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Nils Pipenbrinck
03-24-2006, 10:39 AM
There's something I stumbled over while testing some gradients:

The gradients inherit the transform of the objects they are applied to. That makes perfect sense for me. In the spec this is only defined for the pathes, but ok - it makes sense to do the same for images as well.


However, the spec clearly states that paint transforms are affine only, and image transforms might be projective.

Now if I apply a gradient paint on a image drawn with perspective projection I don't know what results to expect. The reference implementation draws something, but it's hard to find out/guess how exactly the gradient should be drawn.


Question:


Do we have to mimic the behaviour of the reference implementation for this special case? After all it's not defined in the spec.
[/*:m:xeh44fix]
If so - how exactly should the gradient sampling work? Do we really have to sample the gradient projective?
[/*:m:xeh44fix]
If we have to sample the gradient affine, how should we get rid of the projective transform?[/*:m:xeh44fix]

Hope someone can help me with these issues. After all - there's little to none traffic on this board..

Thanks,
Nils Pipenbrinck

Nils Pipenbrinck
03-24-2006, 11:00 AM
Here's a bit of code. draws a test image with radial gradient in perspective and affine mode.


void Gradient_WTF (void)
{
VGfloat stops[10] =
{
0, 0.8,0.2,0.1,1,
1, 0.1,0.2,0.8,1,
};

VGfloat gradient[5] =
{
128,128,128,128,60,
};

VGfloat ImageMatrix[9] =
{
-0.4477, 0.8941, 0.0015,
-0.8941, -0.4477, 0.0018,
370, 140, 1,
};

// make checkerboard image:
VGImage img = vgCreateImage (VG_sL_8, 256, 256, VG_IMAGE_QUALITY_BETTER);
for (VGint y=0; y<256; y+=32)
for (VGint x=0; x<256; x+=32)
{
VGfloat white[4] = { 1,1,1,1 };
VGfloat gray[4] = { 0.4,0.4,0.4,1 };
vgSetfv (VG_CLEAR_COLOR, 4, (x^y)&32 ? white:gray);
vgClearImage (img, x,y,32,32);
}

// make paint:
VGPaint paint = vgCreatePaint();
vgSetPaint (paint, VG_FILL_PATH);
vgSetParameteri (paint, VG_PAINT_TYPE, VG_PAINT_TYPE_RADIAL_GRADIENT);
vgSetParameteri (paint, VG_PAINT_COLOR_RAMP_SPREAD_MODE, VG_COLOR_RAMP_SPREAD_REPEAT);
vgSetParameterfv (paint, VG_PAINT_COLOR_RAMP_STOPS, 10, stops);
vgSetParameterfv (paint, VG_PAINT_RADIAL_GRADIENT, 5, gradient);

// draw projective image:
vgSeti (VG_IMAGE_MODE, VG_DRAW_IMAGE_MULTIPLY);
vgSeti (VG_MATRIX_MODE, VG_MATRIX_IMAGE_USER_TO_SURFACE);
vgLoadMatrix (ImageMatrix);
vgDrawImage (img);

// draw affine image:
vgLoadIdentity ();
vgTranslate (400, 10);
vgDrawImage (img);

vgDestroyImage (img);
vgDestroyPaint (paint);
}

Nils Pipenbrinck
03-30-2006, 04:29 AM
anyone?

Nils Pipenbrinck
03-30-2006, 06:04 AM
Ok - if anyone is ever interested, this is what the reference implementation does:

Projective image matrices become unprojected before multiplication with the paint matrix. The unprojection works like this:

build 3 vectors that form a unit coordinate system in 2d:
vector v[3];
v[0] = vector(0,0);
v[1] = vector(1,0);
v[2] = vector(0,1);

transform these vectors by the image matrix (extend 2d vectors to homogenous 2d vectors by adding an implicit z=1). after transformation divide out the vectors x,y by z.

Then subtract projected origin from projected axis:

v[1] -= v[0];
v[2] -= v[0];

The output matrix directly built from these 3 vectors.

- translational part is equal to v[0],
- first column of upper 2x2 submatrix equals to v[1]
- second column of upper 2x2 submatrix equals to v[2]
- projective part becomes 0, 0, 1 (as usual for affine matrices)

I still scratch my head if this is fine with the standard.

muratmat
03-30-2006, 09:12 AM
...
I still scratch my head if this is fine with the standard.

OpenVG specifications do not clarify the correct behavior for perspective image matrixes coupled with a paint.
For radial gradient, i think that the correct result should be, in your case, that gradient circles had to "fit" (perspectively) the image outline.

Nils Pipenbrinck
03-31-2006, 07:44 AM
...
I still scratch my head if this is fine with the standard.

OpenVG specifications do not clarify the correct behavior for perspective image matrixes coupled with a paint.
For radial gradient, i think that the correct result should be, in your case, that gradient circles had to "fit" (perspectively) the image outline.

Hm..

I doubt that this is intended.. For pattern images that wouldn't be such a big deal since it simplifies down to just a perspective image mapping, but the radial gradients are already hard enough to calculate.

Making them perspective correct would make almost all good optimizations impossible (e.g precomputed dda-method for scanlines).

muratmat
03-31-2006, 09:19 AM
Hm..

I doubt that this is intended.. For pattern images that wouldn't be such a big deal since it simplifies down to just a perspective image mapping, but the radial gradients are already hard enough to calculate.

Making them perspective correct would make almost all good optimizations impossible (e.g precomputed dda-method for scanlines).

I was talking about the correct behaviour that every user could expect, and that i think is the most correct one.
From an implementation point of view, it's normal to accept some compromises to not loose too much speed.
Anyway, i think that guys who wrote OpenVG specifications did not consider a such case, and that's why in my vector graphics framework (Amanith, do you know it?) i left gradient/pattern matrix independent from the "modelview" matrix.

Espen
12-13-2006, 09:26 AM
When there is a projective transform on an image, the image mode falls back to normal mode.