[cairo] Trouble resizing (scaling) an XImage

[Justin Anderson]

Hi Taekyun,

Thank you for your response.  I actually was copying the width and height
from my memory and forgot to include the (double) cast.  It doesn't make a
difference in practice.  Even trying a hard-coded 0.75 to presumably get a
75% image does not work.

I'm actually now having cairo paint the scaled image into a Pixmap instead
of directly onto the window, so I can inspect the pixels.  They seem to be
zero.

I wonder if it's a depth issue.  As I said, the source is an 8 bit grayscale
image but the screen is 24 or 32 bits.  I'm not sure if cairo can handle
paining from one depth to another.


On Mon, Jun 27, 2011 at 10:43 AM, Taekyun Kim <podain77 at gmail.com> wrote:

> I think you did integer division. Something like "(double)width /
> (double)xImage->width" might solve your problem.
>
> 2011. 6. 28. 오전 12:05 Justin Anderson <justin at rayonix.com> 작성:
>
> I should mention that the source image is an 8 bit grayscale.  Thanks.
>
>
> On Mon, Jun 27, 2011 at 10:00 AM, Justin Anderson < <justin at rayonix.com>
> justin at rayonix.com> wrote:
>
>> Hello,
>>
>> I'm trying to resize an X11 XImage which is a 640x480 pixel image from a
>> video grabber, to be displayed on a window in an X11/Motif application.
>>
>> If I comment out the scale command in the following code, the image is
>> displayed fine, albeit clipped on my window.  If I uncomment the scale
>> operation, there is no image in the window.
>>
>> Any ideas?  Thanks!
>>
>>
>>
>>    //Create a simple X11 window for displaying the XImage...
>>    window = XCreateSimpleWindow(
>>       display,
>>       RootWindowOfScreen(screen),
>>       0, // x
>>       0, // y
>>       width,
>>       height,
>>       0,                          // border width
>>       BlackPixelOfScreen(screen), // border
>>       BlackPixelOfScreen(screen)  // background
>>    );
>>    XMapRaised(display, window);
>>
>>    //cairo stuff...
>>    cairo_surface_t *surface_dest = cairo_xlib_surface_create(display,
>> window, XDefaultVisualOfScreen(screen), width, height);
>>    cairo_t *cr_dest = cairo_create(surface_dest);
>>
>>    cairo_surface_t *surface_source =
>> cairo_image_surface_create_for_data((unsigned char*)(xImage->data),
>> CAIRO_FORMAT_RGB24, xImage->width, xImage->height, xImage->bytes_per_line);
>>
>>    cairo_scale(cr_dest, width / xImage->width, height / xImage->height);
>>
>>    cairo_set_source_surface(cr_dest, surface_source, 0, 0);
>>    cairo_paint(cr_dest);
>>    cairo_surface_destroy(surface_source);
>>
>>    //Put the scaled XImage on the window...
>>    XPutImage(display, window, gc, xImage, 0, 0, 0, 0, width, height);
>>     cairo_surface_destroy(surface_dest);
>>    cairo_surface_destroy(surface_source);
>>
>
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