author Sam Lantinga <slouken@libsdl.org>
Thu, 01 Aug 2013 09:15:36 -0700
changeset 7563 c128ed448c30
parent 4462 5cb6424a5676
child 7578 4b43278168be
permissions -rw-r--r--
Fixed bug 1968 - SDL_RenderCopy stretch loses proportion on viewport boundaries for 3D renderers


SDL_RenderCopy clips dstrect against the viewport. Then it adjusts the
srcrect by "appropriate" amount of pixels. This amount is actually
wrong, quite a lot, because of the rounding errors introduced in the "*
factor / factor" scale.

real_srcrect.x += (deltax * real_srcrect.w) / dstrect->w;
real_srcrect.w += (deltaw * real_srcrect.w) / dstrect->w;

For example:

I have a 32 x 32 srcrect and a 64 x 64 dstrect. So far the
stretching is done perfectly, by a factor of 2.

Now, consider dstrect being clipped against the viewport, so it becomes
56 x 64. Now, the factor becomes 1.75 ! The adjustment to "srcrect"
can't handle this, cause srcrect is in integers.

And thus we now have incorrect mapping, with dstrect not being in the
right proportion to srcrect.

The problem is most evident when upscaling stuff, like displaying a 8x8
texture with a zoom of 64 or more, and moving it beyond the corners of
the screen. It *looks* really really bad.

Note: RenderCopyEX does no such clipping, and is right to do so. The fix would be to remove any such clipping from RenderCopy too. And then fix the software renderer, because it has the same fault, independently of RenderCopy.

[attached patch]
this leaves Software Renderer buggy, as it does it's own clipping later on
     1 0afe0e38e02cf2048e93582f01c52fbb91d3c7bb release-1.2.7
     2 230b156829ed13b31134d96f689c917981f57b84 release-1.2.5
     3 27cab50ec9c746e886ce0f3fdaa0b0cdc55a594f release-1.2.11
     4 2fe3fbd2bff50165b3cad33bf40d70b3bb3c9fd0 release-1.2.3
     5 3c052d3bcc76c899dfd4846be76243a78e8c7180 release-1.2.4
     6 3c5eed71a3320962551af3b3dfbee0c99fcf0086 release-1.2.10
     7 4867f7f7dd3426d1dbbeef48b3f3b3aa19590cc4 release-1.2.12
     8 6e28dae59e3baf4447c83e833a8d2ac912536f5b release-1.2.1
     9 7c2589fb8d4df54c6faabd3faebd0c0e73f67879 release-1.2.13
    10 86de11faf082881ad9b73a1a1d78733ca07f8db8 release-1.2.6
    11 bb051fa871aa0b53ea57df56a446cec3bb85924c release-1.2.2
    12 cfcb2e1c36ebe9809577adf768b0ec53e8768af9 release-1.2.8
    13 e044e7c70a50a2f54d14ee20d0933e904e5853b6 release-1.2.9
    14 f14cf9d71233934811774f941d0de121d5f96ccf release-1.2.14
    15 39c22a953456f6c9e2c8993c8ff973824104102a pre-touch-removal