(and thanks to Cengiz for many of the previous Unreal-related
patches! They were generically credited to Epic Games, but a large
amount of that work was his contribution.)
Fixes Bugzilla #3067.
This is for corner cases where a multi-window app is activated and wants to
make a decision about where focus should go.
This patch came from Unreal Engine 4's fork of SDL, compliments of Epic Games.
Adam M.
When setting a texture alpha mod other than 255 and also specifying a flip mode in the software renderer, the rendering fails. When the texture has an alpha channel, it becomes invisible when flipped. When the texture does not have an alpha channel, it is flipped but the colors are wrong: the alpha mod makes the texture darker rather than more translucent.
0) Initialize a software renderer.
1) Load 16-bit 565 or 32-bit texture.
2) Set texture blend mode to BLEND.
3) Set texture alpha mod to 150.
4) Draw the texture flipped horizontally and/or vertically.
Adam M.
There are three problems in the code that I see.
1. SW_RenderCopyEx enables a color key on surface_scaled even if the source surface didn't have a color key.
2. SW_RenderCopyEx doesn't copy blend mode, color mod, or alpha mod from src to surface_scaled.
3. When SDL_BlitScaled(src, srcrect, surface_scaled, &tmp_rect) is called, it blends the src pixels into surface_scaled instead of overwriting them (if src has blending, etc. enabled).
I've attached a patch that 1) fixes the three problems that I mentioned, 2) adds the requested performance improvement of using the regular blit function if no rotation or flipping is needed, 3) avoids cloning the source surface if no stretching is required, and simplifies the rotation code slightly.
Adam M.
Okay, here is the problem, I think.
During the first blit, RLEAlphaSurface is called to do RLE conversion of the RGBA source into a format allowing it "to be quickly alpha-blittable onto dest". Since the destination is the screen, it has no alpha channel. RLEAlphaSurface calls copy_opaque(dst, src + runstart, len, sf, df) (where copy_opaque is copy_32), which has this code:
SDL_RLEaccel.c:984:
RGBA_FROM_8888(*src, sfmt, r, g, b, a);
PIXEL_FROM_RGBA(*d, dfmt, r, g, b, a);
On the first line, it reads the source pixel 0xFFFFFFFF. The second line drops the alpha value (because dfmt for the screen has no alpha channel) and writes 0x00FFFFFF. Later, when the RLE conversion is being undone, uncopy_32 is called, which has the following code:
SDL_RLEaccel.c:1001:
Uint32 pixel = *s++;
RGB_FROM_PIXEL(pixel, sfmt, r, g, b);
a = pixel >> 24;
PIXEL_FROM_RGBA(*dst, dfmt, r, g, b, a);
However, the the alpha channel has already been dropped by copy_opaque (= copy_32), so pixel = 0x00FFFFFF and 'a' becomes 0. Thus, all opaque pixels lose their alpha channel when being unRLE'd. (I don't know what happens to pixels with alpha from 1-254, but they should be checked too.)
So, that seems to be the problem, but I'm not sure what the solution should be. Since opaque pixels have alpha == 255, I'm thinking to create another uncopy function for opaque pixels that simply uses 255 for alpha.
However, there may be other problems here. For translucent pixels, uncopy_32 assumes the alpha channel is stored in the upper 8 bits, but copy_32 doesn't store it there. Instead, it stores it in whatever location is appropriate for the destination surface. Isn't one of their behaviors incorrect, given the other? I'm not sure which to change, however.
For translucent pixels, it seems that the blit function uses do_blend, which is the BLIT_TRANSL_888 macro, which also assumes alpha is in top 8 bits. It has the comment "we have made sure the alpha is stored in the top 8 bits...", but it seems that's not true (copy_32 doesn't make sure the alpha goes there).
Perhaps the correct fix is to make copy_32 put the alpha there, but then that seems to require that RLE conversion be limited to destination surfaces that don't use the upper 8 bits. However, looking further, it seems that has already been done: if (masksum != 0x00ffffff) return -1; /* requires unused high byte */
Philipp Wiesemann
Eric Wing
Ryan C. Gordon
Alex Szpakowski
Sam Lantinga
Ethan Lee