Is StretchBlt HALFTONE == BILINEAR for all scaling?

Question

Can anyone clarify if the GDI StretchBlt function for the workstation Win32 API performs bilinear interpolation for scaling to both larger and smaller images for 24/32-bit color images? And if not, is there a GDI (not GDI+) function that does this?

The SetStretchBltMode fn has a setting HALFTONE which is documented as follows:

HALFTONE Maps pixels from the source rectangle into blocks of pixels in the destination rectangle. The average color over the destination block of pixels approximates the color of the source pixels.

I've seen references (see follow-up to first answer) that this performs bilinear interpolation when scaling down an image, but no clear answer of what happens when scaling up.

I have noticed that the Windows Mobile CE SDK does support a BILINEAR flag - which is documented exactly opposite of the HALFTONE comments (only works for scaling up).

Note that for the scope of this question, I'm not interested in pursuing GDI+ (which has numerous interpolation options), OpenGL, DirectX, etc. as alternatives, so please don't bother with follow-ups regarding these other APIs or alternate image libraries.

What I'm really hoping to find is some definitive MS/MSDN or other high-quality documentation that clearly documents this behavior of the Win32 (desktop) GDI behavior.

Meanwhile, I'll try some experiments comparing GDI vs. Direct2D (which does have an explicit flag to control this) and post my findings.

Thanks!

Answer 1

HALFTONE mode performs a very blocky halftone dithering on the image, based on varying the conversion thresholds over a defined square. I have never seen a situation where it would be considered the best choice.

COLORONCOLOR is the best mode for color images, but as you've seen it doesn't give great results.

GDI does not support a bilinear mode (except in Windows Mobile CE as you discovered). The naive implementation of bilinear does not do very well when shrinking an image, as it simply tries to interpolate between two adjacent input pixels without trying to draw from a larger area.

Answer 2

I've been looking into this same problem for the past couple of weeks.

As far as I can tell, there does not exist any definitive documentation on this behaviour from Microsoft.

However, I've run some tests myself, to try and establish the degree to which StretchBlt can be trusted to perform consistently with respect to up- and down-scaling images in halftone mode.

My findings are:

1) StretchBlt does produce adequate quality up- and down-scaled images. It might be a touch below Photoshop quality, but probably OK for most practical purposes.

2) It seems to depend upon hardware acceleration, whenever it's available. I haven't been able to confirm this, but I have a slight fear that this may lead to different outputs on different types of hardware. However, on the 5 or 6 different systems I've tried it on, old and new, the performance has been consistent and fast.

3) If you use the call on a 16-bit color device, or lower, StretchBlt will automatically dither your image. If you run it on a 24-bit color device, it will not dither.

4) If you use it to scale small images (smaller than 150x150px), it will randomly fall back to nearest neighbour interpolation. This can be remedied in your own software, by padding the bitmap before scaling, doing StretchBlt on it, and then removing the padding afterwards. Kind of a hack, but it works.