bytes for color, plus 1 byte for
something else.  This should be in the docs"

I know exactly how much memory a GD image uses. I can't say (well I can but not easily) how much memory is used by a PHP resource, its variable, the code used to initialize it.

Not a bug > bogus remains.