gdImageRotateInterpolated() converts the source image to truecolor

[cmb69]

If a palette image is passed to gdImageRotateInterpolated() it is converted to truecolor. That is at the very least unexpected, because gdImageRotateInterpolated() is supposed to return a modified result, but not to change the source image.

Test Program

#include <stdio.h>
#include <assert.h>
#include <gd.h>


int main()
{
    gdImagePtr src, dst;
    int black;

    src = gdImageCreate(10, 10);
    black = gdImageColorAllocate(src, 0, 0, 0);

    printf("src is truecolor: %d\n", gdImageTrueColor(src));

    dst = gdImageRotateInterpolated(src, 30.0, black);
    assert(dst != NULL);

    printf("src is truecolor: %d\n", gdImageTrueColor(src));
}

Expected Output

src is truecolor: 0
src is truecolor: 0

Actual Output

src is truecolor: 0
src is truecolor: 1

[pierrejoye]

All interpolation converts to TC yes. It makes thing too complicated with palette while massively reducing the quality.

[cmb69]

This is not about the output (or the internal calculation), but rather the fact that the source image is converted to true-color. This might have better been solved by only supporting true-color images in the first place (and failing for palette images), but we cannot change that anymore. However, we should document that this conversion is happening.

[pierrejoye]

oh, the original image is converted? sorry I misread. That should not happen. I will look into it.

[cmb69]

libgd/src/gd_interpolation.c

Lines 1882 to 1887 in 9af6d12

	if (src->trueColor == 0) {
	if (bgcolor < gdMaxColors) {
	bgcolor = gdTrueColorAlpha(src->red[bgcolor], src->green[bgcolor], src->blue[bgcolor], src->alpha[bgcolor]);
	}
	gdImagePaletteToTrueColor(src);
	}

Solution would be simple: just create a copy of the src image before converting to palette. I'm not sure whether it's worth it. Users can do that themselves if they need to retain the source image (often there is no need to).

[pierrejoye]

It is needed to be converted anyway.

The fix is:
gdImageClone ion line 1885
gdImagePaletteToTrueColor(clone)
back_src = src;
src = clone;

At this stage I would like to go with the "fix" rather than making it non obvious. If the src changes, the parameter should be ** at the very least (to make it clear and not const *).

[cmb69]

That's fine for me as well. :)

[pierrejoye]

PR done, please check :)