Question

I need to manually find DPI of new image.

$input_width = 361;
$input_height = 413;

$input_dpi_x = 72;
$input_dpi_y = 72;

$output_width = 800;
$output_height = $input_height * $output_width / $input_width;

$output_dpi_x = ceil(($input_dpi_x / $input_width) * $output_width);
$output_dpi_y = ceil(($input_dpi_y / $input_height) * $output_y_res);

echo "Outpud_dpi_x = " . $output_dpi_x;
//Outpud_dpi_x = 160

Why when i resize image i get 802 instead of 800?

and i must use DPI dont ask why

Answer 1

The answer is all in the math… Let’s just focus on the width for simplicity.

Pulling back the layers a little bit, start with scaling operations (reordered to help with loss of precision). Here, I’m calculating the output DPI value and verifying the result by solving the original equation for $output_width.

$output_dpi_x = $output_width * $input_dpi_x / $input_width;  // 159.5567867...
$output_width = $output_dpi_x * $input_width / $input_dpi_x;  // 800

You can see with this non-corrected value for DPI, we arrive back at 800 for the width value. When we round up to the next number (using the ceil operator), it changes the results. By unwinding the math, we can see why you end up with 802px in the output:

$output_dpi_x = ceil($output_dpi_x);  // 160
$output_width = $output_dpi_x * $input_width / $input_dpi_x;  // 802.22222222...

Of course, images can’t contain partial pixels, so your resized image is rounded down to 802px.